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Title: Increased long-flight activity triggered in beet armyworm by larval feeding on diet containing Cry1Ac protoxin

item JIANG, XING FU - Chinese Academy Of Agricultural Sciences
item JIAN, CHEN - Chinese Academy Of Agricultural Sciences
item ZHANG, LEI - Chinese Academy Of Agricultural Sciences
item Sappington, Thomas
item LUO, LI ZHI - Chinese Academy Of Agricultural Sciences

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2013
Publication Date: 5/13/2013
Citation: Jiang, X., Jian, C., Zhang, L., Sappington, T.W., Luo, L. 2013. Increased long-flight activity triggered in beet armyworm by larval feeding on diet containing Cry1Ac protoxin. PLoS One. 8(5):e63554.

Interpretive Summary: The beet armyworm, Spodoptera exigua, is an important pest of cotton in many parts of the world, including China and the US. It is a migratory pest, and part of the difficulty in managing it is the unpredictability of location and intensity of infestations of crops after long-distance movement. Transgenic Bt cotton expressing the Cry1Ac toxin controls several species of Lepidoptera (moth) pests, but beet armyworm is naturally rather tolerant of the toxin. Using laboratory flight mills, we expected to find that BAW larvae surviving and developing on diet containing Cry1Ac would have lowered reproductive output and flight capacity as adults. However, the results showed no significant decrease in reproductive output. Even more surprisingly, flight activity was actually increased in moths surviving the Cry1Ac diet as larvae. Thus, moths surviving Bt cotton may be more prone to migrate, which could increase the area receiving damaging infestations of immigrants. This information will be used by university and government scientists in the U.S., China, and throughout the world to better understand the mechanisms controlling migration and reproduction in many related migratory insect pests, as well as the possible role of Bt crops in changing patterns of infestation in some species.

Technical Abstract: Evaluating ecological safety and conducting pest risk analysis for transgenic crops are vitally important before their commercial planting. The beet armyworm, Spodoptera exigua, is not a direct target of transgenic cotton in China but nevertheless recently has become an important pest. In laboratory experiments, we identified (i) sub-lethal effects of Cry1Ac toxin on larval development and adult reproduction, and (ii) migratory behavior triggered by Cry1Ac toxin which may contribute to increased larval outbreaks elsewhere. Our results indicate sub-lethal effects, including significant decreases in larval development rate, larval and pupal weight, and adult lifetime fecundity. The negative sub-lethal effects on these traits suggest they are irrelevant to the severity of S. exigua outbreaks observed after feeding on Cry1Ac toxin. However, S. exigua has low susceptibility to Cry1Ac toxin. No significant differences in larval mortality, pupal rate, adult emergency rate, longevity, preoviposition period, or oviposition period were observed between controls and larvae fed on artificial diet incorporating a low concentration of Cry1Ac toxin. Furthermore, the percentage of long fliers increased significantly when larvae were fed on low Cry1Ac diet, indicating an increased migratory propensity triggered by Cry1Ac toxin. The migratory population arrives in other appropriate habitat in a short time, followed by a subsequent larval outbreak. Negative effects on development and reproduction caused by Cry1Ac in the diet are offset by increased flight propensity triggered by the poor food conditions, thereby improving the chances of escaping declining local conditions. Increased migratory propensity in turn amplifies the area damaged. This phenomenon might be common in other migratory insect pests receiving sub-lethal doses of Bt toxins and warrants further study.