Biological Control Laboratory
J. E. Corrigan
c/o Department of Environmental Biology
University of Guelph
Guelph, Ontario, Canada, N1G 2W1
Telephone (519) 824-4120, Ext 8591
Fax (519) 837-0442

Classical Biological Control of Weeds:

When a plant from one continent finds its way to another, it often leaves behind its usual complement of natural enemies. Freed from these encumbrances, the plant multiplies rapidly, often displacing the native vegetation. The purpose of bio-control is to reunite a weed with its natural enemies in hope of causing enough damage to control its spread. The object is not to eradicate the weed, but to reduce it to ecologically tolerable levels (Powell 1982).

The general criteria for biological control agents of weeds are that they: i) should be extremely specific to the host plant that they feed on; ii) won't complete their life cycle on any other host plant; and iii) can be expected to have a strong, deleterious impact on their host.

Purple Loosestrife:

Purple loosestrife, Lythrum salicaria, is a herbaceous, upright, perennial that was imported to eastern North America from Eurasia early in the nineteenth century. It has spread westward from its original range and can now be found across Canada. From July to early September, the plant is easily recognized by the presence of spikes of bright reddish-pink flowers that stand on top of 4 to 7 foot stalks. A single, mature root system will send out 30-50 shoots and can produce more than 2.5 million seeds per season. The seeds float, hence are easily dispersed in aquatic ecosystems and they can germinate after more than three years of quiescence. Burning, mowing, flooding or uprooting of the plant often will not kill the root system.

Purple loosestrife is an aggressive invader of wetlands, marshes, pastures and riparian meadows. It forms dense, monospecific stands that displace native plant species, reducing the size and diversity of natural plant communities. It promotes the encroachment of firm land into marshes. Few Canadian birds, mammals or fish utilize it as a food source, so it forces the native wildlife out of the habitat. On an annual basis in North America, it is estimated that 190,000 hectares are lost to purple loosestrife with an estimated economic impact of 45 million dollars (Thompson et al. 1987).

The Biological Control Agents:

Two species of leaf-feeding insects of the family Chrysomelidae, Galerucella calmariensis and G. pusilla have been approved for release against purple loosestrife in North America. The adult beetles are light brown and about 3 to 5 millimeters in length. The larvae look like tiny caterpillars with black heads and a yellowish bodies. The biology of the two species is very similar. They overwinter as adults in the soil, becoming active in May and feeding on the leaves and new shoot growth of the plants. The eggs are laid in small batches on the basal parts of the stems, shoot axils and lower sides of the leaves in June and July and each female is capable of producing 500-1500 eggs. The eggs hatch in about one week and the larvae feed on the leaves, moving from the top to the bottom of the plant. On completion of feeding they pupate in the soil. Egg to adult development takes about 6 weeks, after which the adults emerge and feed prior to going into the soil to overwinter. There is usually one generation per year.

In Europe, a complex of natural enemies, including the Galerucella species, prevent the plant from dominating aquatic ecosystems. Galerucella cause high plant mortality, reduce shoot growth, prevent or delay flowering and can reduce seed production by up to 99% (DeClerck-Floate 1992). Total defoliation of purple loosestrife plants by Galerucella is observed in natural situations in Europe (DeClerck-Floate 1992). In North America, it is expected that the combined impact of the Galerucella species and a European root borer, Hylobius transversovittatus will result in reductions of purple loosestrife abundance of approximately 80%, but it may take 5 to 10 years after they become established before the insect populations will be large enough to have this impact (Malecki et al. 1993).

The History of Importations, Screening Procedures and Specificity Tests:

In the mid 1980's, biologists began to conduct a search for biological control agents of purple loosestrife and the Galerucella species were thought to have excellent potential. Before they were approved for release in North America, feeding trials were conducted from 1987-1989 in Europe and from 1989-1991 in North America. The beetles were exposed to approximately 50 species of plants from four general classes: i) plants related to purple loosestrife; ii) plants found in a similar habitat as purple loosestrife; iii) plants of commercial importance; iv) plants that are associated ecologically with important wildlife species. The beetles were tested in several experiments: i) no-choice, starvation trials with single plant species; and ii) choice experiments, in which the beetles were exposed to combinations of all plant species to be tested. They were scored for; i) adult or larval feeding; ii) oviposition on the plant species; iii) the ability to complete development from egg to adult. The Galerucella species were unable to complete development on any species except purple loosestrife and Lythrum alatum. Some feeding was observed on Decodon verticillatus in no-choice situations, but the beetles did not prefer to feed on this species, or L. alatum, if allowed to choose between them and purple loosestrife. The researchers concluded that damage to L. alatum or D. verticillatus by Galerucella would be negligible and that these plant species are much more threatened by habitat displacement by purple loosestrife itself. After evaluating the test results, the two Galerucella species were approved for release by both the Canadian and American governments.

Proposed Release and Monitoring Protocol:

At each release site, 200-500 mated pairs of one or both of the Galerucella species will be released into sleeve cages (100 pairs per cage) covering a clump of purple loosestrife. The cages will be removed after 24 hours, allowing the beetles to disperse. Site characteristics (habitat type, soil type, size of infestation, water levels) will be recorded at the time of release. Follow-up studies will be conducted later in the release season, and in following years, to determine if the beetles survive and to evaluate their impact on the plant population.

References:

DeClerck-Floate, 1992. The desirability of using biocontrol against purple loosestrife in Canada. Agriculture Canada.

Malecki et al., 1993. Biological control of purple loosestrife. BioScience. Volume 43.

Powell, 1982. Each according to his weed. Harrowsmith. Volume 44.

Thompson et al., 1987. Spread, impact and control of purple loosestrife in North American wetlands. US Fish & Wildlife.

Collaborators:

Ontario Ministry of Transport Ducks Unlimited

Agriculture Canada Royal Botanical Gardens

Ontario Pesticides Advisory Committee

Pest Management Alternatives Office - Green Plan

National Science and Engineering Research Council