Quantitative assessment of predation of eggs and larvae of Galerucella pusilla in Iowa

In Iowa, predators may be limiting Galerucella species to densities below levels needed for biological control of purple loosestrife (Lythrum salicaria). A 2-year assessment of predatory arthropods and quantification of predation in the field provided an understanding of the trophic interactions that occur within L. salicaria-dominated Iowa wetlands. Whole-plant samples and presence/absence counts assessed canopy-dwelling arthropods, whereas pitfall traps were used to identify ground-dwelling spiders and insects weekly from May to August. Thirty-five families of spiders and 45 families of predatory insects were collected from L. salicaria-infested wetlands. Carabidae and Lycosidae were the most abundant ground-dwelling predators. Coccinellidae and Salticidae were the most abundant predators in the canopy. Both egg and larval field predation were assessed. Egg predation was evaluated by attaching sentinel egg masses to L. salicaria plants for 48h. Average egg predation was 26% of egg masses. Paired open and closed cages containing 20 G. pusilla first instars on L. salicaria were placed in Iowa wetlands for 10 days (larval developmental time from first to third instars) to coincide with the natural occurrence of Galerucella larvae in the field. Larval predation was high, 46% for the first generation and 38% during the second generation. The observed levels of predation are likely a factor in decreasing the populations of Galerucella released for the biological control of L. salicaria.