Lethal and sublethal effects of Bacillus thuringiensis var. kurstaki on aquatic insects in laboratory bioassays and outdoor stream channels.

Abstract

The microbial insecticide Bacillus thuringiensis is generally considered to be more environmentally acceptable than many broad-spectrum chemical insecticides (Laird et al. 1990), and Bacillus thuringinesis var. kurstaki (Btk) is now widely used in forest management for controlling defoliating insects (van Frankenhuyzen 1990). Broadscale applications of Btk to forested areas may result in contamination of watercourses, and may pose some hazard to aquatic organisms. Although the impact of B. thuringiensis var. israelensis (a dipteran-active serotype used for biting-fly control) on non-target aquatic fauna has been extensively investigated (e.g., Merritt et al. 1989), few published studies have reported the effects of Btk (the variety used in forest pest management) on aquatic invertebrates. Previous risk assessments of Btk to aquatic invertebrates have been largely based on static bioassays (Eidt 1985) and unpublished data from field investigations (Surgeoner and Farkas 1989). Our objective was to improve the risk assessment for stream insects exposed to Btk by providing further toxicity data for an array of insect taxa under more natural exposure regimes and environmental conditions. We conducted experiments to measure the acute lethal response of aquatic insects to Btk (Dipel 8AF) in recirculating laboratory bioassays, and to determine lethal and behavioural (drift) effects of Btk on aquatic insects in outdoor stream channels. MATERIALS AND METHODS Test organisms were collected from streams with an electroshocker and acclimated at the test sites for 24 hr before the treatments. No attempt was made to determine specific stages of development for the specimens, but most (except Taeniopteryx nivalis and some groups of Lepidostoma sp.) were in late or final instar. Btk was applied as formulated product (Dipel 8AF, Abbott Laboratories, Chicago, IL) at a maximum concentration of 600 IU/mL. As a means send reprint requests to D.P. Kreutzweiser at above address