Host spectrum and damage

Table 2.3. Larval developmental time (days; mean ± SE) of M. brassicae at different temperatures (°C) and two different artificial diets

Temperature (°C) Developmental time (mean ±

S.E.)

Reference

10.5 98.3 ± 5.4 Johansen (1997a)

12.5 70.2 ± 3.7 Johansen (1997a)

15.0 77.2 ± 0.9 Van De Steene (1987)

15.5 50.1 ± 6.0 Johansen (1997a)

17.0 55.7 ± 0.1 Van De Steene (1987)

18.0 39.8 ± 4.6 Johansen (1997a)

19.0 35.9 ± 0.2 Van De Steene (1987)

21.0 31.2 ± 0.2 Van De Steene (1987)

23.0 29.8 ± 0.1 Van De Steene (1987)

2.1.3.5 Host spectrum and damage

Larvae of the cabbage moth are polyphagous but are economically most important in cabbage (Johansen, 1997a,b). They are able to feed on more than 70 host plant species belonging to 22 families, such as beet (Beta vulgaris [L.]), lettuce (Lactuca sativa [L.]), sweet pepper (Capsicum annuum [L.]), and chrysanthemum (Chrysanthemum). They even can be found on trees, such as Salix and Quercus (de Brouwer, 1974; Turnock and Carl, 1995; Waring and Townsend, 2006; Chougule et al., 2008).

Larvae cause yield loss by eating large holes in the leaves and the harvestable parts of the crop. Besides this, they make the plants dirty with their faeces, thereby also causing yield loss.

The food consumption increases with the age and instar of the larvae (Table 2.4). Larvae of the last three instars are the most destructive. Female larvae consume significantly more than male larvae (Theunissen et al., 1985).

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Table 2.4. Leaf area consumption (in cm2 ± SE and % of the leaf area) by the different instars of M.

brassicae on Brussels sprouts (Theunissen et al., 1985)

Instar Leaf area consumption

cm² %

1+2 0.31 ± 0.12 0.29

3 1.23 ± 0.87 1.15

4 3.21 ± 0.76 3.01

5 12.36 ± 3.42 11.58

6 89.61 ± 20.69 83.97

2.1.3.6 Natural enemies of the cabbage moth

The prevalence of M. brassicae as a pest is variable and depends on both biotic and abiotic mortality factors (Johansen, 1997b). When considering the biotic factors, there is a wide range of natural enemies, including predatory birds, coleopterans, and chrysopids, and hymenopteran and dipteran parasitoids which are able to use the cabbage moth as prey or host (Klingen et al., 1996; Johansen, 1997b). According to Johansen (1997b) predation by birds is low, but could be underestimated as birds often consume the whole larvae. Polyphagous staphilinid beetles like Philonthus atratus (Gravenhorst) and carabid beetles like Bembidion tetracolum (Say), Pterostichus melanarius (Illiger), Calosoma chinense (Kirby) and Harpalus rufipes (Degeer) were reported as key mortality factors of M. brassicae larvae, especially of the youngest instars (Johansen, 1997b; Suenaga and Hamamura, 2001). Further, Vasconcelos et al. (1996) indicate that predaceous beetles need to be more mobile (climb or fly onto plants) to exert a higher mortality on the M. brassica population. Eggs and first instar larvae of M.

brassicae were also preyed on by larvae of the chrysopid Chrysoperla sp. (Klingen et al., 1996; Johansen, 1997b; Pfiffner et al., 2009). Further, Bianchi et al. (2005) observed the anthocorid bug Orius niger (Wolff) feeding on eggs of the cabbage moth. Besides predators, eggs and larvae of M. brassicae are also attacked by parasitoids. The reported impact of egg parasitoids on M. brassicae varies.

