Patterns of venom synthesis and use in the fire ant, Solenopsis invicta

In the life of the fire ant, Solenopsis invicta, venom plays several important roles, including prey capture, defense, and anti-microbial action. Although this venom's chemistry, pharmacology, and effects on humans have been extensively studied, its patterns of synthesis and use have not. We determined the ability of different-aged workers to synthesize venom, and measured the amount of venom workers delivered per sting. Newly eclosed workers contained little or no venom in their reservoirs. The rate of synthesis was highest in workers aged 1 day after adult eclosion (1.17 μg venom/day for workers with headwidths of 1 mm), declined by 75% in workers aged 15 days (0.30 μg venom/day), and was negligible thereafter. Inducing ants to sting dummies as surrogates for humans, workers were found to deliver an average of 0.66 nl per sting, amounting to 3.1% of the average individual venom supply. Older workers (foragers) delivered less venom per sting than mid-age workers (reserves) and venom dose from nest-defenders was 55% higher in the spring than in the rest of the year. Thus, fire ant venom synthesis is limited to early life, and injected venom dose appears to be modulated. Economic explanations for the observed venom-use patterns are discussed.