Fly responses to food colour, orientation and toxic bait composition in Drosophila suzukii

Toxic baits comprising a combination of food attractants and a toxicant could contribute to the control of the spotted wing drosophila, Drosophila suzukii (Diptera: Drosophilidae), a major invasive pest of soft fruit and berries. Laboratory cage experiments revealed that flies of both sexes were significantly more attracted to dried red droplets of 0.3% sucrose solution and were more likely to consume red-coloured droplets compared to blue, green or colourless droplets. Flies of both sexes showed a tendency to feed on dried droplets placed on the floor of the cage rather than droplets presented upside-down on the roof or on the cage side-wall. When offered commercial insecticides (5–50 parts per million active ingredient) in dried sucrose solution, fly mortality of both sexes was highest in the spinosad treatment, lowest in abamectin and intermediate in deltamethrin and spinetoram-based products. Male flies had significantly higher mortality than females. A mixture of 25 ppm spinosad with 1.3% sucrose +1.3% corn syrup +1.3% glycerol (named 4% mixture) was consumed by female flies more than any of the components alone. Addition of 1% apple juice to the 4% mixture resulted in an additional increase in spinosad-induced mortality. In contrast, the addition of apple cider vinegar, increasing concentrations of apple juice (5%–10%), or the addition of the yeasts Saccharomyces cerevisiae or Hanseniaspora uvarum (5 × 10⁶ cells/mL) did not increase fly mortality for reasons that were unclear. We conclude that the combination of 4% mixture +1% apple juice could prove to be a useful bait for the delivery of spinosad or other biorational insecticides for D. suzukii control, although this requires field testing in commercial fruit production settings.