Galerucella calmariensis L. is a leaf-feeding beetle introduced into North America for biological control of purple loosestrife, Lythrum salicaria L. It is well known that existing natural enemy communities can reduce herbivore establishment and limit their impacts as weed biocontrol agents. Although such biotic interference can often initially be avoided through inundative releases or use of exclusion cages, successful biological control of L. salicaria will ultimately rest on natural colonization of new host patches. The objectives of this study were: 1) identify potential arthropod predators in Michigan wetlands; 2) test predator propensity to attack G. calmariensis; and 3) assess importance of biotic interference by simulating G. calmariensis colonization in wetland habitats. Field surveys showed that Coleomegilla maculata De Geer, Coccinella septempunctata L., and Harmonia axyridis L. (Coleoptera: Coccinellidae) were most abundant potential predators of G. calmariensis, followed by Pterostichus melanarius Illiger (Coleoptera: Carabidae) and Podisus maculiventris Say (Hemiptera: Pentatomidae). In laboratory tests, 24-h attack rates on eggs and larvae ranged from 10 to 100%, with C. maculata and Forficula auricularia L. consuming the greatest number of G. calmariensis. Increasingly realistic laboratory trials demonstrated that several predators effectively located and consumed immature stages of G. calmariensis under more complex conditions. In the field study, predator abundance and attack rates were low, ranging from 0.63 to 1.63 egg masses consumed (10–27%). Survival of individuals to the adult stage in caged (53–90%) versus open treatments (24–42%) revealed a significant effect of predation. However, even the lowest survival rate observed is probably adequate to ensure effective colonization. We conclude that predation can significantly reduce G. calmariensis population increase in the field, but is unlikely to prevent establishment and spread under the conditions studied.
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