Rate of Dispersal of Spotted Knapweed Biocontrol Beetles ( Larinus spp. Curculionidae) in Wisconsin

Eurasian seed-head weevils, Larinus obtusus Gyllenhal and Larinus minutus Gyllenhal (Coleoptera: Curculionidae), were released annually from 2004-2013 across Wisconsin for biological control of the invasive forb spotted knapweed (Centaurea stoebe L. ssp. micranthos (Gugler) Hayek). In 2014, Larinus spp. presence/absence surveys were conducted at 18 previous releases sites between 7/18/2014 – 8/16/2014 from 2007 (n=3), 2008 (n=3), 2009 (n=3), 2010 (n=6), 2011 (n=1) and 2012 (n=2). We found a significant, linear relationship between time since release and the natural log of the observed dispersal rates of Larinus spp. (F_1,10 = 18.8, P = 0.002, R² = 0.65), suggesting an increasing dispersal rate through time. Modeled result suggested the following relationship: LN (Dispersal distance in km) = 0.258*time since release + 0.741. Because the model intercept (i.e. dispersal rate) did not pass through zero at age zero, which was expected from a biological basis, I warn against extrapolating the modeled dispersal rate to the period from release (t=0) to the end of Year One. Applying these dispersal rates to all 326 previous Larinus spp. releases in Wisconsin we predict that by 2017 low level populations of Larinus spp could be found state-wide.