Diamondback moth (Plutella xylostella L.)

Figures 3.4 and 3.5 show that larvae and pupae of the diamondback moth are mainly present in the beginning of the growing season of Brussels sprouts (i.e. June and July). Their population built up from June on, reaching a peak end June/mid-July, whereafter it declined.

From August on, the insect was only found sporadically in the field. In temperate areas, three generations of the diamondback moth may appear (Groen kennisnet, 2009; Van De Steene, 1994). Our results clearly show only one generation during our two monitoring seasons. On the other hand, like in the study of Leskinen et al. (2011), a first generation of caterpillars could already have occurred on brassica weeds before we started to monitor and we may only have seen the next generation of the resulting adults. Further, it can be seen that there is a great variance in presence of larvae and pupae among locations (Beitem, Kruishoutem and SKW) and seasons (2007 and 2008).

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Like for the cabbage moth, there was a significant interaction in the number of diamondback moth larvae and pupae between location and year (p = 0.002). In 2007, significantly more larvae and pupae were recorded in Kruishoutem than in Beitem (p = 0.015) and SKW (p <

0.001), while in 2008 significantly more larvae and pupae were observed in Beitem and Kruishoutem compared to SKW (p < 0.001 for both locations).

Further, the number of individuals in Beitem and SKW was significantly lower in 2007 compared with 2008 (Beitem: p = 0.044; SKW: p = 0.002). These differences may be a result of the higher rainfall during June 2007 in Beitem and SKW compared with June 2008 (Annex I), as rainfall is reported as a major mortality factor for diamondback moth (Harcourt, 1986;

Talekar and Shelton, 1993). Only for Beitem rainfall was significant higher in June 2007 compared with June 2008 (p = 0.030). Besides rainfall, interspecific competition could also have been responsible for differences among locations. Hambäck et al. (2009) found that larvae of the cabbage moth consume eggs of other caterpillars. The higher abundance of larvae of the cabbage moth in 2007 in Beitem (section 3.3.1), may thus be related to lower numbers of diamondback moth. As reported for the cabbage moth, differences in diamondback moth abundance among locations could also be assigned to different predation rates or agricultural diversification at the research locations (Hooks and Johnson, 2003;

section 3.3.1). Variations in appearance of P. xylostella across the growing seasons and locations were also reported in an earlier study by Van De Steene (1994). These variations highlight the importance of monitoring across different areas and during several years and the difficulty to predict the phenology of this insect.

In the pan traps of 2007, adults were only caught in June for the three locations (Figure 3.6), confirming the early presence of larvae in the Brussels sprouts. During this period, there were no significant differences in the numbers of moths caught in the pan traps among locations (p

≥ 0.104). The relatively higher abundance of adult moths in the pan-traps of Beitem and the low number of larvae and pupae on the plants in Beitem, indicate that either females were not able to lay eggs or larvae and pupae were not able to survive on the plants, which both could be a result of the higher rainfall in June 2007 in Beitem. In 2008, diamondback moths were not monitored using pan traps.

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Analogous to the presence of the diamondback moth, parasitism of this pest varied among locations and seasons. However, the number of parasitized larvae in 2007 was too low to allow comparison among locations and years. Further, in 2008, no significant differences were found among locations (p ≥ 0.235) (Table 3.3).

Parasitism rate in 2008 (51 – 84%) was similar to that reported for Diadegma semiclausum (Hellén) by Winkler et al. (2010) in the Netherlands (67%). Different parasitism rates could be a result of differences among locations in suitable ecological infrastructures for parasitoids within the landscape, such as provision of food, alternative prey, and shelter from adverse conditions (Landis et al., 2000).

Laboratory collected (hyper)parasitoids were sent to Dr. K. Horstmann for identification. The following parasitoids and hyperparasitoids were found: D. semiclausum, Diadegma armillatum (Gravenhorst), Diadegma insulare (Cresson), Diadromus collaris (Gravenhorst) and Mesochorus olerum (Curtis), Bathynthrix decipiens (Gravenhorst), respectively.

From these parasitoids, D. semiclausum showed the highest abundance. This is in accordance with the literature which considers D. semiclausum to be one of the most important species attacking P. xylostella in the world (Talekar and Shelton, 1993). Only a few individuals of P.

xylostella appeared to be hyperparasitized.

Figures 3.7 and 3.8 show the flights of Diadegma sp. adults in 2007 and 2008, based on pan trap catches. There was no interaction in their number between location and year (p ≥ 0.062).

Further, there were no significant differences in the number of Diadegma individuals caught among locations for both years (p ≥ 0.072). However, in 2008, a significantly higher number of Diadegma sp. was caught in comparison with 2007 for all locations (p < 0.001), which probably is a result of the higher diamondback moth abundance in 2008. In contrast to 2007, the occurrence of Diadegma sp. in 2008 followed that of the diamondback moth (Figures 3.5 and 3.8) with a delay of 2-3 weeks, which is consistent with the development time of D.

semiclausum (Abbas, 1988).

61 Figure 3.4. Number of larvae and pupae of the diamondback moth (mean ± SE) at three locations (Beitem, Kruishoutem and SKW) from 26/06/07 to 23/10/07.

Figure 3.5. Number of larvae and pupae of the diamondback moth (mean ± SE) at three locations (Beitem, Kruishoutem and SKW) from 10/06/08 to 14/10/08.

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Figure 3.6. Number of diamondback moths (mean ± SE) caught in pan traps at three locations (Beitem, Kruishoutem and SKW) from 05/06/07 to 23/10/07.

Table 3.3. Mean number of larvae found per replicate and parasitism rate of the diamondback moth in three areas in Flanders (Beitem, Kruishoutem and SKW) for the monitoring periods 2007 and 2008

Year Location No. of larvae ^a n^b Parasitism rate (%)^c

2007 Beitem 0.02 ± 0.02a* 2 50.00 ± 50.00

Kruishoutem 3.80 ± 1.07b 15 80.44 ± 7.86

SKW 0.06 ± 0.03a* 1 0.00

2008 Beitem 1.96 ± 0.42a 28 71.47 ± 6.09

Kruishoutem 3.00 ± 0.54a 35 83.69 ± 4.36

SKW 0.91 ± 0.25b 22 50.45 ± 9.52

a: mean ± SE; means within a column and year followed by the same letter are not significantly different (Wald test, p > 0.05, Bonferroni correction); means within a column and location followed by an asterisk are significantly different (Wald test, p < 0.05, Bonferroni correction)

b: number of replicates whith P. xylostella larvae and pupae used for calculation of the parasitism rate

c: mean % ± SE; means within 2008 are not significantly different (Wald test, p > 0.05, Bonferroni correction)

63 Figure 3.7. Number of Diadegma sp. adults (mean ± SE) caught in pan traps at three locations (Beitem, Kruishoutem and SKW) from 05/06/07 to 23/10/07.

Figure 3.8. Number of Diadegma sp. adults (mean ± SE) caught in pan traps at three locations (Beitem, Kruishoutem and SKW) from 03/06/08 to 28/10/08.