Spatial utilization patterns by the weed biocontrol weevil Mogulones crucifer differ between target and non-target plants

Background/Question/Methods

Biological control can effectively suppress invasive plants, but includes risk of non-target impact. Insects known to utilize (feed, oviposit or develop on) non-target relatives of their target weed in a limited way have been approved for release, and current concerns over non-target impact threatens the future of weed biocontrol. However, damage to individual plants may not translate to population-level effects. Within-patch, spatially uniform utilization is likely to have more impact than spatially aggregated damage, because aggregated utilization generates refuge plants that can maintain population viability. We performed two experiments to assess target and non-target spatial utilization patterns at biocontrol release sites in Western Canada.

Field experiment: Biocontrol weevils (Mogulones crucifer) were released on six rangeland sites in Alberta, Canada with natural populations of a native non-target plant, Hackelia micrantha growing with abundant or rare numbers of the confamilial target weed, Cynoglossum officinale. Plants within 4-14 m were visually assessed after 4-7 weeks to detect spatial utilization patterns.

Mark-release-recapture (MRR) experiment: Weevils were released on patches of potted H. micrantha 2 m, 10 m or 100 m from H. micrantha or C. officinalecollection patches. Weevils recaptured on all patches were counted every two days for eight days after release. 

Results/Conclusions

Mogulones crucifer expressed distinctly different spatial utilization patterns on target and non-target plants. In the field experiment, C. officinale plants were utilized throughout the 14 m sample radius, whereas H. micrantha utilization was essentially restricted to within 4 m of release points, independently of whether target plants were common or rare. In the MRR experiment, dispersing weevils were over an order of magnitude more likely to be recaptured on C. officinale than on H. micrantha, and located plants closest to the release points with highest frequency. These data suggest that non-target compared to target utilization may be more dependent on 1) high weevil density, and 2) the random chance of encountering non-target plants, both of which decrease with distance.

Our data indicate that M. crucifer target utilization is significantly more spatially uniform than non-target utilization. These patterns suggest a potential mechanism for the biocontrol insect’s demonstrated population-level control of C. officinale, and could explain negligible or absent population-level impacts on non-targets. These results will be discussed with respect to risks to non-target species in biocontrol systems.