Synthetic pheromone exposure increases calling and reduces subsequent mating in female Contarinia nasturtii (Diptera: Cecidomyiidae).

BACKGROUND: Pheromone-mediated mating disruption, which uses large amounts of synthetic female pheromones to interrupt insect reproductive behavior, has been successful for managing important agricultural pests. While multiple mechanisms have been discovered explaining how synthetic pheromone treatments prevent males from finding females, it is less clear how unnaturally large doses of synthetic sex pheromone impact the behavior of female insects, particularly nonlepidopteran females. In some species, 'autodetecting' females possess pheromone receptors and respond to ambient pheromones by altering their mating behavior. Here, we test whether exposure to stereospecific and racemic synthetic pheromones influences calling and subsequent propensity to mate in female swede midge (Contarinia nasturtii Kieffer; Diptera: Cecidomyiidae), a pest of Brassica crops. RESULTS: In both laboratory and field settings, females exposed to stereospecific and racemic three-component pheromone blends called significantly more frequently and for longer durations than midges in control treatments. In the field, midges were twice as likely to call in pheromone-treated plots versus nontreated plots. Additionally, pheromone pre-exposure reduced subsequent mating: while 68% of female midges mated following control conditions, only 42% and 35% of females pre-exposed to stereospecific and racemic three-component blends mated, respectively. CONCLUSION: While more frequent calling within pheromone-treated backgrounds may increase the likelihood that females are detected by males, a reduction in female propensity to mate would increase the efficacy of a pheromone-mediated mating disruption system. Our work presents the first known investigation of autodetection behavior in Cecidomyiidae. Additional research is necessary to understand the implications of female autodetection for swede midge management.

Racemic Pheromone Blends Disrupt Mate Location in the Invasive Swede Midge, Contarinia nasturtii.

Swede midge, Contarinia nasturtii Kieffer, is an invasive cecidomyiid pest that causes serious losses of Brassica oilseed and vegetable crops in the Northeastern U.S. and Canada. Currently, few alternatives to systemic insecticides exist for its management. Because a single feeding larva can render heading Brassica crops unmarketable, management strategies that prevent oviposition are needed urgently. Pheromone-mediated mating disruption is a promising management approach for swede midge because it prevents mating and subsequent crop damage. While the swede midge pheromone has been identified, one of the major barriers to using it in mating disruption is the high cost of synthesis. Racemic blends, consisting of natural and non-natural stereoisomers, could be useful for mating disruption because they are cheaper to produce. However, it is not clear whether racemic pheromone blends attract males and/or prevent them from locating and mating with females. Here, we studied the behavior of male swede midge in Y-tube and wind tunnel bioassays to pheromone blends. Specifically, we tested whether males: (1) are attracted to different doses of pheromone, (2) discriminate between blends comprising natural stereospecific or racemic components, or a combination thereof, and (3) are able to locate and copulate with females in pheromone-permeated olfactometers. We found that picogram amounts of pheromone attracted males and prevented them from locating females in y-tube olfactometers. While males were more attracted to stereospecific blends, compared to racemic blends, all blends tested prevented nearly all males mating with females. Therefore, low dose racemic blends may be promising for pheromone-mediated mating disruption.

A Single Swede Midge (Diptera: Cecidomyiidae) Larva Can Render Cauliflower Unmarketable.

Swede midge, Contarinia nasturtii Kieffer (Diptera: Cecidomyiidae), is an invasive pest causing significant damage on Brassica crops in the Northeastern United States and Eastern Canada. Heading brassicas, like cauliflower, appear to be particularly susceptible. Swede midge is difficult to control because larvae feed concealed inside meristematic tissues of the plant. In order to develop damage and marketability thresholds necessary for integrated pest management, it is important to determine how many larvae render plants unmarketable and whether the timing of infestation affects the severity of damage. We manipulated larval density (0, 1, 3, 5, 10, or 20) per plant and the timing of infestation (30, 55, and 80 d after seeding) on cauliflower in the lab and field to answer the following questions: 1) What is the swede midge damage threshold? 2) How many swede midge larvae can render cauliflower crowns unmarketable? and 3) Does the age of cauliflower at infestation influence the severity of damage and marketability? We found that even a single larva can cause mild twisting and scarring in the crown rendering cauliflower unmarketable 52% of the time, with more larvae causing more severe damage and additional losses, regardless of cauliflower age at infestation.