Studies on the ecology and disease transmission of the psyllid vectors of the citrus greening disease, with special reference to the South African vector, Trioza erytreae [Del Guercio] [Homoptera : Psyllidae]

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S t u d i e s on t h e e c o l o m and d i s e a s e t r a n s m i s s i o n of t h e p s y l l i d v e c t o r s of t h e c i t r u s greqnin:,. d i s e a s e , w i t h s p e c i a l r e f e r e n c e t o t h e South A f r i c a n v e c t o r , ~ r y t r e a e

el

~ u e r c i o ) (HO- mop t e r a : ~ s y l l i d a e

)

H.D. C a t l i n g l4.S~.

( I n s t i t u t e f o r Zoological Research, Department of Zoology, Potchef s t r o o n U n i v e r s i t y f o r C.H.E., ~ o t c h e f s t r o o n )

THESIS

PRESENTED FOR THE DEGREE OF

DOCTOR SCIEMTIAE

IN

THE

POTCHEFSTROOM UNIVERSITY FOR C.H.E.

Promoter : D r D.P. Annecke February, 1970

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A comparatively new v i r u s - l i k e disease c a l l e d greening has becone a very s e r i o u s t h r e a t i n many c i t r u s producing c o u n t r i e s of the vrorld. The disease i s transmitted by two i n s e c t v e c t o r s belonging t o the P s y l l i d a e ( ~ o n o ~ t e r a ) namely,

ziizg

~ a t r e a e

el

~ u e r c i o ) , the African vector, and m o h . i % c i t r i Kuw., the O r i e n t a l vector.

-

F i e l d s t u d i e s were made on the e c o l o g , biology'and c o n t r o l of

2.

_erytr_eae in the northern Transvaal and i n Swaziland during 1965

-

1970. This v e c t o r i s extremely fecund b u t has weak powers of d i s p e r s a l . The nain e c o l o g i c a l fac- t o r s found t o r e g u l a t e populations of t h i s i n s e c t were the f l u s h i n g rhythm and f l u s h q u a l i t y of c i t r u s , extremes of weather, and n a t u r a l enemies. Other f a c t o r s involved included i n t e r s p e c i f i c coi.ipe t i t i o n with c i t r u s aphids, and i n some seasons i n t r a s p e c i f i c competition f o r breeding s i t e s . Fundar~entally f l u s h i n g r h y t h determines the p o t e n t i a l population d e n s i t y of the vector, while the occurrence and sequenoe of l e t h a l weather extremes m i n l y r e g u l a t e

the p o p u l a t i o ~ l s i z e during c i t r u s growth periods. A weather index was found t o e x p l a i n the p e s t s t a t u s of the veotor i n southern A f r i c a and p a r t l y accounts f o r previous outbreaks of v e c t o r and disease. A population nodel i s given which e x p l a i n s the seasonal abundanoe of t h e i n s e c t . S t u d i e s were a l s o made on the general biology i n c l u d i n g sex r a t i o , egg laying, mating behaviour, and the i n f l u e n c e of temperature on the duration of the immature stages.

The greening disease r e p r e s e n t s a severe l i m i t i n g f a c t o r t o c i t r u s pro- duction in the highe-lying regions of the e a s t e r n Transvaal and Swaziland. The h o t t e r lowveld a r e a s a r e v i r t u a l l y f'ree of d i s e a s e symptoms and v e c t o r populations though p r e s e n t a r e u s u a l l y low. Transnission s t u d i e s showed t h a t To i s the p r i n c i p a l vector, s e v e r a l o t h e r p s y l l i d species feeding

CI

on c i t r u s were n o t found t o be t r a n s m i t t e r s . It appears t h a t a f a i r l y m a l l proportion of f i e l d a d u l t s a r e i n f e c t i v e and t h a t t h e r e i s a seasonal f l u c t u - a t i o n i n t r a n s n i s s i o n e f f i c i e n c y . Single males and females were a b l e t o t r a n s n i t the disease. Unsuocessful a t t e n p t s were made t o screen a d u l t s f o r i n f e c t i v i t y using a chronatographic method, and t o i s o l a t e t h e c a u s a l orga- nism f r o n excised s a l i v a r y glands.

The c o n t r o l of

2.

e r y t r e a e depends an the use of i n s e c t i c i d e s . Experi- nents t o s e l e c t a s u i t a b l e r x i t e r i a l and t o determine i t s c o r r e c t t i n e o f ap- p l i c a t i o n a r e described. A spray prosanme f o r the c o n t r o l of c i t r u s p s y l l a

has been used with apparent success by farmers i n the Malkerns d i s t r i c t of Swaziland.

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(

ii)

Comparative f i e l d observations and surveys f o r both Asian greening disease and t h e O r i e n t a l v e c t o r a r e described, a d the world d i s t r i b u t i o n of greening and of t h e two p s y l l i d v e c t o r s a r e a v e n .

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(

iii) S t u d i e s on the e c o l o ~ and d i s e a s e t r a n s m i s s i o n of t h e p s y l l i d

v e c t o r s o f the c i t r u s greening d i s e a s e , w i t h s p e c i a l r e f e r e n c e t o t h e South A f r i c a n v e o t o r , p i o z a e r y t r e a e (Del Guercio) ( ~ o - mop

tera

: P s y l l i d a e

)

H.D. C a t l i n g l¶.So.

I n s t i t u t e f o r Zoological Researoh, Department of Zoology, P o t o h e f s t r o o n U n i v e r s i t y f o r C.H.E., Potchefstroom C O N T E N T S __r- General

. . . o . . .

.

1 P e s t s t a t u s of

2.

e w t r e a e . . . ~ a ~ . o ~ ~ ~ o . . 1 The greening d i s e a s e of c i t r u s

...

2 L i t e r a t u r e review

...

2 ( i )

g.

e r y t r e a e

.

. .

.

. . .

. . . .

.

. . . .

.

. .

.

. . . .

.

. .

.

...

.

2 v-. ( i i ) South A f r i c a n greening

.

. . .

.

. . .

.

. . .

.

3

The c i t r u s i n d u s t r y i n Swaziland

...

4

Other p s y l l i d s p e c i e s on c i t r u s

...

4

( i )

2.

p u n o t u l a t a

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.

. .

..

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.

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.

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.

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.

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5

(ii)

2.

zebra.na . . .

... ...

.

5 SECTION A a -E o o l o g i c a l s t u d i e s cn Trioza, e r y t r e a e (Del Guercio)

...

. . .

..

7

Notes on

x.

e p t r e a e . . . O . . . . O . . . O . . . O .

7

F i e l d nethods

...

7

( i ) Experimental s i t e s

. .

. . .

. .

.

. . .

. .

* . o . a * .

.

e . a o .

8

( i i ) P o p u l a t i o n a s s e s s n e n t s o f

2.

s t r e a e

.

. .

.

. . .

. .

.

9

( i i i ) Assessments of f l u s h i n g rhythm

...

.

..

...

11 ( i v ) Assessments o f p a r ~ s i t e a o t i v i t y

. .

.

,

.

. . .

. .

.

. .

13

( v )

,z

s i t u c o u n t s

.

. . .

.

o. o . * . ., a o . * .

.

...

.

14

( v i ) Greening assessments

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.

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.

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.

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.

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.

...

.

....

14

I n f l u e n c e o f f l u s h i n g r h y t h - ...~.o.a~a.o~....

Uc

( i ) F l u s h i n g rhythm and p o p u l a t i o n f l u c t u a t i o n f o f

2. 9-

. . . 0 . . 0 . . 0 9 0 0 . . . 0 . . .

1 5 (ii) I n f l u e n c e of f l u s h i n g on o v i p o s i t i o n

. . . .

.

. . .

.

. .

.

18

(iii) I n f l u e n c e o f t r e e age and v a r i e t y on f l u s h i n g rhythm

.

. .

18

( i v ) 11 vv the greening d i s e a s e " n I I

...

1 9

( v )

D i s t r i b u t i o n o f f l u s h nocording t o t r e e a s p e o t

.

,

. .

.

20

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Influence of f l u s h q u a l i t y

...a,...,...

24 ...

( i ) E f f e c t of n u t r i t i o n on nymphal development 24 ( i i ) Fluctuations i n nitrogen content of young f l u s h i n tho

f i e l d . . . a . . . e O O O ~ . . . 25

...

( i i i ) Discussion 27 Influence of p a r a s i t e s

...

30

...

.

( i ) P a r a s i t e s of

2

s t r e a e

..

31 ...

( i i ) f i e l d s t u d i e s a t Malkerns Research S t a t i o n

33 ...

