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Population ecology of Trichoplusia ni in greenhouses and the potential of Autographa californica nucleopolyhedrovirus for their control Cervantes, Veronica Beatriz

Abstract

The cabbage looper Trichoplusia ni is the major lepidopteran pest in tomato, cucumber and pepper greenhouses in British Columbia. T. ni has developed resistance to Bacillus thuringiensis (Btk) in some greenhouses and this compromises the effectiveness of this widely used bioinsecticide for the control of loopers. The production in greenhouses continues almost year round and loopers can be present in the crop very early in the season. Few other biological control options are available for this important pest, so new alternatives that are compatible with integrated pest management programs are required. In this work I explored the overwintering success of cabbage loopers and found that populations could survive winter in greenhouses but not outside. This poses a great risk for continued resistance of loopers to Btk. I then evaluated the potential of Autographa californica nucleopolyhedrovirus (AcMNPV) in controlling cabbage looper larvae that are resistant to Btk. I found that larvae resistant to Btk were twice as susceptible to AcMNPV as were those from a non-resistant colony. I also studied if host plants could alter the susceptibility of T. ni to AcMNPV and the speed of kill of the virus. I conducted bioassays with susceptible cabbage loopers on the three main crops cultivated in greenhouses as well as on artificial diet. Susceptibility did not vary among the hosts. Despite the fact that larval growth was highest on cucumber and lowest on pepper no biologically meaningful differences were found in speed of kill. These parameters could be further influenced by the behaviour and differential consumption of host plants by larvae. Observations in greenhouses showed that on tomato plants larvae tended to forage in the lower portion of the plants while on peppers they tended toward the top of the plants. This agrees with experimental results that showed larvae tended to go to the top of pepper plants, the middle of cucumber plants and the bottom of tomato plants. Position was not affected by whether larvae were infected with AcMNPV but the behaviour of larvae could influence the distribution of virus on different crops. I conclude that AcMNPV has much potential as an alternative to Btk and that greenhouse level evaluations should proceed.

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