Soybean Maturity Group and Incidence of Velvetbean Caterpillars (Lepidoptera: Noctuidae) and Mexican Bean Beetles (Coleoptera: Coccinellidae)

Abstract

Five soybean varieties from maturity groups IV-VIII were routinely monitored in the 1993 and 1994 growing seasons to examine the effect of plant maturity on the seasonal abundance of velvetbean caterpillars, Anticarsia gemmatalis Hubner, and Mexican bean beetles, Epilachna varivestis Mulsant. Differences were detected in the population density of pest populations, percentage of defoliation, and yield. The early maturing varieties (groups IV and V) had lower mean infestations, although the peak A. gemmatalis populations occurred in all varieties at the same time in Georgia. During the population peak in mid-September, the group IV-V varieties were already maturing and no longer attractive to the damaging population densities of 30-60 A. gemmatalis per 25 sweeps that were present on the group VI-VIII varieties. Similar trends were note for E. varivestis populations. Population peaks occurred in all varieties in mid-September, with highest densities in the group VIII variety and much lower populations in all the earlier-maturing varieties. Plots treated with diflubenzuron at 0.28 kg (AI)/ha in mid-Auguest had lower insect populations and percentage defoliation and higher yields than corresponding untreated plots for all varieties except the group IV variety, in which no differences were detected because of low insect populations. Significant linear regressions were obtained between peak A. gemmatalis population densities and percentage defoliation for all 5 maturity group soybeans. Significant linear regressions also were obtained between population peaks and yield reductions for the group V-VIII varieties. The economic injury levels (ElL) for A. gemmatalis were lower for the group IV and V entries (25-30 per 25 sweeps) than for the group VI-VIII entries (35-40 per 25 sweeps). Thus, it appears that a standard ElL for A. gemmatalis cannot be established across all maturity group soybeans in the southern region. The soybean maturity group affected the density of both the A. gemmatalis and E. varivestis population peaks, but the timing of these peaks was similar across all varieties. The group IV and V varieties can be planted to escape high populations of these pests and resultant plant injury, and these varieties had yields comparable to the later maturing entries.