Evaluation of Seedling Transgenic Cotton Containing Bacillus Thuringiensis Toxins to Saltmarsh Caterpillar

The saltmarsh caterpillar, Estigmene acrea (Drury), is an occasional pest of cotton, Gossypium hirsutum L., in the U.S.  Although saltmarsh caterpillar is most often encountered late in the season, the most damaging populations are usually dispersing late instars infesting seedling cotton.  Three cotton varieties, a non-Bt, a Bollgard 2 (Cry1Ac + Cry2Ab), and a Widestrike (Cry1Ac + Cry1F), were evaluated at the two true-leaf stage for resistance to feeding by neonate and late fourth-instar saltmarsh caterpillars.  The Bollgard 2 and Widestrike varieties were very resistant to neonate saltmarsh caterpillars, killing 100% with no visible damage after 3 days of exposure.  Mortality on the non-Bt variety was 0% and damage was evident.  When exposed to fourth-instar larvae, the Widestrike and Bollgard 2 varieties killed 80 and 90%, respectively, after 7 days of exposure.  Mortality by the non-Bt variety was 10%.  Leaf consumption by fourth-instar saltmarsh caterpillars on the Bt varieties was negligible, while a mean of 19.7-cm² of the non-Bt variety was consumed per larva.  Based on relative leaf area of cotton on the Texas High Plains and the estimate that cotyledon- to two true-leaf-stage cotton can withstand 75% damage without significantly impacting yield, treatment thresholds for late-instar saltmarsh caterpillars may approximate 0.5, 1.0, and 1.5 larva per plant on cotyledon, one true-leaf, and two true-leaf-stage cotton, respectively.