Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2003
Publication Date: 4/1/2004
Citation: Gore, J., Adamczyk Jr, J.J. 2004. Selective feeding of soybean looper (lepidoptera: noctuidae) on meridic diet with different concentrations of the bacillus thuringiensis cry1ac protein. J. Entomol. Sci. 39(2): 243-250 (April 2004) Interpretive Summary: Genetically engineered Bollgard® cotton became commercially available during the 1996 growing season. Since that time Bollgard® cultivars have provided acceptable control of the tobacco budworm, the target caterpillar pest. However, Bollgard® cotton only provides limited control of other caterpillar pests, such as the soybean looper. Laboratory experiments were conducted to determine if soybean looper larvae can selectively feed on diet with sub-lethal concentrations of the Cry1Ac protein in Bollgard® cotton. In a choice feeding experiment, soybean looper larvae fed on diet with sub-lethal concentrations of Cry1Ac. The sub-lethal concentrations were based on no-choice experiments and produced less than 50% mortality. This novel finding demonstrates that certain caterpillar species can distinguish varying levels of Bt within Bollgard® cotton, a mechanism that possibly helps explain why soybean loopers survive in this type of cotton.
Technical Abstract: Cotton cultivars (Bollgard®) that produce the Cry1Ac protein from Bacillus thuringiensis (Bt) Berliner have been commercially grown in the United States since 1996. Although Bollgard® cotton provides good control of the tobacco budworm, Heliothis virescens (F.), other occasional pests such as the soybean looper, Pseudoplusia includens, are not adequately controlled with this technology. The ability of soybean loopers to selectively feed on meridic diet with sub-lethal concentrations of Cry1Ac was investigated in a choice assay. Neonates selectively fed on non-treated meridic diet and diet with concentrations of Cry1Ac ranging from 0.1 to 1.0 µg/ml compared to diet with Cry1Ac concentrations of 5.0 and 10.0 µg/ml. Averaged across rating periods, means of 11.9 to 17.6 percent of larvae were observed feeding on diet with Cry1Ac concentrations ranging from 0.0 to 1.0 µg/ml compared to 3.4 and 4.3 percent on diet with Cry1Ac concentrations of 5 and 10 µg/ml, respectively. In a no-choice study, concentrations ranging from 0.1 to 1.0 µg/ml produced 4.3-40.8% mortality in soybean looper; while, concentrations of 5.0 and 10.0 µg/ml produced 58.5 and 90.8% mortality, respectively. Also, diet with Cry1Ac concentrations of 5.0 and 10.0 µg/ml delayed the development of soybean looper larvae compared to diet with the other concentrations of Cry1Ac. Based on results of this study, soybean loopers can select diet with sub-lethal concentrations of Cry1Ac. In Bollgard® cotton, selective feeding could potentially increase the pest status of soybean loopers. Furthermore, these findings demonstrate that certain Lepidopteran species can distinguish varying Cry1Ac levels within the transgenic cotton plant. Thus, this may be a mechanism for survival of soybean looper larvae in Bt cotton by preferentially feeding on plant structures with lower amounts of Cry1Ac.