Effects of Plant Density and Late-season Defoliation on Yield of Field Beans

Abstract

Effects of stand density and late-season defoliation by Mexican bean beetles, Epilachna varivestis Mulsant, on three yield components of beans were estimated from field studies conducted during 1983 and 1984 in four fields of pinto beans in northeastern Colorado. Number of beans per plant, mean weight of a bean, and mean weight of beans per plant were calculated for 100 samples per field, and regressed against three independent variables: plant density, percentage of defoliation, and density × defoliation interaction. Results of the analyses showed that plant density negatively affected number of beans per plant and weight of beans per plant, but not average weight of a bean. Responses were not linear; slope decreased at higher densities. Defoliation negatively affected all three yield variables. Average weight of a bean was linearly related to defoliation. Number of beans per plant and weight of beans per plant showed linear responses above a threshold level of defoliation. A piecewise regression model indicated that yields were not affected by defoliation until levels reached approximately 19% foliage loss. Laboratory consumption studies were conducted to determine age-specific damage potential for Mexican bean beetles and two occasional defoliators of pinto beans, saltmarsh caterpillars, Estigmene acrea (Drury), and yellow woollybears, Spilosoma virginica (F.). Insect consumption potentials and field studies of yield responses to defoliation were used to estimate threshold densities.