Density dependence in induced plant resistance to herbivore damage: threshold, strength and genetic variation

The density-dependent effect of induced plant resistance on herbivore popul ations depends on the relationship between the amount of herbivore damage a nd the level of induced resistance produced by the plant. This relationship should influence the interaction of induced resistance and herbivore popul ation dynamics, and if the relationship Varies among plant genotypes, it co uld be subject to natural selection by herbivores. In this study the relati onship between percent leaf area damaged and level of induced resistance wa s characterized for four genotypes of soybeans grown in a greenhouse. Damag e ranging from 8 to 92% of leaf area was imposed using Mexican bean beetle larvae, and induced resistance was measured by bioassay using Mexican bran beetle adults. The level of induced resistance was significantly affected b y the amount of damage, and the level of induced resistance varied signific antly among the four genotypes. There was also a marginally significant int eraction of damage and plant genotype, suggesting that the form of density dependence varies among these four genotypes of soybeans. These results sug gest that these genotypes of plants might affect herbivore populations diff erently. If this variation is heritable, the form of density-dependent effe cts of induced resistance has the potential to evolve in this system.