( i i i ) F i e l d s t u d i e s a t Ross C i t r u s E s t a t e 35 ( i v ) Other s i t e s in Swaziland

...

35

...

( v ) Discussion 35

...

Influence of predators 39

...

.

( i ) Predators of'

$

e r y t r e a e 40

( i i ) Experiments on the effeotiveness of predators a s l i m i t - i n g f a c t o r s

...e.eo...

42 ( i i i ) Predator a c t i v i t y i n f i e l d populations

...

43 ...

...

( i v ) Discussion

..

45

...

Influence of extremes of weather on s u r v i v a l

46 (i)

Praliminary evidence of the letlzal e f f e c t af high ten-

p e r a t u r e s

...

47

( i i ) Survival of eggs and f i r s t i n s t a r nymphs f'rom

&

s i t u

counts

...

47 ( i i i ) Population f l u c t u a t i o n s of

2 .

e s y t r e a ~ i n r e l a t i o n t o

p r e v a i l i n g weather

...

51

( i v ) Discussion

...

53

Effeot of weat!ier extremes on d i s t r i b u t i o n and outbreaks of

...

.

T

z x t r e a e 56

== ...=

-...

( i ) Derivation of a weather index

...

57

...

( i i ) Regional m o r t a l i t y assessments 57

( i i i ) An explanation f o r outbreaks of

2 .

3 r y t r e a e and green-

.

i n g : 1938 1968

...o...

58 ( i v ) Disoussion

...

59

...

Duration of immature stages 60

Influence of population density. l e a f s i z e and l e a f placement

...

on egg hatoh and nymphal s u r v i v a l

..

62

...

( i ) ~ n f l u e n c e of population density 62

...

( i i ) Influence of l e a f l e n g t h and p o s i t i o n 63

...

( i i i ) Disoussion

..

63

...

...,...

Sex r a t i o

....

64

( i ) Sex r a t i o i n f i e l d populations a t Malkerns

...

64

...

( i i ) Sex r a t i o i n laboratory 2 u l t u r e s 65

...

( i i i ) Discussion 65

...

Mating behaviour. oviposi t i o n and longevity 66

...

( i ) Mating behaviour

66 ...

( i i ) Oviposition 67

...

( i i i ) Pre-oviposition

..

71

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...

( i v ) Longevity

71

13 .

D i s p e r s a l

...

72

...

( i ) D i s p e r s a l a t Malkerns Research S t a t i o n 72

...

( i i ) Discussion

..

...

73

...

14 .

P o p u l a t i o n ilynanics o f 2

.

e r y t q e a g 75

...

( i ) N w e r i c a l f l u c t u a t i o n 75

...

( i i ) F i e l a g e n e r a t i o n s 76 ( i i i ) Fecundity

...

77 ...

(

i v ) S u r v i v a l

78 ...

( v )

S i g n i f i c a n c e of t h e main e c o l o g i c a l f a o t o r s

78 ...

( v i ) A p o p u l a t i o n model 82

15 .

Summary

...

83

SECTION B

...

S t u d i e s on d i s e a s e t r a n s m i s s i o n and c o n t r o l o f 2

.

e r y t r e a e 86

...

I n t r o d u c t i o n 86

Inoidenoe o f greening in Swaziland

...

86

...

( i ) Malkerns. Swaziland Middleveld 86

...

( i i ) Swaziland Lowveld

..

88

Vector t m s n i s s i o n s t u d i e s

...

88 ( i ) Screening of a d u l t s and nymphs of

2.

e _ r y t t and t h e

c i t r u s a p h i d

...

89

...

( i i ) Screening of o t h e r p s y l l i d s p e c i e s 9 1

...

(

iii) Seasonal f l u c t u a t i o n i n t r a n s m i s s i m e f f i c i e n c y 9 1 ( i v ) P r o p o r t i o n of i n f e c t i v e i n d i v i d u a l s i n f i e l d p o p u l a t i o n s

93

( v ) R a t e of spread of greening i n t h e f i e l d

...

94

...

Location of t h e greening pathogen i n t h e v e c t o r

95

D e t e c t i o n o f ~ e e n i n g n a r k e r s u b s t a n c e s i n t h e v e c t o r

...

96

...

Control of t h e v e c t o r

.

...

96

(

i) P r e l i m i n a r y s c r e e n i n g of i n s e c t i c i d e s

...

97

(ii)

S o i l - a p p l i e d systemics v s f o l i a r s p r a y s

...

9 7 (iii) F o l i a r s p r a y s o f dimethoate

...

98 ( i v ) Laboratory t r i a l s w i t h dimethoate

...

99

...

( v ) Discussion 100 SECTION . C

.

Observations on giaphoy+-~citri Kuw

...

104 1

.

I n t r o d u c t i o n ...~.OO.O.OOOOOOOOOOOO.O... 1 0 4

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...

3 .

Bionomics of

2 .

&trA

106

( i ) Population f l u c t u a t i o n s and f l u s h i n g rhythm of c i t r ~ s

.

106

...

( i i ) Natural enemies

LO7

... ...

( i i i ) Extremes of weather

.

108

4 .

Surveys f o r

2 .

z i t r i

and greening i n Asia and t h e Far E a s t

...

108

...

(i)

P h i l i p p i n e s 108

...

(

i i ) Other c o u n t r i e s 110

5.

World d i s t r i b u t i o n of greening v e c t o r s

...

111

...

6 .

c o n t r o l c f

2 .

ci2iri 112

7 .

Suumary and conclusions

...SO...O...

113

References

...

117

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1. GENERA-L

S t u d i e s on the e c o l o , y of e r y t r e a e

e el

~ u e r c i o ) were begun i n 1965 i n the Letaba d i s t r i c t of t h e northern Transvaal and continued u n t i l February, 1967, in this d i s t r i c t . P a r t of t h i s work formed the b a s i s of a master's t h e s i s e n t i t l e d "Studies on the ecology of the South A f r i c a n c i t r u s p s y l l a , Trioza e r y t r e a e

e el

~ u e r o i o ) ( ~ o m o ~ t e r a : ~ s ~ l l i d a e ) " . This study was continued in g r e a t e r depth and extended t o include i n v e s t i g a t i o n s i n t o v e c t o r transmission and epidemiology of the c i t r u s greening d i s e a s e f r o n Fe- bruary, 1967, t o January, 1970, i n ~ w a z i l a n d .

For t h r e e months during 1968 t h e a u t h o r was employed by t h e Food and A g r i c u l t u r a l Orsanization of t h e United N a t i o n s t o a s s i s t t h e P h i l i p p i n e Bu-

r e a u of P l a n t I n d u s t r y i n i n i t i a t i n g r e s e a r c h on the O r i e n t a l v e c t o r of the greening disease. Surveys f o r the d i s t r i b u t i o n of t h i s vector, Diaphorina c i t r i Kuw., and oomparative observations on symptom of d e c l i n i n g c i t r u s groves,

I

were made i n t h e P h i l i p p i n e s and s e v e r a l o t h e r c o u n t r i e s in A s i a and t h e Far East. During Novenber, 1969, t h e a u t h o r a t t e n d e d t h e 5 t h Conferenoe of the I n t e r n a t i o n a l Organization of Citrus V i r o l a g i s t s h e l d in Japan and p a r t i c i p a t e d in a Post-Conferenoe t o u r of s e v e r a l citrus-producing c o u n t r i e s of South-East Asia.

Ecological s t u d i e s on the two p s y l l i d v e c t o r s a r e of v i t a l importance i n gaining an understanding of t h e n a t u r e of t h i s s e r i o u s and widespread c i t r u s disease. U n t i l the start of t h e Transvaal study t h i s a s p e c t had been l a r g e l y neglected i n southern Africa, while work on t h e O r i e n t a l v e c t o r was s t a r t e d i n the P h i l i p p i n e s i n 1968. Likewise, l i t t l e a t t e n t i o n has been p a i d t o t h e v e c t o r transmissiun of the disease. Fron a n academic p o i n t of view the study on

2.

e r y t r e a e may c o n t r i b u t e t o our h o w l e d g e of the ecology of the P s y l l i d a e . 1Naloff (1968) recognizes t h e work of Clark (1962, 1963a, b etc.) on Cardiaspina a l b i t e x t u r a , and VJatmough (1968a) on two Arytaina spp. a s the only b a s i c s t u d i e s on t h e e c o l o ~ r of t h i s superfamily. To t h e s e may be added the work of Moran (1967) on the p s y l l i d , Paurocephala c a l o d e n d r i Moran.

