Baccharis halimifolia is host to many species of insect herbivore, including myrmecophilous aphids. Previous studies with B. halimifolia have revealed strong effects of competition by an early-feeding beetle, Trirhabda bacharidis, and nitrogen enrichment. The effects of ant mutualists, however, have not been evaluated for their potential influence on interspecific interactions among insects on the host plant. I have employed a series of experiments aimed at answering the following general questions. (1) How does the density of aphid-tending ants affect common insects on the host plant? (2) What are the relative effects of (a) competition from T. bacharidis, (b) nutrient enrichment, and (c) ant presence on common insects of the host plant? (3) How do the effects of exotic ants differ from those of native ants? The density of aphid-tending ants had positive effects on myrmecophilous aphids and aphid predators. However, given a choice between tended and untended aphid populations, aphid predators preferred to forage in the absence of ants. The density of aphid-tending ants also increased predation on leaf miners although this did not necessarily translate into reduced densities of leaf miners. Competition by early-season feeding of T. bacharidis negatively affected later-feeding herbivores but the effects of competition were unaffected by nutrient enrichment. Nutrient enrichment had positive effects on some herbivores, often only in the absence of early season herbivory. Trirhabda bacharidis larvae showed evidence of nitrogen and phosphorus limitation and suffered no predation by aphid-tending ants. Ant presence increased host plant survivorship from stemborer damage in 2004. Ant species identity was an important factor determining the densities of myrmecophiles and non-myrmecophiles on the host plant in addition to affecting the responses of herbivores to increases in host plant quality. Aphids were more abundant in the presence of the exotic Linepithema humile (Hymenoptera: Dolichoderinae) versus the native Camponotus floridanus (Hymenoptera: Formicinae). Aphid predators also had higher densities in the presence of L. humile versus C. floridanus. Only L. humile acted as predator on leaf mines although predation did not always translate into reduced seasonal abundances for both species of leaf miner.

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