Host resistance to an insecticide and selection at larval stage favour development of resistance in the parasitoid, Cotesia plutellae

Laboratory selection for fenvalerate resistance was conducted in two colonies of the parasitoid, Cotesia plutellae, reared on two colonies of its host, Plutella xylostella, which differed in resistance to the insecticide. In the selection regime, the insecticide was applied to the host larvae that harboured 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. Comparison of detoxifying enzymes between the insect colonies revealed that fenvalerate resistance was positively related to increased monooxygenase activity, but was unrelated to carboxylesterases and general esterase in both the host and the parasitoid, indicating that the two insects shared a major metabolic mechanism for resistance to fenvalerate. This information can help improve selection procedures for the development of insecticide resistance in parasitoids.