2. PEST STA!KJS OF T. mYTRJUE

U n t i l

i t

was shown t o be a v e c t o r of the q e e n i n g disease,

2 .

e-wtreae was regarded a s a minor p e s t of c i t r u s i n southern A f r i c a . Closely-related

groups of the Hornoptera, such a s aphias, leaf-hoppers and white f l i e s , have been known a s v e c t o r s of v i r u s - l i k e d i s e a s e s of p l a n t s f o r nany years. But a p a r t from the " p s y l l i d yellowsv condition, which i s a l o c a l i z e d c h l o r o s i s caused by t o x i n s s e c r e t e d d u r i n g feeding ( C a r t e r , 1962), i t i s only conpara- t i v e l y r e c e n t l y t h a t p s y l l i d s have been shown t o be involved i n d i s e a s e

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transmission. I n 1964. Jensen

&

FA.,

c l e a r l y demonstrated that P s g l l a p.yricola F o e r s t e r was the v e c t o r of the p e a r d e c l i n e d i s e a s e i n C a l i f o r n i a , Following

t h i s discovery, f i r s t

x.

&';y+reae t h e n

go

c i t - i were shown t o be v e c t o r s of t h e greening d i s e a s e of c i t r u s . Both of t h e s e d i s e a s e s a r e graft t r a n s m i s s i b l e b u t a r e spread more e f f e c t i v e l y 5y t h e i n s e o t v e c t a r .

3.

THE

GREENING

D I S F ~ E OP -.-_._I_._ CITEXJS

I n t h e l a s t three decades a nurber of c l o s e l y r e l a t e d v i r u s - l i k e d i s e a s - e s have appeared i n most r e g i o a s of Yfie world where c i t r u s i s grown. Many workers now b e l i e v e t h a t the greening d i s s a s e of South Africa, l e a f m o t t l i n g

of the P h i l i p p i n e s , an3. a major cornp.:nent i n %he c i t r u s die-back of I n d i a , a r e caused '$y t h same ?at.?.?gen c- p a t h o g m cor$ex. I t i s a l s o becoming in- c r e a s i n g c l e a r ?!:EL

t

t h s s'L.--.':5orn < i s e a c s of Br.er!.cs, vein-phloem degeneration

. -

of Indone sj.a, y r ? l L o : ~ - z':oo2: c? r::,;~.:-.?.r-~ Ci:i:-?.

,

and lik&ia of Taiwan, a r e v e r y s i m i l a r t o g?s?ni.g. Il>:c;--4:.?.y g~.:aning h a s A".; 3 Seen r e p o r t e d from B r a z i l

( ~ o s s e

tti,

1559). I n NoveAer, 1959, i - k was t h e considered opinion of de- l o g a t e s t o t h e 5 t h Coxt'e=.c:-,-e o f t h e I r . t s r r ~ t L o n a 1 Organization o f C i t r u s V i r o l o g i s t s t h t , tl3.s c o q s r a t l v e l y r:;:7 v i l . r ~ ~ = - l i k e d i s e a s e c o n s t i t u t e s the

most s e r i m s d i s e z s s p r o ' k m frtcLrn

-

cqA-.' .,- . A -- L_l 1 t u r e a t the p r e s e n t time.

S e v w a l workers c w Se1.i.e-.-e *;:;at fhe g;-eezing pathogen, which has always been a soz3wh3-5 laO_qFc=i.l vi.~-.:;;~, 1i3.y k 3 a q c o p ? a s m and n o t a t r u e v i r u s a t a l l , This IT-2-d e?nou::l for: G:i-J CC .i'~e ~ . n c ~ . a l i r s a s s o c i a t e d with the

d i s e a s e c o ~ q l e x . I n the c v s of' <. ::s pe?-: d e s l i n e 2 i s e a s e , which c l o s e l y

paralle1.s c i t r c s grer ..ling, :ace;r.",leecY.-im c l :: :osc cp:;. s t::die s have r e v e a l e d t h e presance of myco;-~3.asas j.n t t 3 sali;.;:-y g - . n d s c:? inf'e2ted i n s e c t s and

i n

t h e s i e v e t u b e s of p e r t r e z s ( ~ o h e i d e ? , H. 1969, p e r s o n a l communication). Since m y c o p l a s ~ s a r e s u s c e p - 5 b l e '3 t e t x - s c y c l h e s , t h e r e i s some hope o f a

c u r a t i v e t r e a t m e n t b e i n g dsvcloped i n t k s fukure.

( i )

2. ertae.

C i t r u s psyXLa was f i r z 5 r e p o r t e d in southern Africa by Lounsbury i n 1897, Van d e r Merwe (l9!!3) p z b l i s h e d a g e n e r a l account of

t h e i n s e c t d i s m s s i n g such & , p e s t s a s t>,xor_:zy, p e s t ~ t . biology, distri- 5 ~ ~ ~ bution, end c o n t r o l measura s , AddF-3or nL bio: ? s i c 3 1 n o t e s were c o n t r i b u t e d from l a b o r a t o r y s t u d i e s by Anneck3 & Cil!-iers (1963). The l a t t e r a u t h o r s a l s o d i s c u s s e d t h e p a r a s i t i c spec:-es a t h c k i n g

2.

srytreae-.

F i e l d s t u d i e s by Schwarz (1964) suggested that e i t h e r

2.

e w t r e a e

or

the b l a c k c i t r u s aphid, Gxop&eza , ~ ~ i , t ; - l ~ ~ i $ u . ~ ( K i r k , ) , w a s a v e c t o r of greening, while f i e l d o b s e r v a t i o n s by Oberhoizer

2 -

gL,,

(1965) i m p l i c a t e d t h e former. S h o r t l y a f t e r w a r d s McClean & OSerholzer (1965b) were f i n a l l y a b l e t o demon-

s t r a t e t h a t T. e ~ t r e a e

-

_*-= Ss i n f a o t a n itcpoAan-5 v e c t o r of t h e d i s e a s e .

(10)

Immature stages

o f

the p s y l l a a n d x . c i t r i c i d u s d i d n o t t r a n s m i t t h e d i s e a s e i n t h e i r experiments, Subsequently,

a

large number of t r a n s m i s s i o n t e s t s have confirmed t h e s e r e s u l t s ( ~ c c l e a n A.P.D. & C a t l i n g

H.D.

-

unpublished r e s u l t s ) .

The e l e v a t i o n of

2.

e r y t r e a e t o

m a j o r

p e s t s t a t u s encouraged s e v e r a l workers t o b e g i n b a s i c s t u d i e s on t h i s c i t r u s p e s t . Moran & Blowers (1967) d e s c r i b e d t h e egg and nymphal s t a g e s , and under c o n t r o l l e d , f l u c t u a t i n g con- d i t i o n s in t h e l a b o r a t o r y s t u d i e d t h e l i f e h i s t o r y and noted t h e e f f e c t s of t h r e e temperature regimes on development and s u r v i v a l . I n t h e same y e a r t h e s e a u t h o r s d e s c r i b e d t h e s t r u o t u r e and development o f the female repro- d u c t i v e system

l lowers

& Moran, 1967). From f i e l d s t u d i e s i n t h e n o r t h e r n Wansvaal, C a t l i n g (1967) gave a g e n e r a l ttcoount of t h e biology and bionomics of t h e i n s e c t and enumerated t h e main e o o l o g i o a l f a c t o r s . A b r i e f ac- oount of t h i s study was p u b l i s h e d t h e f o l l o w i n g y e a r ( c a t l i n g & Annecke, 1968). Moran (1968a) compared the development of

2.

e w t r e a e on f i v e d i f - f e r e n t h o s t p l a n t s , and i n a companion paper

ora an,

1968b) d e s o r i b e d pre- l i m i n a r y s t u d i e s on t h e choice of h o s t p l a n t s by t h e a d u l t . The work o f that a u t h o r was brought t o g e t h e r in a Ph.D. t h e s i s

oran an,

1967) which in- oluded a d d i t i o n a l b i o l o g i o a l n o t e s and a d i s c u s s i o n on t h e d i s t r i b u t i o n of both v e c t o r and greening d i s e a s e ,

C a t l i n g (1969a, b, o) published t h r e e papers on t h e bionomics of

g.

t r e a e and d e s c r i b e d s t u d i e s on t h e chemical o o n t r o l of t h e i n s e c t (1969d).

I

The i n f l u e n o e of weather-induced m o r t a l i t y on t h e d i s t r i b u t i o n of t h e v e c t o r , and a n e x p l a n a t i o n f o r p a s t outbreaks of b o t h v e o t o r and d i s e a s e were given by Green & C a t l i n g ( i n p r e s s ) . A l l

o f

t h e above f i v e p a p e r s a r e included i n t h i s t h e s i s .

