Improving Artificial Selection Of Insecticide Resistance In The Parasitoid Cotesia Plutellae: Integrating Host Resistance Into The Selection Process

One of the major challenges in implementing integrated pest management strategies has been integration of biological and chemical controls. The use of naturally or artificially selected insecticide-resistant strains of natural enemies can be one way to enhance the compatibility of these two kinds of control methods. We have used Plutella xylostella (Lepidoptera: Plutellidae) - Cotesia plutellae (Hymenoptera: Braconidae) - spinosad as a host-parasitoid-insecticide model system to investigate ways to improve the selection methods. Selection was conducted with parasitoid strains that were reared on susceptible or resistant strains of the host. Selection was also conducted by applying selection pressure on different development stages of the parasitoid. The outcomes of the different selection methods were then compared.(1) Establishment and rearing of SS strain and RR strain of the hostASS strain highly susceptible to spinosad and a RR strain highly resistant to spinosad were established by hybridizing a spinosad-susceptible strain with a spinosad-resistant strain and following-up selection. The LC50 to spinosad of the SS strain was 0.011mg/L, and that of the RR strain was 265.55mg/L. After rearing for over 30 generations and regular selection of the RR strain every 4-5 generations, the LC₅0 of SS strain remained in the range of 0.01mg/L-0.10mg/L, and the LC₅0 of the RR strain remained in the range of 50.0mg/L-80.0mg/L. Compared to the SS strain, the RR strain always had several thousand fold resistance.(2) Establishment and selection of different strains of the parasitoidResistance selection was conducted for strains of the parasitoid that were reared using the SS or RR strain of P. xylostella as hosts. Resistance selection for C. plutellae was also conducted by applying selection pressure on different development stages of the parasitoid. An unselected strain of C. plutellae, named as NS, was established as the control and reared using the SS strain of P. xylostella as hosts. A strain of C. plutellae named as SSH-A was reared with the SS strain of P. xylostella as hosts and selected at the adult stage. Another three strains of C. plutellae, named as SRH-L, SRH-P, SRH-A respectively, were reared with the RR strain of P. xylostella as hosts and were selected at larval, pupal, oradult stage. Compared to the NS strain, the SSH-A and SRH-A strains did not acquire a significant level of resistance after more than 30 selection cycles, but the SRH-L strain acquired 21.69 fold resistance. The levels of resistance did not differ significantly between the SSH-A and SRH-A strains. Similarly the levels of resistance between the SRH-A and SRH-P strains did not differ significantly either. These results demonstrate that the high level of resistance in the host favours the development of resistance in the parasitoid when the selection was applied to parasitoid larvae via the host larvae. However, host resistance did not show favourable effects when selection was applied to the parasitoid adults.(3) Lethal and sublethal effects of spinosad on the parasitoidTo observe the lethal and sublethal effects of the insecticide on various stages of C. plutellae, cohorts of the parasitoid reared on the RR strain of the host were treated with 25mg/L spinosad (the recommended rate for field application) at the egg or larval stage by applying the insecticide to the host larvae, or at the pupal stage by direct application. In addition, the toxicity of the insecticide on adult wasps was bioassayed. Compared to the controls, when the parasitoids were exposed to spinosad at the egg or larval stage via their host larvae, they suffered mortality within the hosts and in the process of egression out of the hosts, resulting in reduction in cocoon formation and pupation by 43-56%, adult emergence by 19-33% and adult female ratio by 11-25%, while adult body size, longevity and ability of parasitism were unaffected; when the parasitoids were exposed to spinosad at the pupal stage, percent adult emergence was reduced by 10% and adult longevity was decreased, wh...