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

Texas A&M Univ. Agric. Res. and Ext. Ctr., Box 1658, Vernon, TX 76385
Dep. of Agron., Purdue Univ., 1150 Lilly Hall of Life Sci., West Lafayette, IN 47907-1150
Texas A&M Univ., Agric. Res. & Ext. Ctr., 1509 Aggie Dr., Beaumont, TX 77713-8530
USDA-ARS, Stoneville, MS 38776

^* Corresponding author (j-sij{at}tamu.edu).

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.

Received for publication February 2, 1998.

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