Abstract. 1. Superparasitism occurs in Cotesia glomerata (Hymenoptera: Braconidae), a gregarious endoparasitoid of Pieris spp. (Lepidoptera: Pieridae). The response of P. brassicae larvae to superparasitism and the consequences for the parasitoid were examined in order to elucidate the ecological significance of this behaviour. 2. Field surveys of a Swiss population revealed that C. glomerata brood sizes from P. brassicae larvae ranged from three to 158, and both the female ratio and the body weight of emergent wasps correlated negatively with brood size. In the laboratory, single oviposition on P. brassicae larvae did not produce any brood size larger than 62, but brood size increased with superparasitism. 3. Laboratory experiments demonstrated that both naive and experienced female wasps were willing to attack hosts that had been newly parasitised by themselves or conspecifics. Superparasitism reduced survivorship but increased food consumption and weight growth in P. brassicae larvae. Superparasitism lengthened parasitoid development and prolonged the feeding period of host larvae. 4. Despite a trade‐off between maximising brood size and optimising the fitness of individual offspring, two or three ovipositions on P. brassicae larvae resulted in a greater dry female mass than did a single oviposition on the host. Thus, superparasitism might be of adaptive significance under certain circumstances, especially when host density is low and unparasitised hosts are rare in a habitat.
Summary 1. The codling moth Cydia pomonella (L.), largely regarded as a sedentary species, shows great variation in flight capacity among individuals in the laboratory, as has been reported for tethered flight. The occurrence of individuals with the ability to fly over long distances is considered ecologically significant for the colonisation of new habitats in response to deteriorating environmental conditions. The work reported here was designed to investigate the importance of the genetic component in regulating dispersal in C. pomonella. 2. Bi‐directional selection on mobility measured by actograph was carried out in the laboratory. Both male and female codling moths responded significantly to the upward and downward selections. Divergence between the two selected strains occurred after one generation of selection and increased as further selections continued. The realised heritabilities for mobility were estimated as 0.43 in males and 0.29 in females when averaged from the two selection directions. 3. The dispersal capacity of the selected mobile and sedentary strains was verified by mark–release–recapture experiments in the field, though only the released male moths were captured effectively. Flight distances by males differed significantly between the two strains, with the mobile strain showing a greater dispersal capacity than the sedentary strain. 4. This study demonstrated a positive correlation between mobility measured by actograph and field dispersal capacity in the codling moth. Furthermore, it provides the first experimental data to show field performances of different genotypes in insect dispersal.
In Australia, canola is subject to attack by at least 30 species of invertebrate pests, although the composition of this pest complex can vary between regions. Mites (e.g. the redlegged earth mite Halotydeus destructor and the blue oat mites Penthaleus spp.), lucerne flea (Sminthurus viridis) and false wireworms (e.g. the grey false wireworm Isopteron punctatissimus and the bronzed field beetle Adelium brevicorne) are the major pests threatening the seedling establishment, whereas aphids (the cabbage aphid Brevicoryne brassicae, the turnip aphid Lipaphis erysimi and the green peach aphid Myzus persicae), the native budworm (Helicoverpa punctigera), the diamondback moth (Plutella xylostella) and the Rutherglen bug (Nysius vinitor) can cause irregular and unpredictable damage to the flowering and podding plants. Current tactics of pest management for canola rely largely on the use of synthetic pesticides, but this single-technology approach is likely to incur negative effects on natural enemies and the risk of pest resistance. Thus, the sustainable production of canola requires integrated pest management (IPM) strategies, in which cultural control, crop resistance and biological control are used as important components, with chemical inputs applied only when absolutely needed to restrict pests from reaching economically damaging densities. Such IPM strategies should be built around a fundamental understanding of pest ecology at both regional and local farm levels and the integration of renewable technologies. Therefore, future research efforts need to be focused on the canola-cropping system, with a particular emphasis on the impact of pest species, natural enemies of the pests, varietal resistance to pests and the spatial ecology of pest species.
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