Biocontrol of the oriental armyworm, Mythimna separata, by the tachinid fly, Exorista civilis, is synergized by Cry1Ab protoxin

Authors: JIANG, XINGFU - Chinese Academy Of Agricultural Sciences; ZANG, LEI - Chinese Academy Of Agricultural Sciences; YANG, HAIXIA - Chinese Academy Of Agricultural Sciences; Sappington, Thomas; CHENG, YUNXIA - Chinese Academy Of Agricultural Sciences; LUO, LIZHI - Chinese Academy Of Agricultural Sciences

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2016
Publication Date: 6/21/2016
Citation: Jiang, X., Zang, L., Yang, H., Sappington, T.W., Cheng, Y., Luo, L. 2016. Biocontrol of the oriental armyworm, Mythimna separata, by the tachinid fly, Exorista civilis, is synergized by Cry1Ab protoxin. Scientific Reports. 6:26873. doi:10.1038/srep26873.

Interpretive Summary: The oriental armyworm, Mythimna separata, is one of the most important pests of corn in China. Like many closely-related caterpillar pests of corn in the U.S., including the sister species Mythimna unipuncta, it is only imperfectly controlled by Bt corn, and is not the primary target pest of Bt varieties. However, we showed that parasitism by a tachinid fly, Exorista civilis, is compatible with the Bt toxin and even increases mortality of M. separata beyond that expected from simple additive effects of both agents. Our data also show that there is no negative effect of the parasitoid developing in M. separata larvae fed Bt toxin in an artificial diet. Overall, our results demonstrate that use of Bt corn to control M. separata not only is compatible with the use of the tachinid parasitoid as a biocontrol agent, but that the two methods can act synergistically to manage this destructive insect pest. This, together with the lack of evidence for negative effects on E. civilis, provide support for the safety of transgenic Bt plants expressing Cry1Ab in China. This information will be used by university and government scientists in the U.S., China, and throughout the world, and to regulatory agencies in China tasked with assessing the environmental risks of Bt corn to non-target and beneficial insects.

Technical Abstract: Tritrophic interactions between the oriental armyworm Mythimna separata, its larval tachinid parasite, Exorista civilis, and the Cry1Ab protoxin of Bacillus thrunginenis, were examined using a laboratory-based system. Although M. separata sixth (last) instar mortality increased with increasing Cry1Ab concentration in artificial diet, relatively low susceptibility to Cry1Ab was observed. Likewise, parasitization by E. civilis contributed to only 14% mortality of sixth instar M. separata reared on toxin-free control diet. However, larvae exposed to the combination of Cry1Ab and E. civilis parasitization showed a significant Cry1Ab dose-dependent increase in mortality over that of either alone, including a 52-56% synergistic increase in efficacy at the two concentrations tested. Likewise, pupal weight, adult emergence and lifetime fecundity of M. separata derived from larvae surviving both agents were negatively affected. The ability of E. civilis to parasitize and subsequently develop on the host larva was not adversely influenced by Cry1Ab ingested by the host at any concentration tested. No significant differences in parasitism rate, adult emergence rate or lifetime fecundity were observed when E. civilis developed on M. separata reared on diet containing different concentrations of Cry1Ab. E. civilis pupation rate increased significantly in host larvae reared on diet containing the lowest concentration of Cry1Ab tested (3.125 µg/g), the only significant interaction observed. Overall, our results demonstrate that use of Cry1Ab Bt maize to control M. separata not only is compatible with the use of the tachinid parasitoid as a biocontrol agent, but that the two methods can act synergistically to manage this destructive insect pest. This, together with the lack of negative effects on E. civilis, provide support for the safety of transgenic Bt plants expressing Cry1Ab in China.