Host-Plant Selection by the Mexican Bean Beetle, Epilachna varivestis1

Abstract

The Mexican bean beetle, Epilachna varivestis Mulsant, feeds selectively on certain species of Phaseolus to the exclusion of the remaining representatives of this and other genera of beans. In a study of the chemical factors underlying this selective feeding, the phagostimulant fraction in the seeds of P. vulgaris was isolated and identified as sucrose. Bioassay of 13 sugars and 15 amino acids showed that only sucrose and, to a lesser degree, its hexose components induced feeding by the beetle. The seeds as well as leaves of the nonresistant species of Phaseolus were found to have a higher concentration of sucrose, glucose, and fructose, thereby revealing the role these sugars play as arrestants in host-plant selection by the beetle. Bioassay of the volatile fraction from foliage of host-plants yielded positive olfactory responses from Mexican bean beetle larvae, and only indirect evidence of similar responses from the adults. These findings indicate the presence of a short-range attractant in the volatile constituents of hostplants. The influence of both attractants and arrestants in host-plant selection by insect pests is discussed, and the significance of quantitative differences in essential nutrients in host-plants is stressed. Phytogeographical factors relating to the 10 common species of Phaseolus also are discussed, emphasizing that in host-plant selection their role, if any, is only subordinate to stimulant chemicals in plants.