EFFECT OF SOIL-MOISTURE AND SOYBEAN GROWTH STAGE ON RESISTANCE TO MEXICAN BEAN BEETLE (COLEOPTERA, COCCINELLIDAE)

Although plant breeders and entomologists have been working on the dev elopment of insect-resistant soybean, Glycine max (Merrill), lines and cultivars over the past 20 yr, only 4 cultivars have been released. Q uestions have been raised regarding the potential for insect resistanc e in soybeans. One concern is whether resistance can persist at the de sired level under a range of environmental conditions and soybean grow th stages. This study sought to elucidate the relationship between the expression of Mexican bean beetle, Epilachna varivestis Mulsant, resi stance in soybean and high and low soil moisture. In addition, we exam ined the effect that soybean physiological growth stage has on the exp ression of resistance. Four soybean genotypes that differed in their i nsect resistance levels were grown at 3 soil moistures in the greenhou se. 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 moist ure decreased. Decreased expression in resistance was observed in plan ts grown in high soil moisture during the R3-R5 bioassay compared with the V4-R2 growth period. Resistance expression was lost for the moder ately resistant line, HC83-193, and decreased for the highly resistant line, HC83-123-9, during the R3-R5 bioassay. Ramifications are discus sed for growers concerning lower levels of resistance with increasing soil moisture. These results also are discussed as they relate to expl aining problems related to breeding programs and expression of resista nce in greenhouse and field grown plants.