Pheromonal control over worker execution of sexual larvae in fire ants (Solenopsis invicta)

I studied the phenomenon of worker execution of sexual larvae in the red imported fire ant, Solenopsis invicta. Both fertilized and unfertilized queens as well as queen corpses were equally effective in inducing executions by 96 h. Colonies receiving acetone (control) or queen extracts in acetone did not differ in execution rates. In sharp contrast, however, colonies receiving an extract of queens in a buffered solution killed significantly more sexual larvae than those colonies receiving only the solution (control). The extract was as effective as live queens, and, as in all tests, were introduced once every 12 h at concentrations of single-queen equivalents. In addition to having full activity, the extract was stable, as the percentage of larvae killed did not differ between jfresh (Id) and old (21 d) extracts maintained in clear vials at room temperature. The poison sac was identified as the likely storage site of the pheromone. This sac also houses the venom alkaloids and queen retinue pheromone, but these chemicals are readily extracted with strong organic solvents, unlike the 'execution' pheromone. There was no correlation between ratios of the two most abundant alkaloids and queen age, but ratios of saturated to unsaturated alkaloids were significantly greater in wingless queens than in winged queens. The queen venoms are less derived than those of workers perhaps because the execution pheromone has constrained their evolution.