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

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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%.