Maturity and Environntental Effects on Soybeans Resistant to Mexican Bean Beetle (Coleoptera: Coccinellidae)

Abstract

Entomologists and breeders have met with limited success in the development and release of insect resistant soybean cultivars. Although numerous germplasm lines resistant to defoliating insects have been released, 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 impact of plant maturity. This study examined the relative resistance levels of four advanced germplasm lines during the vegetative, flowering, and pod-fill growth stages of soybean development. Resistance was measured by rearing Mexican bean beetle, Epilachna varivestis Mulsant, on excised, field-grown leaves and comparing larval mortality and developmental periods. The study was conducted in two locations for 2 yr. In the 1st yr, mortality was significantly greater, and developmental periods lengthened, on the four resistant soybean lines compared with two susceptible cultivars during the vegetative and flowering stages; however, these differences were slight to nonexistent during the pod-fill stage. Although similar differences were evident the 2nd yr, the levels 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 attributed to much higher than normal rainfall in July at both locations. We discuss the consequences of lower levels of resistance in maturing plants and under conditions of high rainfall, both for breeding programs and field resistance as required by growers.