Conditional mutualism: Density- and quality-dependent responses of aphids to tending by ant

Experimental objectives were to determine if the effect of ant tending on aphid population growth varies with aphid density and host plant quality, and to consider possible mechanisms explaining the conditionality of the association. Four experiments involving aphids (Aphis varians) and ants (Formica cinerea) on fireweed (Epilobium angustifolium) were conducted.;Ant tending significantly improved the growth of small aphid populations. However, the benefit from ant tending decreased as aphid density increased, as there was no difference in growth between tended and untended populations at high aphid densities.;Possible mechanisms of this density-dependence were considered. The following results were observed: (1) the relative number of ants tending declined as aphid density increased, (2) some evidence suggested that ant tending inhibited alate production at high aphid densities, (3) the effect of ant tending on predator abundance did not vary with aphid density, and (4) ant tending reduced predator abundance in only one experiment.;Aphid population growth was significantly greater on higher quality (i.e. taller) fireweed shoots. The effect of ant tending on population growth did not vary with plant quality. Therefore, the ant-aphid association is not dependent on plant quality.;Density-dependence of the ant-aphid association is important in generating stability in the system. A lack of benefit from ant tending at high aphid densities may slow population increase and prevent aphids from over-exploiting their host plant. (Abstract shortened by UMI.).