The ichneumonid Diadromus collaris (Gravenhorst) (Hymenoptera: Ichneumonidae) is a major solitary, pupal endoparasitoid of the diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae). Experiments to examine parasitism of the host pupae of different ages by the parasitoid were conducted in the laboratory. Diadromus collaris preferred host pupae that were in the first half of their pupal development. Survival from larva to adult, and size and parasitizing capacity of the resultant female adults decreased dramatically as host pupal age increased. When ovipositions were made into host pupae that were in the last quarter of their development, all parasitoids died before adult emergence. The performance of D. collaris, as affected by host pupal age, agrees with the simple diet theory that predicts female wasps should select hosts of higher nutritional quality for oviposition.

Hellén [Hymenoptera: Ichneumonidae], Apanteles ippeus Nixon [Hymenoptera: Braconidae], and Oomyzus sokolowskii Kurdjumov [Hymenoptera: Eulophidae]) and one species of pupal parasitoid Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae) attacked immature P. xylostella. The most abundant groups of predatory arthropods caught in pitfall traps were Araneae (Lycosidae)＞Coleoptera (Carabidae, Coccinelidae, Staphylinidae)＞Neuroptera (Chrysopidae)＞Formicidae, whereas on crop foliage Araneae (Clubionidae, Oxyopidae)＞Coleoptera (Coccinelidae)＞Neuroptera (Chrysopidae) were most common. The abundance and diversity of natural enemies was greatest at sites that adopted IPM, correlating greater P. xylostella mortality at these sites. The efÞcacy of the natural enemy complex to pest mortality under different pest management regimes and appropriate strategies to optimize this important natural resource are discussed.

Elevated jasmonic acid (JA) concentrations in response to herbivory can induce wounded plants to produce defences against herbivores. In laboratory and field experiments we compared the effects of exogenous JA treatment to two closely related cabbage species on the host-searching and oviposition preference of the diamondback moth (DBM), Plutella xylostella. JA-treated Chinese cabbage (Brassica campestris) was less attractive than untreated Chinese cabbage to ovipositing DBM, while JA-treatment of common cabbage (B. oleracea) made plants more attractive than untreated controls for oviposition by this insect. Similar effects were observed when plants of the two species were damaged by DBM larvae. In the absence of insect-feeding, or JA application, Chinese cabbage is much more attractive to DBM than common cabbage. Inducible resistance therefore appears to occur in a more susceptible plant and induced susceptibility appears to occur in a more resistant plant, suggesting a possible balance mechanism between constitutive and inducible defences to a specialist herbivore.

Foraging adults of phytophagous insects are attracted by host-plant volatiles and supposedly repelled by volatiles from non-host plants. In behavioural control of pest insects, chemicals derived from non-host plants applied to crops are expected to repel searching adults and thereby reduce egg laying. How experience by searching adults of non-host volatiles affects their subsequent searching and oviposition behaviour has been rarely tested. In laboratory experiments, we examined the effect of experience of a nonhost-plant extract on the oviposition behaviour of the diamondback moth (DBM), Plutella xylostella, a specialist herbivore of cruciferous plants. Naive ovipositing DBM females were repelled by an extract of dried leaves of Chrysanthemum morifolium, a nonhost plant of DBM, but experienced females were not repelled. Instead they were attracted by host plants treated with the non-host-plant extract and laid a higher proportion of eggs on treated than on untreated host plants. Such behavioural changes induced by experience could lead to host-plant range expansion in phytophagous insects and play an important role in determining outcome for pest management of some behavioural manipulation methods.

Laboratory selection was conducted for fenvalerate resistance in two colonies of the parasitoid, Cotesia plutellae Kurdjumov, each reared on a different colony of its host, Plutella xylostella (L.), with different level of resistance to the insecticide. In the selection regime the insecticide was applied to the host larvae that harbored the parasitoid larvae. Compared with the unselected parasitoid colony, the parasitoid colony selected with the susceptible hosts acquired 4.5-fold resistance after 14 selection cycles, while the colony selected with the more resistant hosts acquired 13.6-fold resistance after 13 selection cycles. These results demonstrate that parasitoid larvae could be exposed to insecticide selection via the hosts, selection with more resistant hosts could accelerate development of resistance in the parasitoid, and resistance genes selected during larval development could be expressed at the adult stage. This information can help improve selection procedures for insecticide resistance in parasitoids.