Interactions of arthropod predators and Cry1Ac-transgenic cotton
Torres, Jorge Braz
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Interactions of predators and Bt-cotton plants (containing a gene from Bacillus thuringiensis that expresses Cry1Ac toxin) were investigated in the field, greenhouse, and laboratory. Abundance of predatory arthropods was monitored from 2002 to 2004 in three pairs of adjacent Bt and non-Bt fields (5 to 15 ha each). Analysis of predator abundance and dynamics showed variation among sampling dates and among seasons for some specific taxa collected through whole plant and drop cloth sampling in favor of either cotton. However, when averaged over three years, differences were nearly all eliminated. Of 65 ground-dwelling arthropods collected in pitfall traps, no differences were found between cotton types for abundance, diversity, and species richness. Field-collected materials (plant-herbivore-predator) were assayed for Cry1Ac toxin using ELISA. Bt-cotton and lepidopteran larvae were positive on all sampling dates, while among seven predator species only Podisus maculiventris and Chrysoperla rufilabris were positive (on one and two sampling dates, respectively) concurrent with high abundance of lepidopteran larvae in the fields. Ingestion of Cry1Ac toxin by four common predatory heteropterans (Geocoris punctipes, Nabis roseipennis, Orius insidiosus, and P. maculiventris) was studied using prey fed Bt-cotton in a greenhouse or dilutions of purified Cry1Ac in the laboratory (Geocoris punctipes). Predatory heteropterans were unable to pick up toxin directly from the plant, despite plant feeding behavior, but may acquire Cry1Ac from prey fed Bt-cotton. The amount of prey consumed by small predatory heteropterans (Orius, Geocoris, and Nabis) seems to limit ingestion of Cry1Ac below detectable levels in their bodies. G. punctipes was able to pick up toxin from Cry1Ac purified dilutions in water at detectable levels but from concentrations higher than levels detected in cotton plants and greater than conveyed by prey fed Bt-cotton. Most of the ingested Cry1Ac, however, was excreted and was not detected in the predators’ bodies or feces more than 48 to 72h after feeding. Prey- and/or plant-mediated effects on the omnivorous predator G. punctipes were studied in the field. Predators were exposed to a combination of prey with and without toxin, and Bt and non-Bt plants from egg hatch until death. The results showed no effect of prey fed-Bt or direct effect of Bt-cotton plants on life history parameters of the predator.