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Soybean Maturity Effect on Expression of Resistance to Lepidopterous Insects

Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602

John N. All and James W. Todd

Dep. of Entomology, Univ. of Georgia, Athens, GA 30602
Dep. of Entomology, Coastal Plain Stn., Tifton, GA 31794

^* Corresponding author.

The development of soybean [Glycine max (L.) Merrill] cultivars with resistance to defoliating insects requires characterization of genotypes for the degree of defoliation by natural insect populations in several field environments. Combined analysis of variance over environments for insect defoliation often shows genotype x environment interactions. This research documents the importance of plant maturity and insect seasonality on this interaction. Fourteen (seven resistant and seven susceptible) soybean genotypes, ranging across Maturity Group(s) (MG)V to VIII, were evaluated in four field environments [three with populations of velvetbean caterpillar (VBC), Anticarsia gemmatalis (Hübner) or mixed populations of VBC and soybean looper (SBL), Pseudoplusia includes (Walker), occurring late in the growing season, and one with a mid-season population of beet armyworm (BAW), Spodoptera exigua (Hübner)], four greenhouse experiments [14-d-old plants artificially infested with either VBC, BAW, corn earworm (CEW), Helicoverpa zea (Boddie), or SBL], and two field-cage experiments [45-d-old plants artificially infested with CEW]. In the vegetative and early reproductive stages of development, soybean maturity was unrelated to insect defoliation (r = –0.21 to 0.35). In three field experiments with late season VBC or mixed VBC and SBL populations, the early maturing genotypes had greater defoliation than late maturing genotypes (r = –0.74 to –0.90). This negative association of maturity and defoliation resulted in greater defoliation of resistant MG V genotypes than susceptible MG VII and VIII genotypes.

Received for publication July 13, 1992.