Performance of Galerucella calmariensis (Coleoptera: Chrysomelidae) on different North American populations of purple loosestrife

The success of a biological weed control program depends on the ability of control agents to develop on various genotypes of their host plants, thereby reducing the competitive ability of the target plant species. We studied the performance of the chrysomelid beetle Galerucella calmariensis L., introduced as a biological control agent of purple loosestrife, Lythrum salicaria L., on plants from 3 different North American populations. Plants for the experiments were obtained from Ithaca, NY; St. Paul, MN; and Salem, OR, representing different climates and colonization histories. We studied the survival and impact of G. calmariensis at 3 herbivore densities with plants grown under identical conditions at Ithaca. Plant growth and biomass allocation patterns were significantly different among the 3 tested populations. Survival rates of the leaf beetles (egg to adult) were not significantly different among sites or among herbivore attack levels. Herbivory caused significant reductions in plant height, leaf, shoot, root, and total plant biomass with increasing attack levels. We found no significant interaction of herbivory and plant origin. Plants compensated for the loss of photosynthetic tissues by replacing foliage at the expense of below ground storage, thus, herbivory altered the biomass allocation pattern of L. salicaria. This might have important implications for other control agents attacking the roots of L. salicaria.