Effects of Repeated Applications of Bacillus thuringiensis israelensis on the Mosquito Predator Erythemis simplicicollis (Odonata: Libellulidae) from Hatching to Final Instar

Abstract

Nymphs of a common dragonfly, Erythemis simplicicollis (Say), were exposed to Bacillus thuringiensis israelensis de Barjac at 1.2 ppm once a week for an entire life cycle in a controlled laboratory environment. Eight weekIy applications were administered to 2 treatment groups: external contact only, and external + internal contact using prey (mostly anopheline larvae) that had fed on B. thuringiensis . Each B. thuringiensis -treated group and a control group consisted of 15 nymphs. Mortality was not affected by B. thuringiensis applications. Repeated B. thuringiensis applications did not affect development to the adult stage, morphology, or maiden flight capability. Nymph size of the external contact group, measured by hind femur length and head width, was substantially smaller compared with the control group in most instars from 4–12. However, adult size, based on head width and hind wing length comparisons, did not differ among the 3 groups. Prey consumption and instar duration, which were highly correlated, did not account for the differences in size. Sex ratio (lower proportion of large females in the external group) and initial size (slightly smaller in instar 2 in the external group) appeared to be the major factors contributing to the size differences in the external group. However, these variables were not responsible for the external + internal contact group being smaller in instar 10 compared with the controls, as sex ratios and initial size were equal. If repeated B. thuringiensis applications affect size in E. simplicicollis , the effect may be insignificant in terms of reproductive success, as published studies do not show a positive relationship between size and reproductive success in dragonflies.