SOYBEAN MATURITY EFFECT ON EXPRESSION OF RESISTANCE TO LEPIDOPTEROUS INSECTS

The development of soybean [Glycine max (L.) Merrill] cultivars with r esistance to defoliating insects requires characterization of genotype s for the degree of defoliation by natural insect populations in sever al field environments. Combined analysis of variance over environments for insect defoliation often shows genotype x environment interaction s. 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 popula tions of velvetbean caterpillar (VBC), Anticarsia gemmatalis (Hubner) or mixed populations of VBC and soybean looper (SBL), Pseudoplusia inc ludes (Walker), occurring late in the growing season, and one with a m id-season population of beet armyworm (BAW), Spodoptera exigua (Hubner )], four greenhouse experiments [14-d-old plants artificially infested with either VBC, BAW, corn earworm (CEW), Helicoverpa zea (Boddie), o r SBL], and two field-cage experiments [45-d-old plants artificially i nfested 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 m ixed VBC and SBL populations, the early maturing genotypes had greater defoliation than late maturing genotypes (r = -0.74 to -0.90). This n egative association of maturity and defoliation resulted in greater de foliation of resistant MG V genotypes than susceptible MG VII and VIII genotypes.