The organization of colony defense in the ant Pheidole dentata mayr (Hymenoptera: Formicidae)
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Colonies of Pheidole dentata employ a complex strategy of colony defense against invading fire ants. Their responses can be conveniently divided into the following three phases: (1) at low stimulation, the minor workers recruit nestmates over considerable distances, after which the recruited major workers (“soldiers”) take over the main role of destroying the intruders; (2) when the fire ants invade in larger numbers, fewer trails are laid, and the Pheidole fight closer to the nest along a shorter perimeter; (3) when the invasion becomes still more intense, the Pheidole abscond with their brood and scatter outward in all directions (Figs. 1, 4).
Recruitment is achieved by a trail pheromone emitted from the poison gland of the sting. Majors can distinguish trail-laying minors that have just contacted fire ants, apparently by transfer of the body odor, and they respond by following the trails with more looping, aggressive runs than is the case in recruitment to sugar water. Majors are superior in fighting to the minors and remain on the battleground longer.
The first phase of defense, involving alarm-recruitment, is evoked most strongly by fire ants and other members of the genus Solenopsis; the presence of a single fire ant worker is often sufficient to produce a massive, prolonged response (Figs. 2, 5, 6). In tests with Solenopsis geminata, it was found that the Pheidole react both to the odor of the body surface and to the venom, provided either of these chemical cues are combined with movement. Fire ants, especially S. geminata, are among the major natural enemies of the Pheidole, and it is of advantage for the Pheidole colonies to strike hard and decisively when the first fire ant scouts are detected. Other ants of a wide array of species tested were mostly neutral or required a large number of workers to induce the response. The alarm-recruitment response is not used when foragers are disturbed by human hands or inanimate objects. When such intrusion results in a direct mechanical disturbance of the nest, simulating the attack of a vertebrate, both minor and major workers swarm out and attack without intervening recruitment.
KeywordsNatural Enemy Major Worker Inanimate Object Trail Pheromone Mechanical Disturbance
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