Impact of coleopteran-active Bt corn on non-target nematode communities in soil and decomposing corn roots

Abstract

Genetically engineered corn expressing crystalline proteins for insect control and encoded by genes derived from soil bacterium Bacillus thuringiensis (Bt) are widely adopted in the United States. Among the seven different events of Bt corn available commercially, YieldGard® Rootworm (event MON863) expresses a variant of the cry3Bb1 protein in the root tissue to control corn rootworm larvae. Nematodes reside in the rhizosphere and are potentially exposed to Cry3Bb1 toxins exudated from roots of Bt corn. We test the hypothesis that coleopteran-active Bt corn does not affect non-target soil nematodes. Experimental treatments were: 1) a Bt hybrid, 2) a non-Bt isoline treated with a conventional soil insecticide, and 3) a non-Bt isoline without insecticide. Nematodes were extracted from soil samples collected prior to planting (May), at peak anthesis (August), and after harvest (October) in 2003 and 2004, enumerated and identified to genus. A total of 73 nematode genera were encountered in soil and litter combined. During the growing season, maturity index values and relative abundance of fungivorous nematodes were greater in the Bt hybrid than the non-Bt isoline with or without insecticide. Nematode trophic diversity values were greater in the Bt hybrid than non-Bt isoline with insecticide and this effect continued through the following spring. Abundance of nematode predators increased two weeks after insecticide was applied to non-Bt isoline, but decreased without insecticides on either Bt or the non-Bt isoline. In decaying roots of corn treatments, maturity index values and the relative abundance of nematode predators was greater in the Bt hybrid than non-Bt isoline with insecticide. Effects at the overall community structure and nematode genera varied more by seasonal phenology than corn treatment. The isoline with insecticide had more non-target effects on nematode communities than the Bt hybrid. This treatment increased the relative abundance of predaceous nematodes temporarily but eventually reduced successional maturity by harvest time, which continued to decline during the winter in both soil and decaying corn roots.