Mutualisms, interactions between species whereby both benefit, can alter ecosystems by affecting community structure and species’ distributions. Aphid tending behavior by ants, a common mutualism, directly affects aphid population growth and distribution. Aphids consume plant phloem and excrete honeydew that is eaten by ants. In return, ants provide aphids with protection from predators, parasitoids and fungal attack. Foeniculum vulgare is an invasive perennial herb in coastal scrub habitats of California that hosts a mutualism between invasive Argentine ants (Linepithema humile) and three aphid species (Cavariella aegipodii, Dysaphis apiifolia, Dysaphis foeniculus). These associations can modify insect assemblage and affect host plant distribution by impacting herbivory rates. Host plants may experience a net reduction in herbivory caused by release from insects like the native lepidopteran, Papilio zelicaon, whose larvae rely on F. vulgare. This may facilitate the invasive success of F. vulgare. It is important to understand ecological mechanisms that affect plant invasions because species like F. vulgare can have drastic consequences for native plant communities. I used field data from two sites in East Bay Regional Parks to address the questions: : 1) How common is tending behavior? 2) Does tending affect insect assemblage? 3) Does tending affect host plant growth? 4) Does tending affect the distribution of P. zelicaon? I ran a greenhouse experiment aimed at determining the relative effects of aphid and P. zelicaon herbivory on F. vulgare.

Results/Conclusions

In the field, I tracked growth of F. vulgare ant-aphid hosts versus non-hosts and found no effect of aphid presence. I recorded the presence of other fauna and found on ant-aphid host plants 23.3% and 26.3% reductions in abundance and richness of non-mutualists, respectively. P. zelicaon larvae were distributed an average of 80.1% farther from ant-aphid hosts than plants with neither mutualists nor larvae. Greenhouse work showed that P. zelicaon reduced growth of F. vulgare while aphids did not. At the end of the experiment, plants in the P. zelicaon treatment were 65.8% shorter than plants in other treatments. I conclude that the mutualism can increase host fitness through the exclusion of other insects. P. zelicaon is particularly important as it demonstrates the potential to control F. vulgare via larval herbivory. There may be conservation implications for P. zelicaon given the increasing prevalence of L. humile, which clearly affects the distribution of the native. Ant-aphid mutualism may be an important factor in facilitating/maintaining the invasion of F. vulgare.