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

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Purple loosestrife (Lythrum salicaria L.) is an invasive nonindigenous plant that negatively affects North American wetlands. In 1992, four host-specific insect herbivores were introduced from the plant's native range as biological control agents and are now established in over 30 states and 10 Canadian provinces. Severe defoliation of purple loosestrife by Galerucella calmariensis L. and G. pusilla Duft. (Coleoptera: Chrysomelidae) selectively reduced purple loosestrife biomass by as much as 95% at many early release sites. At three sites, mass emergence of new generation Galerucella adults resulted in localized, short-term attack on Rosa multiflora Thunb., Potentilla anserina L., and Decodon verticillatus (L.) Elliott. Individuals of the same plant species away from the immediate emergence areas and at other release sites remained undamaged, and we observed neither feeding nor oviposition on the same plants by overwintered adults. Attack did not persist into the next growing season, and nontarget plants grew and appeared vigorous the following year, while purple loosestrife remained suppressed. Such "spillover" does not constitute a host shift; beetles are unable to complete development on these nontarget plants. Spillover effects have been observed in other biocontrol programs and do not affect distribution or abundance of nontarget species. We anticipate that occasional spillover with transient attack on nontarget species may occur at other release sites with high population densities of the Galerucella species. Careful monitoring is the best means to determine long-term impact.

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Natural Areas Journal