The eoology of t h e p s y l l i d v e c t o r s of c i t r u s greening was reviewed by

Catling

(1969 f and g).

(

i i ) South A f r i w n greenin+ Oberholzer

&.,

(1965) d e s c r i b e d t h e h i s t o r y of greening o u t b r e a k s

i n

South Afrioa. The l a t t e r a u t h o r s ' and MoClean & Oberholzer (19653) gave accounts of t h e h o s t range and symptoms

of t h e d i s e a s e , and l e d evidenoe t o show t h a t greening i s a t r a n s m i s s i b l e v i r u s . S t u d i e s on t h e anatomical a s p e c t of t h e d i s e a s e were made by

Sohneider (1968). A c l o s e c o r r e l a t i o n between t h e incidence of

2.

e r y t r e a e and t h e spread of g r e e n i n g i n southern A f r i c a has been c l e a r l y e s t a b l i s h e d

( C a t l i n g , 1967; Schwarz, 1967; Schwarz

&

s.,

i n p r e s s ) .

Schwarz (1965b; 1968a; 1968b) discovered a f l u o r e s o e n t marker substance i n t h e albedo of

f r u i t

and t h e bark o f twigs of i n f e o t e d branohes. He t h e n developed a r e l i a b l e chromatographio method f o r d e t e c t i n g t h e d i s e a s e i n many s p e c i e s and v a r i e t i e s of c i t r u s . Schwarz ( 1 9 6 8 ~ ) a l s o succeeded i n t h e

(11)

(12)

mechanioal transmission of greening

t o

cucumber. Definite s t r a i n s of green- i n g were reported i n South Africa by the same author (1969). Schwarz & Green (1969) found the degree of symptom expression i n the e a s t e r n Transvaal t o be governed by a h e a t index o o r r e l a t e d with a l t i t u d e , and reported the success- f l i l h e a t i n a c t i v a t i o n of inf'ec t e d budwood.

THE CITRUS INDUSTRY I N S$UILAND

5.

-

C i t r u s i s a young i n d u s t r y jz Swaziland and the f i r s t s i g n i g i c a n t ex- p o r t s of f r u i t began i n 1965. The o l d e s t commeroial groves were planted i n Malkerns i n 19% and 1955. Fig. 1 shows the d i s t r i b u t i o n of c i t r u s i n rela-

t i o n t o the main ecological regions. A s a t March, 1969, there wore 554,000 t r e e s i n Swaziland, 58% s i t u a t e d i n the a r i d Lowveld region and 42% i n the Middleveld. Sweet oranges, mainly Valencias, form 6% of the plantings,

3%

a r e Marsh Seedless g r a p e f r u i t planted mainly i n the Lowveld, and the re- maining 5% i s made xp of several other t@es including lemons.

OTHER PSYLLID SPECIES ON CITRUS

T. e q t r e a e and

2,

c i t r i a r e the only two p s y l l i d speoies known t o breed

-

on c i t r u s . However, a number of other species a r e occasional feeders and some may e x i s t on c i t r u s f o r extended periods when t h e i r normal h o s t i s unattrac- t i v e or dormant. I n Japan, f o r instance, P s y l l a coccinea (KUW.) has been col- l e o t e d on c i t r u s but does not feed on t h i s host

asto top

V.F., 1967, personal communication). The following p s y l l i d species were found on c i t r u s during the p r e s e n t study:-

1. Illphorina p u n c t u l l Pe t t e y , widely i n Transvaal and Swaziland

2. Diaphorina

--

-.TI__ zebrana (capener MS*), widely i n Transvaal and Swaziland

3

n r D e s m i o s t i ~ sp., Swaziland, April, 1968, r a r e

4.

A w t a i n a

nr

mopani Pettey, Letaba and Agatha, December, 1965 5. Agonoscena sp O , Swaziland Lowveld, January and March, 1969

6. nr Pauro_psylla sp., Malkerns, Deoember, 1968

7.

W t a i n a sp., Malkerns, 1968

S i x more species were found on other host p l a n t s adjacent t o c i t r u s groves:- two unde scribed Trioza spp

.

; Ciriacremum sp. ; Paurop s y l l a

nr

t r i c h a e t a Pettey; nr Allceneura sp

.;

P s y l l a sp.

The above two D i a p h o r a - species feed on c i t r u s i n l a r g e numbers i n the Transvaal and have been observed on c i t r u s i n Swaziland. Transmission t e s t s with these two p s y l l i d s a r e described i n Section

B e

I t i s p o s s i b l e t h a t un- detected p s y l l i d s feeding i n t e r m i t t e n t l y on c i t r u s may be responsible f o r

*

t o be published s h o r t l y

Fig. 1 Map of Swaziland showing the main ecological regions and d i s t r i b u t i o n of c i t r u s

(13)

the slow spread of the stubborn disease in C a l i f o r n i a and Arizona ( ~ a l a v a n ,

E.C., Carpenter, J.B., and Allen, R.M. 1969, personal oomrnunication)

.

( i )

g.

punctulata. T h i s species i s widely d i s t r i b u t e d in South Af'rioa and h a s been recorded on over 30 d i f f e r e n t indigenous shrubs and t r e e s (cape-

ner, L., 1967, personal communioation). I t s main breeding p l a n t i s the

ma-

rula,

Sclerooarva o a f f m Sond. I n the Letaba d i s t r i o t

i t

was colleoted by the author on manila a t Ofoolaoo and New A g a t h a , and was p a r t i o u l a r l y abun-

dant i n Ootober of 1966 a t Letaba E s t a t e s when

57.8

n f 110 buds removed

from 22 widely-separated marula t r e e s were found

to

be i n f e s t e d . I t was

a o l l e c t e d on marula a t Mpaka and Sipofaneni, northern Swaziland Lowveld, and on c i t r u s a t W k e r n s and Esulwini.

D. punctulata was p r e s e n t on c i t r u s i n the Letaba d i s t r i c t throughout

-

1)-

-the year. During July, 1966, when m r u l a s were dormant l a r g e numbers of

a d u l t s were found feeding on c i t r u s a t Letaba Estates. Colleotions with

the i n s e o t sampling mohine showed t h a t the p s y l l i d was p r e s e n t i n c i t r u s a t a l l times of the day and night, and t h a t t h e i r numbers s t e a d i l y deolined

a t the end of August when t h e i r breeding h o s t came i n t o flower. Adults

survived f o r

45

days i n the laboratory on c i t r u s seedlings. I n one marula

t r e e studied a t Letaba, overwintering a d u l t s were noticed on the p e t i o l e s

cf

o l d leaves i n e a r l y September. Shortly afterwards, eggs of the f i r s t spring

generation were l a i d i n expanding buds giving r i s e t o oolonies of nymphs between the b r a o t s of young flowers o r i n developing shoots. With f u r t h e r shoot growth some of the darkish nymphs were noticeable on u n f u r l i n g leaves b u t l a t e r the nymphs became enolosed between the upper halves of the leaf' whioh f o l d inwards. D i s t i n c t i v e c o i l s of w h i t e , spaghetti-like excreta ap-

pear on i n f e s t e d flowers and shoots. By the middle of October many t e n e r a l

a d u l t s were observed, i n d i c a t i n g

s

5-6

woek poriod f o r

tho

f i r s t - spring

generation. The main peak of a d u l t s was seen

in

November and the p s y l l i d

was again f a i r l y abundant on the January growth cyole.

Buds examined i n Ootober, 1966, indioated that severe weather extremes

had oaused a high mortality r a t e i n f i r s t and seoond i n s t a r nymphs.

( i i )

2 .

sebrana. Both i n l i f e h i s t o r y and general biology t h i s

g

e c i e s

i s renarkably dimilar t o

2.

punctulata. The c l e a r l y s t r i p e d wings, however,

d i s t i n g u i s h

i t

frompunotulata. Both speoies breed on

the

young growth of

the

manzla

t r e e and a r e f r e q u e n t l y found together on the same l e a f o r shoot.

During the e a r l y summer of 1966

E.

zebrana was found t o be the predominant

speoies on some t r e e s a t Letaba,

This speoies was a o l l s o t e d a t New A g s t h e and Letaba in 1966, and a t

Malksms and Sipofaneni i n Swaziland i n 1967. Like punctulata

i t

appears

ta

overwinter in t h e adult stage on s e v e r a l evergreen trees, inoludipg

citrus.

- - - -

- - -

(14)

When conf'ined on

c i t r u s

i n the laboratory, however, t h i s speoies survived f o r a oomparatively s h o r t period.

(15)

(16)

SECTION A Ecological S t u d i e s on Trioza e t r e a e

e el

~ u e r c i o ) -.1 -=

-

--~...?L..<

1.

