Effect of Red Imported Fire Ant (Hymenoptera: Formicidae) on Rhinocyllus conicus (Coleoptera: Curculionidae), a Biological Control Agent of Musk Thistle

The effect of the red imported fire ant, Solenopsis invicta Buren, on Rhinocyllus conicus Froelich in a musk thistle, Carduus nutans L., biological control program was evaluated in laboratory and field trials in pastures in middle Georgia in 1999 and 2000. In the laboratory, 30 thistle heads containing eggs and larvae were kept as controls and 30 heads were exposed to fire ant colonies. Foraging by fire ants for 72 h did not reduce the number of R. conicus eggs or larvae on musk thistle heads. Foraging by fire ant workers on musk thistle in pasture plots was nearly eliminated following treatment with Amdro bait (hydramethylnon). However, there were no noticeable differences in R. conicus egg, larva, or adult counts for thistle heads in treated and untreated plots. Results suggest R. conicus is largely unaffected by red imported fire ant activity on musk thistle.