From ecological menace to roadside attraction: 28 years of evidence support successful biocontrol of purple loosestrife

Introduction and spread of non-native plants provide ecologists and evolutionary biologists with abundant scientific opportunities. However, land managers charged with preventing ecological impacts face financial and logistical challenges to reduce threats by introduced species. The available toolbox (chemical, mechanical, or biological) is also rather limited. Failure to permanently suppress introduced species by mechanical and chemical treatments may result in biocontrol programs using host-specific insect herbivores. Regardless of the chosen method, long-term assessment of management outcomes on both the target species and associated biota should be an essential component of management programs. However, data to assess whether management results in desirable outcomes beyond short-term reductions of the target plant are limited. Here, we use implementation of a biocontrol program targeting a widespread wetland invader, Lythrum salicaria (purple loosestrife), in North America to track outcomes on the target plant over more than two decades in New York State. After extensive testing, two leaf-feeding beetles (Galerucella calmariensis and Galerucella pusilla; hereafter “Galerucella”), a root-feeding weevil (Hylobius transversovittatus) and a flower-feeding weevil (Nanophyes marmoratus), were approved for field releases. We used a standardized monitoring protocol to record insect abundance and L. salicaria stem densities and heights in 1-m2 permanent quadrats at 33 different wetlands and followed sites for up to 28 years. As part of this long-term monitoring, in 20 of these wetlands, we established a factorial experiment releasing either no insects (control), only root feeders, only leaf beetles, or root and leaf feeders. We documented reduced L. salicaria occupancy and stem densities following insect releases over time, irrespective of site-specific differences in starting plant communities or L. salicaria abundance. We could not complete our factorial experiment because dispersal of leaf beetles to root-feeder-only and control sites within 5 years invalidated our experimental controls. Our data show that it took time for significant changes to occur, and short-term studies may provide misleading results, as L. salicaria stem densities initially increased before significantly decreasing. Several decades after insect releases, prerelease predictions of significant purple loosestrife declines have been confirmed.