Soybean Row Spacing: Effects on Insecticide Efficacy Against Three Common Lepidopteran Defoliators of Different Size Classes

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Abstract
Field studies were conducted in wide- and narrow-row soybean plantings during 1981 (central Mississippi)and 1982 (south Mississippi)to investigate the differential effect of commonly used insecticides in three common species of lepidopteran larvae of three size classes (small, medium, and large). Percentage of larva reduction was calculated based on populations in the untreated control on the same sample date, and data were analyzed separately for each of three species. Results demonstrated clear differences between row-spacing types for each of three pest species (soybean looper, Pseudoplusia includens (Walker); velvetbean caterpillar, Anticarsia gemmatalis Hübner; and green cloverworm, Plathypena scabra (F.) when all size classes are combined. When row-spacing data were combined in a single analysis, differences among size classes were noted; large larvae generally, but not always, were more difficult to control than small or medium larvae. Analyses to segregate differences between row spacings within size classes are presented for each species. Significant differences were recorded in many instances. The differences observed 2 days after application often were not observed at later intervals. Data from these experiments may prove helpful in the development of pest management programs that consider agronomic (row-spacing) and biological (size-class)effects on insecticide effectiveness.