DOES HOST-PLANT QUALITY MEDIATE APHID-ANT MUTUALISM

We investigated the hypothesis that variation in the quality of the host plant (Epilobium angustifolium) mediated the mutualistic interaction between a phloem-feeding homopteran (Aphis varians) and the ant (Formica cinerea) tending it. First, we used a bioassay to define the quality of Epilobium for A. varians. We established cohorts of first-instar A. varians on fireweed shoots of different heights. Development time, size at maturity, and the number and size of embryos per adult were significantly correlated with plant height. Therefore, plant height provided an indirect measure of plant quality. Second, we experimentally tested the effects of plant height and tending by ants on the growth of aphid populations. We conducted this experiment in the absence of natural enemies of A. varians in order to examine the direct link between plants, aphids and ants, rather than the indirect links involving the natural enemies of aphids. As expected, population growth of A. varians on individual Epilobium ramets was positively correlated with plant height. However, in the absence of natural enemies, the effect of ants on the growth of aphid populations did not vary with plant height. The absence of an interaction between tending by ants and plant quality was probably due to two processes. First, although there were more ants tending on higher quality host plants, the absolute number of ants tending aphids was low and the increase in the number of ants with increases in plant quality was small. Second, in the absence of natural enemies and at initial densities of approximately 100 aphids per plant, we were unable to detect a significant effect of ants on the growth of aphid populations. We conclude that host-plant mediation did not occur in this aphid-ant interaction, but we do not know whether this was the result of 1) excluding predators, 2) using initial densities above which ants no longer benefit aphids, 3) the low numbers of ants, or 4) small sample sizes.