Soil Inactivation of the <i>Bacillus thuringiensis</i> Subsp. <i>kurstaki</i> CryIIA Insecticidal Protein within Transgenic Cotton Tissue: Laboratory Microcosm and Field Studies

Abstract

The environmental fate of Bacillus thuringiensis subsp. kurstaki CryIIA insecticidal protein expressed within transgenic cotton plant tissue (= Bt-cotton) was evaluated by determining reduction in the biological activity of the protein incubated in soil for 120 days. Studies were conducted in a laboratory microcosm and under field conditions during the fall and winter of 1995−1996 in St. Louis, MO. An insect bioassay, based on growth inhibition of larval Heliothis virescens (F.), was used to estimate DT50 values (50% dissipation time = “half-life” of bioactivity) of the CryIIA protein. DT50 values were 15.5 and 31.7 days for the laboratory and field, respectively. The percentages of initial CryIIA protein bioactivity remaining after 40 days of incubation were similar for the laboratory and field samples. In both environments, <25% of the initial bioactivity remained after 120 days. These results indicate that the biological activity of CryIIA protein, as a component of postharvest Bt-cotton plants, readily dissipates when cultivated into soil and suggest that laboratory soil microcosms can be useful for estimating the rate at which dissipation occurs in the field. Keywords: Soil degradation; transgenic cotton; CryIIA insecticidal protein; Heliothis virescens