NOTES

ON

2.

ERYTREAE -.

The taxonomy of

2.

e r y t r e a e has been discussed by C a t l i n g (1967) and Moran (1967). The eggs a r e l a i d on young growth, u s u a l l y along the margin

of young l e a v e s ( ~ 1 . I.). There a r e f i v e nymphal i n s t a r s (Fig. 2~). The

f i r s t i n s t a r s e t t l e s on t h e underside of the l e a f and a c h a r a c t e r i s t i o open g a l l begins t o form ( ~ 1 . 11). The a d u l t i s winged and sexes a r e s e p a r a t e (Fig. 2 c,D). The l e a v e s and shoots of h e a v i l y populated p l a n t s may become s p r i n k l e d with white granules of e x c r e t a ( ~ 1 . 11). The f e e d i n g of l a r g e num- b e r s was n o t observed t o cause any s e r i o u s t o x i c e f f e c t t o p l a n t t i s s u e , l e a f

c h l o r o s i s being s l i g h t and extremely r a r e .

T. e r y t r e a e has been r e p o r t e d i n a l l the main c i t r u s a r e a s of southern

.I

A f r i c a and was observed o r c o l l e c t e d by t h e a u t h o r i n Rhodesia, n o r t h e r n and e a s t e r n Transvaal, Swaziland, Zululand, and a t Cape Town and Tulbegh in t h e western Cape Province. Specimens were a l s o r e c e i v e d from the a r i d C i t r u s - d a l a r e a of the western Cape.

The world d i s t r i b u t i o n of

2.

e r y t r e a e , and of the O r i e n t a l veotor, Diauhorina c i t r i Kuw., i s discussed i n S e c t i o n C. I n Appendix 1 s a l i e n t differences i n the biology and appearance of the two v e c t o r s a r e tabulated.

2. PIELD METHODS

This c h a p t e r d e s c r i b e s the experimental s i t e s and the main f i e l d methods used i n the study of

2.

eyytreae. Other methods used in small-scale f i e l d s t u d i e s o r l a b o r a t o r y experiments a r e d e s c r i b e d i n t h e a p p r o p r i a t e chapter. The main method of sampling f o r

2.

g t r e a e , t h e i n s i t u counts, and t h e method of p a r a s i t i s m assessments were d e s c r i b e d by C a t l i n g (1967) b u t a r e included here f o r t h e sake of completeness and because of a d d i t i o n a l datq obtained i n the f i e l d on t h e v a l i d i t y of these methods. S i m i l a r l y , b r i e f d e s c r i p t i o n s of experimental s i t e s i n the Le taba d i s t r i c t a r e included.

SXpLANATION OF FIGURE

Fig. 2 Trioza e r y t r e a e

el ~ u e r c i o ) . A

-

egg;

B

-

f i v e

nymphal

i n s t a r s , t h i r d i n s t a r shown a s newly-moulted; C

-

g r a v i d female; D

-

male i n f e e d i n g p o s i t i o n . A l l s t a g e s t o same soale. _del. _H.D. _{C a t l i n g}

(17)

plate

I.

Eggs of

z.

e w t r e s e on the margin of young oitrus leaves

Plate 11. Citrus seedlings heavily populated with

2.

erytreae

_CI

(18)

8.

(i) b e r i m e n t a l

sites

(Fig.

3)

Letaba D i s t r i o t . Tranmrvaal. The Le taba

d i s t r i o t

i s flanked on the west

by

the Dralcensbsrg esoarpment and extends eastwards i n t o the Lowveld

3-

Bush-

veld

of Aoooks

(1953).

For the t h r e e main elcperimenbl s i t e s

in

this

area,

blooks of 50 o i t r u s t r e e s of oomparable age, size, a s p e o t and distanoe from n a t u r a l bush were s e l e c t e d a t three d i f f e r e n t a l t i t u d e s . None of

the

trees

had reoeived

a

r e g u l a r spray programme f o r a t l e a s t three years. Table 1 shows f u r t h e r d e t a i l s of the s i t e s . A thermohygrograph was operated a t each s i t e

ao-

oording t o s p e c i f i o a t i o n s suggested by tho South African Weather Bureau.

Two minor s i t e s , each with a srnall weather s t a t i o n , were e s t a b l i s h e d i n oommercial groves a t Letswalo and Riverside. Regular surveys were made i n the 400,000 t r e e s planted along the northern s i d e of the Letaba Riv& belonging

t o Letaba E s t a t e s .

Table 1. D e t a i l s of main experimental s i t e s i n the Letaba d i s t r i o t

l e t a b a

1

l?airview

I

Malkerns D i s t r i c t , Swaziland.

-

Malkerns i s s i t u a t e d i n the Swaziland middleveld a t an average a l t i t u d e of 2,500 f a e t and f a l l s within the same v e l d type a s the Letaba d i s t r i o t . The experimental s i t e s oomprised three, f i f e

t r e e blooks and a s i n g l e hundred-tree block (ROSS c i t r u s ) , a l l a t s i m i l a r al- t i t u d e s . Further d e t a i l s of the s i t e s appear i n Table 2. No i n s e c t i c i d e s were applied during the study period. Meteorological d a t a was a v a i l a b l e from a o l a s s 2 weather s t a t i o n a t Malkerns Research S t a t i o n and a thermohygrograph was operated a t Kelly' s Hope from Maroh, 1967, t o Maroh, 1968.

--

-C----.---L--.--- ' I

Table 2. D e t a i l s of experimentsl s i t e s i n the Malkerns D i s t r i c t

I

Altitude..

...

.I

1960 f t -

Mean oanopy area of 279 sq. f

t

Variety

...

raid-seasons Watersupply...

...

i r r i g a t i o n N u t r i t i o n

...

good

-lr --*-.

Rosa

Re searoh Kelly1 s

I

Usutu

/

C i h

S t a t i o n Orohsrda Bsttrtea 1

-

2800

f t

4200 f t 225 sq. f

t

292 sq,

f t

24 Navels Valenoias 26 Valenoias r a i n o n l y r a i n only f a i r poor

-

-land Lowveld. A t a mean a l t i t u d e of approximately 700 f e e t a.s.1. and average annual rainfall of 20-25 inohes, the climate of the S w z i l a n d LOW-v e l d i s 8 l i g h t l y more a r i d than t h a t of Letaba Estates. S--scale s t u d i e s and observations were made a t Tambankulu Estates, Swaziland I r r i g a t i o n Soheme and Big Bend. (Fig. 1 ) .

~ e a n

oanopy area of

trees

.

Varie$y

...

Water

eupply

N u t r i t i o n

...

261 sq. f

t

337

sq. ft 2%

aq,

ft

Valenoias Vslenoias , NBvels

i r r i g a t i o n r a i n only

good very poor

2%

89,

f t

Navels

i r r i e t i o n good

(19)

(20)

(21)

(ii) P o p u l a t i o n a s s e s s m e n t s o f

2.

e r y t r e a e .

-

S e v e r a l methods have been used t o sample t h e immature, r e l a t i v e l y s e s s i l e s t a g e s of p s y l l i d s . Georgala (1957) sampled l e a f c l u s t e r s and rnade o b s e r v a t i o n s on tagged p e a r s p u r s i n a s t u d y o f =la

-

p y r i c o l a F o e r s t e r ; C l a r k (1962) made f i e l d r a t i n g s of t h e l e v e l of a b u n b n c e on s e l e c t e d p a r t s o f t h e e u c a l y p t h o s t of C a r d i a s p i n a a l b i t e x t u r a T a y l o r ; w h i l e Moran (1967) removed l e a v e s and Watmough (1968a) g r e e n twigs, f o r l a b o r a t o r y examination. The l a t t e r a u t h o r used

a

carbon d i o x i d e sampler f o r a s s e s s i n g t h e numbers o f a d u l t s .

T. e r y t r e a e f l u c t u a t e s v i o l e n t l y i n numbers. S i n c e t h e i n s e c t i s a t

-

v e r y low d e n s i t i e s on dormant c i t r u s t r e e s , where p o s s i b l e d e s t r u c t i v e sampl- i n g was c a r e f u l l y avoided.

