Does <i>Aphis asclepiadis</i> colony size mediate <i>Formica rufa</i> and <i>Tapinoma sessile</i> competition for mutualist aphids

Abstract

Mutualisms are the species interaction in which both mutualist partners benefit from the relationship. Most mutualisms involve multiple interacting species and associating with multiple mutualist partners can be beneficial if they provide complementary benefits. Yet, mutualist species may also compete with each other for access to partners, which can alter the benefits the partner receives. Ant-aphid mutualism is a food-for-protection in which aphids produce honeydew to feed ants while ants provide aphids protection against predators. Previous studies of the aphid Aphis asclepiadis and two species of ants, Formica rufa and Tapinoma sessile, show that both ant species provide mutualistic services to aphids. However, aphid colonies tended by F. rufa are five-fold larger than colonies that tended by T. sessile, and the mechanisms driving this pattern are unclear. In this study, we tested whether aphid colonies tended by F. rufa are larger than those tended by T. sessile because (1) F. rufa is a better mutualist or (2) F. rufa competitively excludes T. sessile from tending large aphid colonies. We experimentally established large and small colonies of the aphid Aphis asclepiadis on the host plant Ligusticum porteri in three experimental ant manipulation treatments: open to all ants, F. rufa excluded, and all ants excluded. We recorded the number of aphids on the plants and the number and species of ants tending the aphids every three days for three weeks (21 days). Our results did not provide support for either of the two hypotheses. We found that F. rufa was not significantly more likely to select large aphid colonies to tend, and T. sessile was not significantly more likely to tend aphid colonies when F. rufa was excluded. We also found that the benefit F. rufa provided for the aphid colonies were more variable across the aphid colony life cycle compared to the benefits provided by T. sessile, and the overall benefit the aphid colonies received when they tended by F. rufa and T. sessile were not significantly different. By studying these interactions, we are one step closer to understand mutualism in multi-species context.

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