Mutualistic Interactions of the Aphid Lachnus allegheniensis (Homoptera: Aphididae) and Its Tending Ant Formica obscuripes (Hymenoptera: Formicidae)

Abstract

A mutualistic relationship is described between the aphid Lachnus allegheniensis McCook, which occurs on Quercus gambellii Nutt, and the ant Formica obscuripes Forel. Evidence for the mutualism includes fouling of the aphid feeding aggregation with honeydew in the absence of the ants, the presence of the aphid only on Q. gambellii clones with foraging F. obscuripes present, a decline in aphid numbers at the periphery of the tending ant colony's foraging territory, the defensive behavior of the ant toward intruders, and the construction of a gallery by F. obscuripes in the ground at the base of the plant where the aphid retreats when disturbed or during the heat of the day in midsummer. Adaptations of L. allegheniensis that enhance ant attendance are numerous. The phenology of aphid development coincides strongly with the season-long activity of F. obscuripes , providing to the ant a dependable honeydew supply. The aphid has reduced cornicles, produces a concentrated honeydew, has very limited alate production, feeds in dense aggregations when resources are suitable, and migrates daily to protected galleries constructed by F. obscuripes. Q. gambellii indirectly benefits from ant tending of L. allegheniensis by significant reductions in amount and frequency of damage to leaves by defoliators, thus potentially increasing its long-term suitability as a host plant for the aphid. These considerations lead to the conclusion that persistence of L. allegheniensis populations depends upon the survival of the tending ant colony.