( a ) g i ~ c t f i e l d count of,_cplonie%. Because f e m a l e s l a y exclu- s i v e l y on young f l u s h p o t n t s and t h e nymphs a r e r e l a t i v e l y s e s s i l e , t h e developmental s t a g e s of

2,

s v t r e a e a r e a g g r e g a t e d i n t o d i s t i n c t , e a s i l y - s p o t t e d c o l o n i e s . I n t h i s method, p o p u l a t i o n a s s e s s m e n t s were made by a d i r e c t c o u n t of c o l o n i e s on t h e lower f o u r f e e t of canopy o f e a c h data k e e u s i n g a c o x t a n t team of

3-4

f i e l d s c o u t s . The a r e a s e a r c h e d c o n p r i s e d

40-50%

of t h e k o t a l canopy a r e a of t h e t r e e . Each colony was r e c o r d e d s e p a r a t e l y a c c o r d i n g t o i t s predominant a g e c l a s s , i . e , eggs, i n s t a r s 1-111, i n s t a r s IV-Ve Assessments were u s u a l l y made a t weekly i n t e r v a l s ,

A t

Malkerns t h e 1 0 randomly-chosen d a t a t r e e s used f o r f l u s h s t u d i e s w e x s e a r c h e d f o r

2,

e r y t r e a e , p l u s one oonsecu- t i v e t r e e g i v i n g a t ~ t a l of 20 t r e e s , T h i s means t h a t 16-2% of t h e t o t a l t r e e canopy a t t h e s i t e was s e a r c h e d on each occasion.

Fig. 4. shows t h a t a t Malkerns R e s e a r c h S t a t i o n t h e s e a r c h i n g of 10, 20 o r 50 t r e e s u . % ~ ? l l y made l i t t l e d i f f e r e n c e t o the mean popula- t i o n d e n s i t y recoz-ded. ??;.om c a l c u l a t i o n s of t h e

95%

f i d u c i a l l i m i t s of a s e r i e s of sample s i z e s a t v a ~ i o u s p o p u l a t i o n d e n s i t i e s , t h e s i z e of t h e r a t i o o f confidence interval/mean x 100 was u n a c c e p t a b l e when 1 0 t r e e s were seapled, b u t was below 2@ f o r moderate and h i g h p o p u l a t i o n s a n d from 20-305 f o r low p o p u l a t i o n s of

2.

e r y t r e a e . A t low p o p u l a t i o n s t h e r e w a s l i t t l e improvement by t a k i n g a 50 t r e e sample. The p o p u l a t i o n was e x p r e s s e d a s t o t a l c o l o n i e s p e r 1 0 square f e e t of canopy.

---

EXPLAIUTION OF FIGURES

----

f i g .

3.

Experimental s i t e s i n t h e Letaba and Malkerns d i s t r i c t s

Fig.

b.

Mean p o p u l a t i o n d e n s i t i e s o f

2 ,

e r y t r e a e a t Malkerns Research S t a t i o n a s i n d i c a t e d by %he sargpling of 10, 20, and 50 t r e e s

(22)

10. The number o f i n d i v i d u a l s p e r colony was s t u d i e d from i n sib c o u n t s and from a number of haphazardly s e l e c t e d c o l o n i e s i n t h e f i e l d . Colo- ny s i z e w ~ s extremely v a r i a b l e (from 2-5 t o s e v e r a l hundred i n d i v i d u a l s ) and in t h e egg s t a g e was r e l a t e d t o t h e number of l e a v e s p e r f l u s h p o i n t and t h e r a t e o f egg-laying. A t Malkerns Research S t a t i o n ,

178

c o u n t s made d u r i n g J a n u a r y and from May t o November,

1968,

showed t h e mean egg colony s i z e t o be t h e l o w e s t i n mid-winter ( ~ u n e 20.4; J u l y 60.5) and t h e h i g h e s t in September

(176.7

eggs p e r colony). The mean f o r a l l c o u n t s was

86.7

eggs p e r colony.

Simple t e s t s f o r t h e r e l i a b i l i t y of s e a r o h i n g by this method were o a r r i e d o u t i n t h e Letaba d i s t r i c t a t v a r i o u s d e n s i t i e s of t h e i n s e c t ( c a t l i n g , 1967). A f t e r a normal s e a r c h , t r e e s were searohed f o r

a

se- cond time f o r twice t h e o r i g i n a l p e r i o d , sometimes s w i t c h i n g t h e s c o u t s . The double s e a r c h i n c r e a s e d t h e number of a o l o n i e s found by a mean of

21% a t moderate t o h i g h d e n s i t i e s of

2.

e q t r e a e , w h i l e a t low d e n s i t i e s t h e p o p u l a t i o n was i n c r e a s e d by

157%.

The d i r e c t c o u n t of c o l o n i e s was found t o be a r a p i d and r e l i a b l e f i e l d ne thod f o r a s s e s s i n g p o p u l a t i o n d e n s i t i e s of t h e immature s t a g e s a n d was e x t e n s i v e l y employed a t the main s t u d y s i t e s . D e s t r u a t i v e

sampling was a v o i d e d and t h e method f a c i l i t a t e d r e g u l a r f i e l d observa- t i o n s on a s p e c t s of b i o l o g y and t h e a c t i v i t y of n a t u r a l enenies. Es- t i m a t e s of zge d i s t r i b u t i o n , a n i m p o r t a n t a s p e c t i n p o p u l a t i o n ecology, e n a b l e d t h e c a l c u l a t i o n of f i e l d g e n e r a t i o n s .

A t f o u r o t h e r s i t e s of n a t u r e t r e e s i n t h e Malkerns d i s t r i c t , name- l y Amaswazi E s t a t e s , Lenhaven E s t a t e s , and Ross C i t r u s E s t a t e s ( 2 s i t e s ) , p o p u l a t i o n s were a s s e s s e d a t monthly i n t e r v a l s by examining t h e lower f o u r f e e t of canopy of 20 t r e e s a l o n g two d i a g o n a l s .

( b ) D i r e c t f i e l d c o u n t of a l l s t a ~ . A t F o r e s t H i l l from Janu- a r y ,

1966,

t o January,

1967,

t o t a l c o u n t s were a d e of a l l i n d i v i d u a l s o c c u r r i n g on inf'ested f l u s h p o i n t s e n c l o s e d by t h e frame when making f l u s h counts. P o p u l a t i o n s of eggs, i n s t a r s 1-111, and i n s t a r s IV-V on t h e 1 0 d a t a t r e e s were e x p r e s s e d a s t h e mean number p e r 1 0 s q u a r e f e e t of canopy. I t i s e s t i m a t e d t h a t a p p r o x i m a t e l y

3%

of t h e t o t a l s i t e canopy a r e a w a s searched. T h i s method i n d i c a t e d s i m i l a r p o p u l ~ t i o n t r e n d s and f i e l d g e n e r a t i o n s t o that o b t a i n e d by t h e p r e v i o u s nethod, b u t due p a r t l y t o t h e small a r e a sampled, mean d e n s i t i e s f o r s u c c e s s i v e c o u n t s were extremely v a r i a b l e d u r i n g some p e r i o d s . Also t h e o o u n t i n g of eaoh i n d i v i d u a l was e x o e s s i v e l y t e d i o u s and f r e q u e n t l y i n a c c u r a t e s i n c e many of t h e young s t a g e s i n populous c o l o n i e s may be obscured by l e a f - c u r l i n g .

( c ) I n s e c t sampling machine. The a d u l t s of

-

T. e q t r e a e and t h e a c t i v e s t a g e s of t h e a s s o c i a t e d comelex of n a t u r a l enemies were sampl-

(23)

ed w i t h a Model 1 W a c Vaouum I n s e c t Sampling Maohine a t f o r t n i g h t l y i n t e r v a l s from August, 1967, t o December, 1970, a t Malkerns R e s e a r c h S t a t i o n , and d u r i n g t h e 1967/68 s e a s o n a t Ross C i t r u s E s t a t e s . An e i g h t i n c h band around t h e canopy of e a c h of f i v e h a p h a z a r d l y - s e l e c t e d t r e e s were sampled f o r one minute, t h e mean canopy a r e a of t h e t r e e s a t each s i t e b e i n g w i t h i n

2%

of each o t h e r . U s u a l l y t r e e s b o r d e r i n g t h e e x p e r i m e n t a l s i t e were s e l e c t e d b u t a t high p o p u l a t i o n d e n s i t i e s two o r t h r e e of t h e e x p e r i m e n t a l t r e e s were i n c l u d e d . Samples were mostly t a k e n between 10.00 a.m. and 3.00 p.m.

A d u l t s c o l l e c t e d by t h e i n s e c t sampling machine augmented t h e

data from p o p u l a t i o n a s s e s s m e n t s of t h e s e s s i l e s t a g e s . Fig.

