Spread of Larinus minutus (Coleoptera: Curculionidae), a biological control agent of knapweeds, following introduction to northwestern Arkansas
Alford, Adam M.
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Spotted knapweed, Centaurea stoebe L. (Asteraceae), is an invasive perennial forb that has become economically and ecologically damaging in North America. The weevil Larinus minutus Gyllenhal (Coleoptera: Curculionidae), a biological control agent of invasive knapweeds, was introduced to 37 sites in northwest Arkansas since 2008 as part of a biological control program for spotted knapweed. In 2011 and 2012, 25 of these release sites were surveyed to monitor how L. minutus infestation rates changed in relation to distance and time from release. The initial L. minutus introductions at these sites occurred from 2008 to 2011. Transects were used from the point of initial weevil introduction to establish sampling quadrats in which capitula were collected to document weevil presence and infestation rates. The mean maximum distance of weevil colonization and mean local abundance (within the first 50 m from the release point) were calculated in relation to time (yr) since release. Five sites had > 10 quadrats in both sampling years and were analyzed with an exponential decay function to model localized population growth and spread. Annual increases in mean local abundance and maximum distance of colonization were observed. These results were substantiated by localized growth (at 3 sites) and spread (at 2 sites) that occurred at the 5 sites analyzed with the exponential decay function. These findings suggest that in years following L. minutus introduction, assuming similar release strategies and environmental conditions, consistent increases in weevil infestation rates and spread from the release site may be expected in about half the sites at which populations establish.