Operational implementation of weed biocontrol to reduce risks from herbicides
Project code: PRR03-070
Rob Bourchier - Agriculture and Agri-Food Canada
To re-establish an ecological balance between leafy spurge, an invasive alien weed species and its natural enemies and thereby reduce spurge populations to manageable levels
Summary of Results
Canadian grazing lands are threatened by invasive plants, usually introduced from abroad, that have been able to flourish due to a lack of native herbivores to limit their spread. One of the most serious of these invasive plants is Leafy Spurge (Euphorbia esula L.), introduced from Europe and now infesting an estimated 2 million hectares of rangeland and riverside habitats in North America. The plant contains a sticky white sap that is poisonous to cattle. It also out competes more desirable rangeland plants and is thus a threat both to the grazing value of the land and to biodiversity. The use of chemical herbicides to control leafy spurge can be cost prohibitive due to the difficult terrain and large areas to be treated. In addition, there are environmental concerns with the use of herbicides in many locations because of the close proximity of infested areas to water bodies.
Agriculture and Agri-food Canada (AAFC) is studying classical biological control of leafy spurge. This involves introducing natural insect enemies of the weed into Canada, from their native habitat in Europe. The overall goal is to re-establish an ecological balance between the weed and its natural enemies that will result in the reduction of weed populations to manageable levels. Biological control is an attractive pest management strategy because it is self-sustaining, economical for the large areas requiring treatment and has a minimal environmental impact.
The beetle (Aphthona lacertosa) was first released in Canada in 1991, after intensive host-range screening to ensure it was specific to the target weed, leafy spurge. The beetle larvae feed on the roots of the weed, while the adult beetles, when at high densities, consume the above ground portion of the plant before it produces seeds.
This 3 year project has drawn on previous work carried out by AAFC and the Alberta government, and involved critical collaboration with agricultural fieldmen from participating municipal district and individual landowners. The project focuses on the release, establishment and impact of the beetles on the weed in relation to site conditions.
It includes follow-up monitoring at both the 1997 provincial sites (7-9 years post release) and AAFC release sites established since 2001.
Intensive sweep and vacuum sampling were conducted from late June to the end of July for adult beetles. Sweep samples were counted and re-released in the field. Vacuum samples were returned to the laboratory and beetles were processed in the winter, for species identification, changes in sex-ratio and beetle fecundity and size. Vegetation sampling was conducted in the fall of each year. Changes in spurge density were assessed using presence/absence and spurge density estimates at 1 metre intervals, on 100-150 grid-points, at a release patch scale.
Impact of the leafy spurge beetles at sites with high densities of beetles was very clear. Between 2004 and 2006, there was an average reduction in mean stem density of 1.5 stems per sample point at sites with high beetle densities, whereas sites with low beetle densities experienced an increase of 0.4 stems per sample point. Percent spurge cover declined by 24% at high density beetle sites, whereas it increased by 3.3% at low beetle density sites.
Because of the rapid population growth of the spurge beetles at several Alberta release sites, all releases in 2006 and 2007 were conducted using Alberta beetles. The assumption is that these beetles would be better adapted to local conditions (such as the rapid warming associated with Chinook winds that can occur during winter days) than the beetles coming from Montana. Comparing beetle densities in the year following release for the 2005 releases, where beetles from Montana were used, with the 2006 releases, where Alberta beetles were used, provided an unplanned but useful post-hoc test of this assumption. Comparing emergence trap estimates of beetle density: there were slightly more beetles found at the Alberta releases than at the Montana releases (mean 2.7 beetles per emergence trap versus mean 1.7 beetles per emergence trap, respectively). These means were not statistically different, however the trend merits additional monitoring of these sites in subsequent years. Alberta beetle populations may grow at a faster rate because they are better adapted to the local climate.
Warm winter conditions related to Chinook winds in Southern Alberta were associated with lower population densities of leafy spurge beetles at release sites. However warm overwintering temperatures were not consistently associated with all sites where beetle impact was very clear. Additional manipulative experimentation is planned to understand the relative importance of extremely warm and cold periods during the winter for the spurge beetles.
Site information, including GPS coordinates and site attributes for all releases associated with this project have been entered into a release database. This database will enable appropriate timing of site monitoring to assess longer-term impacts of the biocontrol agents on their target weeds. In addition, all available paper records for historical biocontrol releases in Alberta have been entered into the database. This provides easy access to information on over 1200 releases targeting 19 weeds, with over 50 insect species, dating back to 1965.
Knowledge gained from this research will enable better selection of insect release sites ensuring improved survival and growth of populations of the beetle, and allowing further redistribution of the beetle on public and private lands. The black spurge beetle has the potential to provide an environmentally friendly, cost-effective alternative to conventional herbicides for the control of leafy spurge.
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