江苏沿江棉区3代棉铃虫对棉花的为害及经济阈值

通过人工接虫为害，研究了3代棉铃虫对蕾铃脱落及结铃的影响，观察了不同年份棉铃虫卵和各龄幼虫的自然存活率及棉花蕾、幼铃的成铃率。结果表明，蕾铃的虫害脱落率对结铃的影响最大，直接通径系数达－0.8917，虫量和蕾铃脱落数主要通过接虫期间的虫害脱落率影响棉花结铃，其间接通径系数分别为－0.8894和-0.891，接虫期间的自然脱落率对结铃的直接和间接通径系数均很小，表明33代棉铃虫为害时棉花4的补偿能力已基本丧失，各龄幼虫的为害量观察表明，1－6龄幼虫的单虫为害当量分别为0.02、0.14、0.31、0.47、0.84和1.54个大铃，根据防治的直接和间接收益，得出3代棉铃虫的防治指标为百株累计卵量26粒。

Damage and economic threshold of the third generation of cotton bollworm in the Yangtze River basin cotton region of Jiangsu Province

The impact of the third generation of cotton bollworm Helicoverpa armigera on shedding of buds and bolls and on bolling was studied by inoculating its larvae to cotton plants. The results showed that the shed rate of buds and bolls damaged by the larvae had the greatest impact on bolling, and its direct path coefficient was up to -0.8917. The larval population density and the number of buds and bolls damaged by the larvae impacted bolling mainly through the damaged shed rate during inoculating stage, and their indirect path coefficients were -0.8894 and -0.891 respectively. Both of the direct and indirect path coefficients of natural shed rate during inoculating stage to bolling were small, which showed that the cotton had little capacity of compensation while the third generation cotton bollworm occurring. The investigations of damage number by larvae showed that the damage equivalent of the 1st-6th instar larval were 0.02, 0.14, 0.31, 0.47, 0.84 and 1.54 old bolls respectively. Based on direct and indirect benefits from controlling, the economic threshold of the third generation of the bollworm was worked out to be 26 accumulated eggs per 100 plants.