Whiteflies can excrete insecticide-tainted honeydew on tomatoes

Whiteflies are important insect pests in a wide variety of agricultural crops that are targeted with large quantities of insecticides on a global scale. Chemical control is the most common strategy to manage whiteflies, however, recent studies had reported that whiteflies and other hemipterans can excrete insecticides through their honeydew, which could have unanticipated, non-target effects. The objective of this study was to determine the concentration of imidacloprid in honeydew excreted by whiteflies feeding on tomato plants. Imidacloprid was applied at its labeled rate to soil at the base of whitefly-infested plants. Densities of whiteflies were assessed before insecticide treatment and 21 days after treatment (DAT). Honeydew was collected in Petri dishes from 1 to 4 DAT and from 5 to 8 DAT. The volume of the honeydew was calculated using stereo microscopy and then rinsed with ethanol. The rinsates were analyzed to determine imidacloprid concentration using liquid chromatography coupled to mass spectrometry. Honeydew production was further quantified by using water sensitive papers. Imidacloprid reduced densities of nymph and adult whiteflies by 81.5% and 76.0% compared to the control at 21DAT. The non-metabolized parent compound imidacloprid was detected from honeydew samples at both collection dates. At 1-4 DAT, imidacloprid concentrations were 180 ng/30 mL in a volume of 39 mm³ of honeydew. At 5-8 DAT, the imidacloprid concentration was 218 ng/30 mL in a volume of 25 mm³ of honeydew. Though the volume of honeydew decreased, the concentration of imidacloprid numerically increased. Last, whiteflies were still producing honeydew 22 DAT in both treatments. These results revealing significant imidacloprid concentrations in honeydew suggest a strong potential for negative secondary impacts on beneficial insects.