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Dryad

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

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Oct 18, 2024 version files 1.17 MB

Abstract

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 not just on the target species, but also on 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 G. 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 1m2 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 document 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 five 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. salicara stem densities initially increased before significantly decreasing. Several decades after insect releases, pre-release predictions of significant purple loosestrife declines have been confirmed.