Avermectin B1a: Highly Potent Inhibitor of Reproduction by Queens of the Red Imported Fire Ant (Hymenoptera: Formicidae)1

Abstract

Avermectin B₁a, a natural product derived from the soil actinomycete Streptomyces avermitilis, inhibited reproduction of queens of Solenopsis invicta Buren when fed to laboratory colonies at concentrations as low as 0.0025% in the soybean oil bait (37.5 µg. of AI available). Some worker mortality occurred at concentrations of 0.025% or greater. Good results were obtained in field tests also. Worker brood was found in only 8 of 928 colonies that fed on the baits which were applied at rates of AI ranging from 0.0077 to 7.41 g/ha. Death of the treated colonies, especially at the lower application rates, was very slow, since the primary effect of avermectin B₁a was on the reproductive capacity of the queen rather than acute toxicity for workers. Therefore, overall effects of avermectin B₁a baits were determined with a population index method that gave a greater numerical value to colonies with functioning queens, as evidenced by the presence of worker brood. With this method, the average population indices in the field plots treated with 0.0077 g or more per ha were reduced 81 to 93%.