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 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.