Effect of soil Moisture and Soybean Growth Stage on Resistance to Mexican Bean Beetle (Coleoptera: Coccinellidae)

Abstract

Although plant breeders and entomologists have been working on the development of insect-resistant soybean, Glycine max (Merrill), lines and cultivars over the past 20 yr, only 4 cultivars have been released. Questions have been raised regarding the potential for insect resistance in soybeans. One concern is whether resistance can persist at the desired level under a range of environmental conditions and soybean growth stages. This study sought to elucidate the relationship between the expression of Mexican bean beetle, Epilachna varicestis Mulsant, resistance in soybean and high and low soil moisture. In addition, we examined the effect that soybean physiological growth stage has on the expression of resistance. Four soybean genotypes that differed in their insect resistance levels were grown at 3 soil moistures in the greenhouse. Insect bioassays were conducted by rearing Mexican bean beetle on excised leaves and comparing larval mortality, developmental periods, and pupal and adult weights. Bioassays were conducted during the V4-R2 growth stages and the R3-R5 growth stage. Total mortality and larval developmental periods increased on all soybean genotypes as soil moisture decreased. Decreased expression in resistance was observed in plants grown in high soil moistuer during the R3-R5 bioassay compared with the V4-R2 growth period. Resistance expression was lost for the moderately resistant line, HC83-193, and decreased for the highly resistant line, HC83-123-9, during the R3-R5 bioassay. Ramifications are discussed for growers concerning lower levels of resistance with increasing soil moisture. These results also are discussed as they relate to explaining problems related to breeding programs and expression of resistance in greenhouse and field grown plants.