8

shows t h a t a t moderate t o high d e n s i t i e s , peaks of a d u l t s corresponded o l o s e - l y w i t h f i e l d g e n e r a t i o n s i n d i c a t e d by a c o u n t of c o l o n i e s . A t low p o p u l a t i o n d e n s i t i e s , however, v e r y few o r no a d u l t s were c o l l e c t e d . L a r g e r s a n p l e s were n o t c o n s i d w e d due t o t h e d e s t r u c t i v e n a t u r e of t h i s

sampling method. Catches fro:n t h e machine a l s o proved t o be v a l u a b l e i n p l o t t i n g p a r a s i t e p o p u l a t i o n f l u c t u a t i o n s and, t o a l e s s e r e x t e n t , i n t h e s t u d y of p r e d a t o r a c t i v i t y . NO a t t e m p t was made t o t e s t t h e r e l i e b i l i t y of t h i s method.

( d ) S t i c b t r a p s . Yellow s t i c k y t r a p s have been used t o s a ~ n p l e t h e a d u l t s of s e v e r a l p s y l l i d s p e c i e s

co as to^,

1961; Wilde, 1962; Moran, 1967) and were shown t o i n d i c a t e t h e a c t i v i t y of

x.

e w t r e a e

i n p r e l i m i n a r y t r i a l s i n Malkerns. S t i c k y t r a p s were used m i n l y t o s t u d y f l i g h t a c t i v i t y i n and aQound t h e Malkerns Researoh S t a t i o n ex- p e r i m e n t a l grove and were s i m i l a r t o t h o s e used by Moran (1967).

They c o n s i s t e d of a 1 2 i n c h square of hardboard p a i n t e d a b r i g h t lemon yellow c o l o u r and covered w i t h t r a n s p a r e n t p l a s t i c s h e e t i n g smeared w i t h 'lOstioo", a s t i c k y tree-banding compound. The y e l l o w c o l o u r was c l e a r l y v i s i b l e through t h e p l a s t i c and s t i c k y c o a t i n g . F u r t h e r d e t a i l s on t h e o p e r a t i o n of t h e s e t r a p s i s given i n t h e c h a p t e r on d i s p e r s a l .

( e ) J a r r i n g of a d u l t s onto s t i c k y p l a t e s . A 1 2 i n c h square g l a s s p l a t e c o a t e d w i t h "Osticow and p l a c e d i n a wooden frame was used t o

sample a d u l t s from f l u s h p o i n t s i n two groves a t Malkerns Research S t a - t i o n between October, 1968, and September, 1969* T h i s method was i n i - t i a l l y used t o sample t h e o i t r u s t h r i p s , S c i r t o t n r i p s a u r a n t i i Faure ( s t a s s e n & C a t l i n g , 1969). F i v e f l u s h p o i n t s on 1 0 haphazardly-seleo- t e d t r e e s were j a r r e d a g a i n s t t h e edge of t h e frame, d i s l o d g i n g and t r a p p i n g t h e e d u l t s on t h e s t i c k y l a y e r . T h i s ncthod gave a f a i r r e - f l e c t i o n of t h e a d u l t a c t i v i t y d u r i n g t h i s p e r i o d .

( i i i ) Assessments of f k u s h i n g rhythm. I n o r d e r t o a s s e s s f l u s h d e n s i t i e s i n t h e f i e l d i t was n e c e s s a r y t o make measurements of t h e canopy a r e a ( sur- faoe a r e a ) of

t n e

d a t a t r e e s . Accordingly, a s u i t a b l e f o m u l a was d e r i v e d

(24)

(25)

by S e r f o n t e i n & C a t l i n g (1968). P r e v i o u s l y

i t

h a s been commonly assumed t h a t t h e shape of a c i t r u s t r e e approximates a hemisphere. However, f u r t h e r ob- s e r v a t i o n s showed t h a t because t r e e h e i g h t and diameter f r e q u e n t l y v a r y in- dependently of one anothor, one o r o t h e r conic s e c t i o n w i l l r e p r e s e n t a more r e a l i s t i c shape f o r most t r o e s .

The r e q u i r s d form was found t o be a n e l l i p s e t h e h a l f of which, stand- i n g on i t s minor a x i s , r e p r e s e n t s a t r e e i n c r o s s - s e c t i o n . The minor a x i s t h e n becomes t h e t r e e diameter and t h e s e m i - m j o r a x i s i t s h e i g h t . I t can be shown t h a t t h e canopy a r e a ( 3 ) i s given by the f o l l o w i n g formula:-

! 2 a b S = 2-ff b

+

a r c s i n e e where a = t r e e h e i g h t b = skirt r a d i u s e = e c c e n t r i c i t y of e l l i p s e

*om measurements of t r e e diameter ( a t s k i r t l e v e l ) and h e i g h t ( f r o n bottom of skirt t o apex), Fig.

5

g i v e s t h e canopy a r e a of any t r e e l i k e l y

t o be encountered i n t h e f i e l d .

F l u s h co~+-&. The canopy a r e a s of a l l e x p e r i m e n t a l t r e e s were c a l c u l a t e d by t h e above method. A f o u r f o o t square metal frame was h e l d a g a i n s t t h e canopy of the t r e e a t random h e i g h t i n each quadrant. The a r e a sampled p e r

quadrant was i n d i r e c t p r o p o r t i o n t o t h e s i z e of t h e t r e e and ranged from f o u r square f e e t f o r small t r e e s , t o 1 6 square f e e t f o r l a r g e t r e e s . A l l f l u s h p o i n t s e n c l o s e d by t h e frame s n d regarded a s b e i n g s u i t a b l e f o r t h e development of

2.

e r y t r e e e were counted end recorded p e r u n i t a r e a . The frame was f i t t e d w i t h a g r i d t o reduce c o u n t i n g ~ h c n dense, e v e n l y - d i s t r i b u t e d

f l u s h p o i n t s were p r e s e n t .

Young f l u s h p o i n t s were c l a s s e d i n t o two c a t e g o r i e s :

C l a s s A poin& ( ~ 1 . I11 A,B) on which eggs of

2 .

e r y t r e a e a r e u s u a l l y l a i d ;

-:

from the f i r s t u n f u r l i n g of the bud t o a s t a g e when t h e m a j o r i t y of t h e l e a v e s a r e l o n g e r t h a n a b o u t

15

i n c h e s ; weight u s u a l l y 0.05-0.50 gm (maximum 1.00~m).

C l a s s B p o i n t s ( ~ 1 . I11 C,D) a r e more advanced and s u p p o r t nymphs of

x,

s-

-

t r e a e - f o r normal nymphal development l e e v e s nust remain s o f t and s u c c u l e n t f o r

-3

a t l e a s t 2-3 weeks

-

t h e monont t h e t i s s u e hardens and i s no l o n g e r "tacky" t o the touoh t h e l e a v e s a r e u n s u i t a b l e ; weight u s u a l l y 0.50 gm (~;mxinum 3.00 gm)

.

Fig,

5

The canopy a r e a of a c i t r u s t r e e from measurements of t r e e r a d i u s and h e i g h t

(26)

P l a t e

111.

_Breeding_{s i t e s o f}

2,

_{ewtreae. A}

and

B

-

olass

A

-

flush

p o i n t s ;

C

and D

-

o l a s s B

flush

p o i n t s ,

Scale

i n

inohes

(27)

Though w a t e r s h o o t s , blossom f l u s h e s and prematurely-hardening f l u s h p o i n t s were sometimes d i f f i c u l t t o c l t r s s i f y , t h e above f l u s h c a t e g o r i e s proved adequate f o r t h e study. Experimental s i t e s were sampled n o s t l y a t weekly i n t e r v a l s . I n t h e Letaba d i s t $ i c t t e n d i f f e r e n t t r e e s were s e l e c t e d randomly a t each sampling d a t e ; a t Malkerns, i n o r d e r t o g a i n a s t r o n g e r c o r r e l a t i o n between f l u s h i n g and f l u c t u a t i o n s of

2.

e r y t r e a e , t h e same t e n t r e e s were sampled on e a c h occasion. Except f o r v e r y low f l u s h d e n s i t i e s , t h e 95% f i d u c i a l l i m i t s of t h e sample s i z e were always below 2% of t h e confidence interval/mean x 100.

