Post-release genetic assessment of two congeneric weed biological control agents


Keever, C.C., Gültekin, L., Bourchier, R.S., Myers, J.H., Cory, J.S. (2021). Post-release genetic assessment of two congeneric weed biological control agents. Biological Control, [online] 152

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The paper reports on a field study to compare the distribution of two closely related biocontrol agents for knapweeds in British Columbia. Both insect species were release for control of spotted and diffuse knapweeds in the early 1990’s. The research questions were 1) have they remained separate species or have the they interbred to formed hybrid species and 2) are the equally distributed on the two target knapweeds or have the specialized based on habitat types or plant species. The two insect species are very difficult to tell apart based on physical characters , thus we used molecular techniques to separate and identify field collected weevils from multiple locations. The results show that both species have persisted since their release for biological control in the early 1990’s . There is no evidence of hybridization or interbreeding. One species, Larinus minutus is established on both spotted and diffuse knapweed whereas the other, Larinus obtuses was only found on spotted knapweed for the sites sampled in this study. Larinus minutus has been shown to be a successful biocontrol agent for diffuse knapweed. The results will assist with monitoring the ongoing spread and impact of the biocontrol agents on both knapweeds and identifying habitat or geographical factors that affect agent impacts in British Columbia


Introductions of biological control agents are unique field experiments to examine the response of novel organisms to new environments. Post-release monitoring is particularly challenging where closely related and morphologically similar biological control agents have been released. Two closely-related phytophagous weevils, Larinus minutus and L. obtusus, were introduced as biological control agents for two major rangeland weeds, diffuse knapweed, Centaurea diffusa and spotted knapweed, C. stoebe subsp. micranthos in North America. The release of the Larinus species coincided with a decline in C. diffusa abundance in many areas. However, it was not clear whether both species of Larinus had become established as they are morphologically very similar. We asked: (1) Could genetic markers be used to identify both the weevil species? (2) Do both weevil species attack both knapweed species in Canada? (3) Are the levels of genetic diversity of the introduced populations of the weevils similar to those of a European source population? (4) Is genetic mixing between the weevil species occurring? Both microsatellite and CO1 mtDNA markers distinguish between the two weevil species. Larinus obtusus was only found on spotted knapweed, while L. minutus was more widely distributed and attacked both weed species. The relatively large initial introductions of weevils to British Columbia (4,300 L. minutus and 5,500 L. obtusus) together with rapid population growth and frequent redistribution among sites has been sufficient to maintain levels of genetic diversity. Results from small samples showed that haplotype, nucleotide diversity and allelic richness of the introduced populations were comparable to those of a European population. We found no evidence for hybridization in the introduced populations.

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