MATURITY AND ENVIRONMENTAL-EFFECTS ON SOYBEANS RESISTANT TO MEXICAN BEAN BEETLE (COLEOPTERA, COCCINELLIDAE)

Entomologists and breeders have met with limited success in the develo pment and release of insect resistant soybean cultivars. Although nume rous germplasm lines resistant to defoliating insects have been releas ed, only three cultivars have been made available to growers over the past 20 yr. Researchers have examined potential limitations of insect resistant soybeans, with most of the studies examining the negative im pact of plant maturity. This study examined the relative resistance le vels of four advanced germplasm lines during the vegetative, flowering , and pod-fill growth stages of soybean development. Resistance was me asured by rearing Mexican bean beetle, Epilachna varivestis Mulsant, o n excised, field-grown leaves and comparing larval mortality and devel opmental periods. The study was conducted in two locations for 2 yr. I n the 1st yr, mortality was significantly greater, and developmental p eriods lengthened, on the four resistant soybean lines compared with t wo susceptible cultivars during the vegetative and flowering stages; h owever, these differences were slight to nonexistent during the pod-fi ll stage. Although similar differences were evident the 2nd yr, the le vels of resistance were greatly lowered. This reduction in resistance as the plants matured is comparable to that found by other researchers . The loss of resistance in the 2nd yr was unexpected and was attribut ed to much higher than normal rainfall in July at both locations. We d iscuss the consequences of lower levels of resistance in maturing plan ts and under conditions of high rainfall, both for breeding programs a nd field resistance as required by growers.