We conducted a series of bioassays to study how queens control worker execution of sexual larvae in colonies of the red imported fire ant, Solenopsis invicta. In all experiments, subset colonies were made from larger colonies, and each unit included 20 sexual larvae that were monitored over the course of 48 to 96 h. The larvae mostly survived in queenless colonies, but were mostly killed in queenright colonies, regardless of whether or not the queen was fertilized. The larvae were also killed when corpses of queens were added to queenless colonies. Whereas queen extracts in acetone did not induce killings significantly, such extracts in a buffered solution led workers to execute most sexual larvae, indicating successful extraction of an execution pheromone. We identified the probable storage location of the chemical as the poison sac, and found both fresh (1 d) and old (21 d) extract kept at room temperature to be equally effective in inducing executions. Extracts of both winged queens (which do not induce killings) and wingless queens were analyzed by polyacrylamide gel electrophoresis. The latter contained two bands not present in the former: a 14 kDa protein and a 1-3 kDa peptide. Either or both of these bands may be related to the execution pheromone. The 14 kDa protein is in the size range of insect pheromone-binding proteins and lipophilic molecule-binding proteins found in secretory glands of other organisms. The 1-3 kDa band is in the size range of peptide pheromones known in various organisms, including insects.
Species 1: Hymenoptera Formicidae Solenopsis invicta (red imported fire ant)
Keywords: poison sac, protein
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