The Effect of Shade and Competition on Emigration Rate in the Ant Aphaenogaster Rudis

Abstract

Aphaenogaster rudis is a common ant in eastern United States forests that exhibits a high rate of colony relocation to new nest sites, even in the absence of nest disturbances. Field and laboratory experiments were conducted to determine if competition or reduced nest isolation, which acts to reduce internal nest temperature, could act as cues triggering colony emigrations. The field experiments tested the effect of increased food supply and artificial shade on the emigration rate. Laboratory experiments tested the effect of increased aggressive encounters with another ant species on emigration rate. Reduction in nest isolation resulted in a dramatic increase in the probability of emigration. Competition resulted in nest relocations only when it involved the invasion of the A. rudis nest site by another ant species. Changes in the amount of available food had no demonstrable effect on the probability of relocation although such changes did affect the intensity and direction of foraging activity. Seasonal trends in emigratory behavior were also observed. Emigration rate in A. rudis reached a maximum in midsummer. Also, the effect of artificially shading nests on emigration rate was the greatest in early summer and significantly less in spring and late summer.