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Wireworm behaviour in response to insecticides

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(Thesis) Ph.D.
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This research examines the behaviour of the Pacific Coast wireworm, Limonius canus, and the dusky wireworm, Agriotes obscurus, in response to insecticides. The ability of wireworms to recover from morbidity induced by dermal contact with insecticides is discussed first. Wireworms enter one of three distinct stages of health intermediate between life and death after exposure. The duration wireworms remain in these health states, their transition between them, ability to recover, and the time required for them to recover fully or die, all depend on the concentration and class of insecticide, post-exposure temperature, and wireworm species. Contact with wheat seeds treated with the synthetic pyrethroid tefluthrin also induces temporary morbidity in L. canus. Both induction and recovery time depend on the duration of exposure, concentration of insecticide, previous exposure, and temperature. The second section examines whether wireworms are repelled by insecticides in soil and soil-less environments, and whether they are capable of learning to avoid insecticides that induce temporary morbidity. In a soil-less environment, larvae of A. obscurus are repelled by droplets of high concentrations of imidacloprid, chlorpyrifos, lindane, and tefluthrin, with the incidence of repellency increasing over time for the last three chemicals. In a soil environment, larvae of A. obscurus and L. canus orient to wheat seedlings treated with insecticide as quickly as to untreated seedlings, but either become moribund in situ (e.g. lindane) or are repelled after 10-15 min. contact (e.g. tefluthrin). Wireworm orientation and contact behaviour, and post-contact health and survival, are described for lindane, clothianidin, imidacloprid, and various rates of thiamethoxam and/or tefluthrin. Wireworms exposed to the odor of the fungicide Dividend XLRTA and/or tefluthrin during morbidity induction may be subsequently repelled from seeds treated with Dividend, but not tefluthrin. Wireworms are not repelled by peppermint odor in soil bioassays after preconditioning with peppermint, suggesting the presence of positive cues (e.g. CO2, suitable host plant) may override a negative cue (e.g. peppermint odor). In the absence of these positive cues, wireworms appear capable of associative learning, but their repellency to peppermint is strongly reduced by repeated morbidity inductions during preconditioning.
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