Success takes time: History and current status of biological control of purple loosestrife in the United States

Abstract

Purple loosestrife (Lythrum salicaria, Lythraceae) is a long-lived forb that has negatively affected North American wetlands for decades. Following the introduction of purple loosestrife from Eurasia in the early 1800s, populations gradually spread across North America, eventually leading to the decline of many native birds, plants, and amphibians. Land managers recognized the widespread ecological harm caused by purple loosestrife and called for sustainable control methods, realizing that traditional methods such as chemical treatments had failed to produce desirable outcomes. In response, research to assess biological control options for purple loosestrife began in 1986 in Europe. This biological control program represented one of the first times a plant was targeted for biological control because of its harm to flora and fauna rather than because of its negative impacts to agriculture. This work led to the release of four host-specific insects: two leaf-feeding beetles (Galerucella calmariensis and Galerucella pusilla; both Coleoptera: Chrysomelidae) and a root-feeding weevil (Hylobius transversovittatus; Coleoptera: Curculionidae) in 1992, followed in 1994 by a flower-feeding weevil (Nanophyes marmoratus; Coleoptera: Curculionidae). The Galerucella leaf-feeding beetles now appear to be widely established and abundant. Data on the abundance and distribution of the root-feeding and flowering-feeding weevils remain sparse. The effect of these insects may vary from site to site, but in many regions across North America, such as the Pacific Northwest, the Great Lakes Region, and the Northeast, biological control of purple loosestrife is now highly effective and economical. For example, long-term data collected from New York document that these insects reduce the density, height, and flower production of purple loosestrife, which in turn allows an increase in native plant diversity. This is the ultimate goal of weed management. Many biological control success stories are anecdotal, and purple loosestrife is one of the first examples for which we have strong evidence that control of invasive plants by insects can result in native plant recovery.