Alarm‐recruitment behaviour in Pheidole militicida (Hymenoptera: Formicidae)

.1 Pheidole militicida Wheeler, a seed‐harvesting species of the southwestern United States, possesses a major worker caste (soldiers) with unusually large heads. Previous work suggested that these large major workers are specialized defenders against large seed‐harvesting species in the genus Pogonomyrmex.2 Experimental introductions of Pogonomyrmex maricopa Wheeler and the omnivorous fire ant, Solenopsis xyloni McCook, demonstrate that P. militicida colonies alarm‐recruit major workers against S.xyloni workers but not against the larger Pogonomyrmex workers. 3 P.militicida is the second species in which enemy‐specific defence against Solenopsis fire ants has been detected. I suggest that the special role of Solenopsis as both competitor and predator in ant communities frequently favours major worker specialization against this genus. 4 Major workers of P.militicida and Pheidole dentata Mayr, the other species with enemy‐specific defence against Solenopsis , are morphologically very different from one another. It is suggested that interspecific variation in major worker morphology is not necessarily associated with variation in behavioural specialization.