Flush s u r v e E . Extensive f l u s h surveys were nade on Letaba E s t a t e s between August, 1966, and February, 1967. B a s i c a l l y t h e surveys were designed t o c h a r t

t h e annual f l u s h i n g rhythm and t o a s s i s t i n t h e s p o t t i n g of

2.

e q t r e a e outbreaks, b u t t h e y were a l s o found t o y i e l d d a t a on f l u s h i n g d i f f e r e n c e s due t o t r e e age and v a r i e t y . S i x t y p l o t s were chosen i n p r o p o r t i o n t o t h e number of t r e e s of e a c h v a r i e t y grown on t h e E s t a t e . A " s t a r t e r " t r e e , r e p r e s e n t a t i v e of t h e p l o t , was s e l e c t e d by a nethod of S e r f o n t e i n , C.M.A., ( p e r s o n a l communica- t i o n , 1366) ~ h o found t h a t t h c p r o b a b i l i t y of a t r e e b e i n g r e p r e s e n t a t i v e of

t h e p o p u l a t i o n i n any p l o t v a r i e s w i t h i t s p o s i t i o n i n t h e p l o t . T h i s v a r i a t i o n , when p l o t t e d a s a f u n c t i o n of %he t r e e 1 s p o s i t i o n ( w i t h r e f e r e n c e t o any base- l i n e , i.e, p l o t edge) gene-ates a f u n c t i o n v e r y c l o s e l y resembling t h e probabi-

l i t y

d i s t r i b u t i o n f u n c t i o n r e s u l t i n g from t h e t o t a l s o b t a i n e d from t h e s i m u l - taneous throwing of t h r e e d i c e . On each s u c c e s s i v e survey a n o t h e r t r e e was t a k e n w i t h r e f e r e n c e t o t h e v s t a r t e r " t r e e . F l u s h was counted a t f o r t n i g h t l y i n t e r v a l s u s i n g a s i m p l i f i e d v e r s i o n of t h e frame method d e s c r i b e d p r e v i o u s l y .

A t f o u r groves i n t h e Malkerns d i s t r i c t f l u s h volume was r a t e d a t monthly i n t e r v e l s on a s c a l e f r o n 0-6.

( i v ) Assessment o f pa-r~zi&e a c t i v i A O Z P s i t i s m . The assessment of pa- r a s i t i s m was based on a microscopic examination of t h e nymphs. _{A t}r e g u l a r in-

t e r v a l s s i n g l e l e a v e s were removed f r o n a number of h a p h a z a r d l y - s e l e c t e d r .

s-

t r e a e c o l o n i e s i n the f i e l d and examined a l m o s t immediately before the nymphs

-

v a c a t ~ d t h e i r l e a f g a l l s . Where t h e p o p u l a t i o n allowed, s u f f i c i e n t l e a v e s were picked t o provide a minimur?. of 500 s u s c e p t i b l e nymphs. _{With t h e e x c e p t i o n} of t h e Ross C i t r u s s i t e sone c o l o n i e s were u s u a l l y s e l e c t e d from t r e e s borde- r i n g t h e e x p e r i m e n t a l t r e e s i n o r d e r t o l i m i t d e s t r u c t i v e sampling. _{A t}_{h i g h e r} h o s t d e n s i t i e s , however, and on each occasion a t Ross Citrus, a l l t h e l e a v e s were removed from t h e e x p e r i n e n t a l t r e e s , A t times of v e r y low h ~ s t d e n s i t y , a s i n midwinter, unsprzyed t r e e s f u r t h e r a f i e l d were searched f o r s u i t a b l e m a t e r i a l .

Nymphs were examined uniler a d i s s e c t i n g microscope a t 16-25x. _{The ventrum} and l e a f g a l l of i n s t a r s 111-V, which a r e t h e s u s c e p t i b l e s t a g e s f o r 2. r a d i a t u s , were examined e x t e r n a l l y f o r p a r a s i t e eggs, l a r v a e and pupae. _{The i n s t a r of} each nymph and, where p a r a s i t i z e d , the s p e c i e s and s t a g e of t h e p a r a s i t e , were recorded. Dried e x o s k e l e t o n s of p a r a s i t i z e d h o s t s which remain a f t e r the emer-

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gence o f t h e p a r a s i t e were i n c l u d e d a s t h e s e s t r u c t u r e s i n v a r i a b l y r e m i n fim- l y a t t a o h e d t o t h e l e a f , b u t f i n a l n o u l t s i n d i c e t i n g t h e emergence of a d u l t s were i g n o r e d as t h e y a r e f r a g i l e and soon l o s t . Nymphs a t t a c k e d by

g.

g u l v i n a t u s become r e c o g n i z a b l e by t h e f o r m a t i o n of a d i s t i n c t i v e mummy. P o p u l a t i o n f l u c t u a t i o n s of p a r a s i t e s . A d u l t p a r a s i t e s were sampled w i t h t h e i n s e c t s a n p l i n g machine a s d e s c r i b e d p r e v i o u s l y . The a f f e c t of p a r a s i t e s on s i n g l e c o l o n i e s was s t u d i e d i n s i t u e i t h e r on experimental t r e e s o r on p o t t e d c i t r u s s e e d l i n g s i n f e s t e d from a l a b o r z t o r y c u l t u r e of

2.

~ r y t r e a e and p l a c e d a t a n e x p e r i m e n t a l s i t e .

A d u l t p a r a s i t e s were r e a r e d from m a t e r i a l used f o r p a r a s i t i s m c o u n t s and from o t h e r l e a v e s c o l l e c t e d s p e c i a l l y f o r t h i s purpose.

( v ) I n s i t u counts. The eggs and nymphal s t a g e s of

2..

e w t r e a e s i t u a t e d on young f l u s h p o i n t s a r c c l e a r l y v i s i b l e i n s i t u . By marking c o l o n i e s i n t h e e g g s t a g e on e x p e r i m e n t a l t r e s s n e a r a weether s t a t i o n and f o l l o w i n g up w i t h d e t a i l e d oounts a t r e g u l a r i n t e r v a l s u n t i l t h e encrgence of a d u l t s , complete l i f e t a b l e s were c o n s t r u c t e d showing t h e i n f l u e n c e of v a r i o u s e c o l o g i c a l f a o t o r s . Counts were done d a i l y o r on a l t e r n a t e days a t Lotaba and u s u a l l y e t 2-3 day

i n t e r v a l s a t t h e o t h e r s i t e s . Using a hand l e n s and a snrtll s t r i p of p a p e r marked w i t h t h e mean l e n g t h s o f i n s t a r s 111-V

i t

was p ~ s s i h l o t o r e c o p i z e eaoh of t h e f i v e nymphal i n s t a r s w i t h r e a s o n a b l e accuracy. The random s e l e c t i o n of c o l o n i e s on a l l p a r t s of t r e e canopy tended t o c o a p e n s a t e f o r d i f f e r e n c e s due t o microclimate and f o l i a g e c o n u i t i o n . When s u i t a b l e c o l o n i e s were n o t a v a i l a b l e

i n t h e f i s l d , a d u l t s from a l a b o r a t o r y c u l t u r e of

x.

e r y t r e a e were c o n f i n e d on a v i g o r o u s c i t r u s s e e d l i n g , allowed t o l a y eggs f o r 24 hours, and t h e s e e d l i n g p l a c e d i n t h e c r o t c h of a t r e e n e a r a weather s t a t i o n .

( v i ) Greening asses.s_m_nts. The i n o i d e n c e of g r e e n i n g was determined by s e a r c h i n g t h e f o l i a g e of t r e e s f o r vein-yellowing and greening-induced d e f i - o i e n c y symptom i n the w i n t e r months, by examinations of t h e f r u i t a t p i c k i n g w i t h t h e h e l p gf t h e "albedo-fluorescence t e s t " of Schwarz (1968a), and by t h e

s c r e e n i n g o f tviigs f o r t h e markcr substance i n d i c a t i v e of g r e e n i n g by t h e same method.

3.

INFLUENCE OF FLUSHING RH?THM

Like most p s y l l i d s ,

g.

e r y t r e a e b r e e d s on the young l e a v e s of i t s p l a n t h o s t and t h u s p o p u l a t i o n f l u c t u a t i o n s a r e s t r o n g l y c o r r e l a t e d w i t h t h e f l u s h i n g rhythm of c i t r u s . A l t e r n a t e h o s t p l a n t s were extremely r a r e i n b o t h s t u d y a r e a s and were n e v e r found t o be inf'csted. _{There y a s t h u s no evidence of any l a r g e} i n d i g e n o u s r e s e r v o i r of

x.

s r y t r e a e c a p a b l e of r a p i d l y r e - i n f ' e s t i n g spr-yea groves. Unlike t h e e u c a l y p t p s y l l i d , C a r d i a s p i n a a l b i t e x t . ( C l e r k ,

_.-

₌

l9"33a)

i n d i v i d u a l c i t r u s t r e e s o r f l u s h p o i n t s r a r e l y d i f f e r e d i n t h e i r attractiveness f o r o v i p o s i t i o n o r f e e d i n g .

(29)

FOREST HILL FLUSH POINTS b

-

PSYLLA COLONIES LETABA C I ,