Effects of Age, Reproductive Activity, Sex, and Prior Exposure on Sensitivity to Cucurbitacins in Southern Corn Rootworm (Coleoptera: Chrysomelidae)

Abstract

Cucurbitacins, toxic to most organisms, are arrestants and phagostimulants for diabroticite chrysomelid beetles. Little is known about variation in the response of diabroticites to cucurbitacins. To quantify the effects of age, sex, reproductive status, and prior exposure to cucurbitacins on the sensitivity to cucurbitacin B of the southern corn rootworm, Diabrotica undecimpunctata howardi Barber, beetles were separated by sex and grouped for treatment with diets including or excluding cucurbitacins. Sensitivity to cucurbitacins was assayed over the first 43 d of adulthood by exposing beetles in groups of five for 24 h to filter paper strips spotted with serial methanol dilutions of cucurbitacin B ranging from 800 to 0.1 ng/ml. The areas consumed by the beetles at each dilution were then measured, and dose-response curves for each treatment were drawn. Treatments were compared in terms of the area under the mean dose response curve (AUC). Sex, age, reproductive activity, and prior exposure to cucurbitacins all significantly affected beetle sensitivity, though prior exposure was the most consistent in its effect. Exposure to cucurbitacins for as little as 6 d permanently reduced sensitivity, whereas beetles without prior exposure remained sensitive to our assay throughout the study. Reproductive activity depressed sensitivity in males but increased sensitivity in females. These results are discussed in terms of their relevance to semiochemical diabroticite baits and also to the proposed role of cucurbitacins in protecting these beetles from predators, parasites, or pathogens.