Induced Resistance in Soybean to the Mexican Bean Beetle (Coleoptera: Coccinellidae): Comparisons of Inducing Factors

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Abstract
Induced resistance in soybean was investigated using mechanical injury and herbivory by the soybean looper, Pseudoplusia includens (Walker), as inducing factors. Dual-choice feeding-preference tests with the Mexican bean beetles, Epilachna varivestis Mulsant, were used to assess the induced resistance. Comparisons of leaves from plants treated by mechanical injury, soybean looper herbivory, and application of soybean looper larval regurgitate on mechanically wounded leaf surfaces revealed that herbivore-feeding injury was a better inducer than mechanical injury. The regurgitate of soybean looper larvae may contain factors that enhance induction of resistance. Tests using various types of mechanical injury as inducing factors showed that the level of induced resistance depended on the total number of wounded cells in contact with healthy cells and not on the total amount of plant tissue lost. We suggest that there is a positive correlation between the intensity of the inducing factor and the level of resulting resistance in soybean. Interplant transfer of inducing signals was tested, but no evidence was found that such transfer occurred.