Foraging on insect mutualists by the thatching ant Formica obscuripes: Interspecific and intraspecific mechanisms and implications

Experimental exclusion of ants confirmed that a mutualism existed between the thatching ant Formica obscuripes and the cynipid gall wasp Disholcaspis perniciosa. The success of all but specialized parasites, Eudecatoma spp that attack before the gall is available for ant attendance, decreased in the presence of ants. Gall wasp survivorship and fecundity schedules fell and the incidence of parasitism increased over a seven year period, including five years of data from galls that were retained on the host. Local persistence of the gall wasp despite increasing parasitism appeared to depend on escape in time due to polymorphic emergence resulting from diapausing prepupae.Formica obscuripes and the aphid Lachnus allegheniensis were also shown to be involved in a strong mutualistic relationship. The phenology of the single-host aphid coincided strongly with the season-long activity of the ant. When compared to control clones lacking aphids and ants, host plant resources lost to aphids were partially compensated for by significant reductions in damage to leaves and the frequency of leaf attack defoliators (presumably because of predation by ants).Chain transport behavior was confirmed for F. obscuripes, and larger workers yielded the task of honeydew collection to smaller workers when the latter were present. Satellite mounds interrupted trails of some colonies, becoming more prevalent as the average size of a colony worker became smaller and as cumulative trail lengths increased. The satellite colonies were used primarily by smaller workers and may serve as recruitment centers, bivouacs to conserve small worker energy, or to reduce trail congestion. A quantitative mode confirmed that small workers are more efficient at collecting honeydew, and large workers are more efficient at transporting honeydew. This provides an explanation for the evolution of chain transport behavior based on colony level selection to improve foraging efficiency.Results also supported the hypothesis that increasing dependence on honeydew would bias caste structure of an ant colony towards smaller workers. Finally, a scenario for colony growth, caste structure, and changes in social structure in rich and marginal habitats summarizes the effects that site-specific resource conditions can have on ant colonies.U of I OnlyETDs are only available to UIUC Users without author permissio