Insect Defoliation and Population Dynamics in Blends of Resistant and Susceptible Soybean Cultivars

Phytophagous insects can cause significant economic yield loss in soybean [Glycine max (L.) Merr.] production areas of the southern United States. Although breeding efforts have resulted in developing insect-resistant cultivars, the yield potential of these cultivars is generally lower than conventional cultivars under conditions of light insect pressure. The objective of this study was to determine if a blend of insect resistant and insect susceptible cultivars affords some degree of insect control while maintaining high yields under high insect pressure environments. Experiments were conducted on a Labelle soil (Oxyaquic Vertic Argiaquolls) near Beaumont, TX, in 1990 to 1992. The 3-yr field study included five population ratios of Maturity Group VIII culfivars Dowling (insect susceptible, S) and Crockett (insect resistant, R): 100 S:0 R, 75 S:25 R, 50 S:50 R, 25 S:75 R, and 0 S:100 R. Plots were sampled weekly for insects and percent leaf defoliation when insect populations began to increase in late summer. Data were analyzed with blend treated as a linear trend. Insect counts of six major pests showed that, of 18 possible year-insect combinations, in only three cases were there significant population differences due to blend. In late season, the percent defoliation of Crockett increased in all 3 yr as the percentage of Crockett in the blend increased. By contrast, as the percentage of Dowling in the blend increased, the percent defoliation of Dowling decreased in 2 of the 3 yr. Therefore, blending Crockett and Dowling did not offer insect defoliation protection for Dowling and may have had an opposite effect. Maximum percent defoliation for Crockett was generally 40 to 50% less than that for Dowling. In 1990 and 1991, blending had no effect on yield or seed quality. In 1992, there was a significant, negative effect on yield (b = −4.46) and seed quality (b = 0.01) as the percentage of Crockett increased in the blend. Blends of these cultivars did not improve yield of Dowling under conditions of insect pressure compared with a pure stand of Dowling. A blend of insect resistant and susceptible cultivars did not appear to be a suitable insect control management strategy in soybean.