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UBC Theses and Dissertations

Ecological interactions between the cabbage looper, a parasitoid, host plants and Bacillus thuringiensis Caron, Valerie


The cabbage looper, Trichoplusia ni, is the only lepidopteran causing economic damage to tomato, sweet pepper and cucumber production in greenhouses in British Columbia. Until recently, T. ni was successfully controlled with Bacillus thuringiensis {Bi), but resistance has developed in some T. ni greenhouse populations. Development of resistance has been associated in other studies with fitness costs such as reduced overwintering ability. Here I assess the overwintering survival of Bt resistant and susceptible T. ni pupae. Resistant and susceptible T. ni had similar survival but resistant populations had reduced fecundity and smaller progeny after cold exposure. This fecundity cost is unlikely to be sufficient to select out the resistant phenotype in the presence of Bt treatment and in the absence of immigration of susceptible moths thus resistance to Bt is unlikely to disappear under current greenhouse management practices. As a potential alternative control, a fly parasitoid, Compsilura concinnata was assessed. In the laboratory C. concinnata readily parasitizes and superparasitizes T. ni, with a preference for later instars. Competition in the host when superparasitism occurred reduced host hemolymph protein, and parasitoids emerged at a younger host stage. Host plant affected T. ni and C. concinnata fitness, as well as the ability of the parasitoid to locate its host. In more complex environments such as in the field and on full grown caged cucumber plants, C. concinnata rarely parasitized T. ni. The effectiveness of the parasitoid depends on T. ni life stage and population, the host plant involved and the complexity of the environment. Although C. concinnata is easy to rear in the laboratory and thus has potential for development as an inundative biological control agent, the complexity of this host/pest/parasitoid system suggests that it may be difficult to effectively use this parasitoid in greenhouses.

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