Nontarget feeding of leaf-beetles introduced to control purple loosestrife(Lythrum salicaria L.)

Purple loosestrife (Lythrum salicaria L.) is an invasive nonindigenous plan t that negatively affects North American wetlands. In 1992, four host-speci fic insect herbivores were introduced from the plant's native range as biol ogical control agents and are now established in over 30 states and 10 Cana dian provinces. Severe defoliation of purple loosestrife by Galerucella cal mariensis L. and G. pusilla Duft. (Coleoptera: Chrysomelidae) selectively r educed purple loosestrife biomass by as much as 95% at many early release s ites. At three sites, mass emergence of new generation Galerucella adults r esulted in localized, short-term attack on Rosa multiflora Thunb., Potentil la anserina L., and Decodon verticillatus (L.) Elliott. Individuals of the same plant species away from the immediate emergence areas and at other rel ease sites remained undamaged, and we observed neither feeding nor oviposit ion on the same plants by overwintered adults. Attack did not persist into the next growing season, and nontarget plants grew and appeared vigorous th e following year, while purple looscstrife remained suppressed. Such "spill over" does not constitute a host shifts beetles are unable to complete deve lopment on these nontarget plants. Spillover effects have been observed in other biocontrol programs and do not affect distribution or abundance of no ntarget species. We anticipate that occasional spillover with transient att ack on nontarget species may occur at other release sites with high populat ion densities of the Galerucella species. Careful monitoring is the best me ans to determine long-term impact.