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Parasitoids belonging to the family of Trichogrammatidae (Trichogramma spp.) and Scelionidae (Telenomus spp.) were found to have a low impact on egg survival (Johansen, 1997b; Bianchi et al., 2005; Pfiffner et al., 2009), whereas Takada et al. (2001) reported that M. brassicae populations were kept at low densities during three years due to high parasitization rates (60-80%) by the egg parasitoid Trichogramma dendrolimi (Matsumura).

Larval parasitoids belonging to several hymenopteran and dipteran families (i.e. Braconidae, Ichneumonidae, Eulophidae, Tachinidae, …) are found to parasitize M. brassicae (Butaye and Degheele, 1995; Turnock and Carl, 1995; Johansen, 1997b). The braconid Microplitis mediator (Haliday) was found to be the main parasitoid attacking M. brassicae in Europe (Turnock and Carl, 1995; Lauro et al., 2005; Pfiffner et al., 2009).

2.1.3.7 Microplitis mediator as natural enemy of the cabbage moth, Mamestra brassicae The solitary endoparasitoid M. mediator is native to Europe and widely distributed across the Palearctic region. This parasitoid has a wide host range, attacking over 40 lepidopteran species within the families Noctuidae and Geometridae (Arthur and Mason, 1986; Pivnick, 1993). The preference of M. mediator for a specific larval instar depends on the host species.

For M. brassicae, first and second instar larvae were found to be the most suitable hosts, whereas third instar larvae were suboptimal (Lauro et al., 2005).

Newly emerged females lay their eggs immediately or after mating in the hemocoel of the host. Most of the parasitoid eggs (i.e. 60-75%) are produced in the first five days of the oviposition period (Luo et al., 2010). Eighteen to 56 hours after oviposition, the first instar larva hatches and starts to cruise the hemocoel in search for other parasitoid larvae or eggs in order to kill them using its strongly developed mandibles. Microplitis mediator has three larval instars, which all feed on the host hemolymph and abdominal tissues (Arthur and Mason, 1986; Qin et al., 1999; Tian et al., 2008). The third instar larva leaves the host to spin a light brown cocoon in the vicinity of the host and pupate (Figure 2.5) (Arthur and Mason, 1986; Pivnick, 1993). A few days after emergence of the parasitoid prepupae, the host dies as a result of abdominal tissue damage, food depletion and/or infection (Pivnick, 1993).

Development from egg to prepupa and pupal development take on average 12.2 days and 7.7 days, respectively, at 22°C (Kim et al., 2008). Development varies depending on the host species, host instar at oviposition, temperature, photoperiod and parasitoid sex (Tanaka et al., 1984; Pivnick, 1993; Qin et al., 1999; Foerster and Doetzer, 2003; Harvey and Strand, 2003;

Li et al., 2006b, 2008).

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Adult longevity varies depending on the sex of the parasitoid and the possibility to oviposit.

In general, female wasps live longer than males (Li et al., 2006a; Luo et al., 2010). During the female's lifetime, Foerster and Doetzer (2003) recorded an average number of 61 parasitized larvae of the wheat army worm Mythimna sequax (Franclemont) at 20°C, which is in line with records for other Microplitis species, such as M. brassicae (Muesebeck) (73 parasitized larvae of the cabbage looper Trichoplusia ni [Hübner]) and M. bicoloratus (Chen) (50 larvae of the tropical armyworm Spodoptera litura [Fabricius]) (Browning and Oatman, 1985; Luo et al., 2007).

The cold tolerance of this wasp differs depending on the life stage. The internal stages are reported to have a lower threshold temperature (i.e. 9.9 °C) compared with the pupae (i.e.

10.3 °C) (Foerster and Doetzer, 2003).

In Europe, where its main host is M. brassicae, the parasitoid usually has two generations per year and survives the winter season as a diapausing cocoon (Arthur and Mason, 1986; Li et al., 2008).

Figure 2.5. Third instar larvae of M. mediator leaving its host M. brassicae (left) and cocoon of M. mediator (right).

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