The effects of two widespread environmental pollutants, perchlorate and hexavalent chromium, were assessed on the efficacy of Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bsph) against fourth instars of Culex quinquefasciatus Say (Diptera: Culicidae) in 24-h laboratory bioassays. Although 250 mg/liter perchlorate, a level somewhat higher than would be considered ecologically relevant, did not affect the control provided by either larvicide, presence of 1.04 mg/liter hexavalent chromium, an ecologically relevant concentration, increased the efficacy of both Bti and Bsph by 21 and 80%, respectively. In the presence of hexavalent chromium, improved suppression could be expected from Bacillus applications at the current label rates. However, because hexavalent chromium has been shown to affect many taxa, we propose that the potential exists for increased susceptibility of nontarget organisms to Bacillus products in polluted habitats.
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Vol. 44 • No. 5