Biological Control

Purple loosestrife, Lythrum salicaria L., is an exotic perennial plant of European origin that is invading and degrading wetland habitats all across North America. This invasive plant can form dense monotypic stands in a variety of wetland and lakeshore habitats replacing native plant species, thus degrading food, shelter and nesting sites for wildlife.

Currently there are no chemical or mechanical methods that provide long-term control of established stands of purple loosestrife. However, biological control, the use of natural enemies to control a pest, shows real promise. Beginning in 1992, a nationally coordinated program introduced four species of European insects (one root-mining weevil, one flower-feeding weevil and two leaf-feeding beetles) into North America.

On June 11, 1993, 2 000 Galerucella pusilla and G. calmariensis beetles were released at a site in southern Ontario. The core area was a virtual monoculture of purple loosestrife with plants over 2 metres (6 feet) tall covering more then 5 000 square metres. Over the next 4 years the insect populations exploded, completely suppressing seed output, and reducing purple loosestrife biomass from over 2 000 grams to less than 20 grams per square metre. In 1996, purple loosestrife plants were about knee high and native plants such as cattails flourished (see picture taken 1996). Purple loosestrife was severely damaged throughout the entire core area (5 000 square metres) and beetles have spread several kilometres from the site. The Galerucella beetles went through 3-5 generations before this damage became obvious.


One year earlier, two species of leaf-eating beetles were released on this site and became established in 1994


By 1995, the leaf-eating beetles defoliated nearly all the loosestrife


In 1996, the purple loostrife biomass was reduced by over 90% and native plants were starting to rebound


The introduction of bio-control insects is not intended to eradicate purple loosestrife, but will significantly reduce its abundance in wetland habitats. The results observed in Ontario were repeated at many purple loosestrife infestations. The level of purple loosestrife reduction depends on individual site characteristics and the insect species introduced.


Nanophyes Marmoratus

Manitoba Conservation Ducks Unlimited City of Winnipeg Invasive Species Council of Manitoba