1. The Argentine ant, Linepithema humile , tends honeydew-excreting homopterans and can disrupt the activity of their natural enemies. This mutualism is often cited for increases in homopteran densities; however, the ant's impact on natural enemies may be only one of several effects of ant tending that alters insect densities. To test for the variable impacts of ants, mealybug and natural enemy densities were monitored on ant-tended and ant-excluded vines in two California vineyard regions.2. Ant tending increased densities of the obscure mealybug, Pseudococcus viburni , and lowered densities of its encyrtid parasitoids Pseudaphycus flavidulus and Leptomastix epona . Differences in parasitoid recovery rates suggest that P. flavidulus was better able to forage on ant-tended vines than L. epona .3. Densities of a coccinellid predator, Cryptolaemus montrouzieri , were higher on ant-tended vines, where there were more mealybugs. Together with behavioural observations, the results showed that this predator can forage in patches of ant-tended mealybugs, and that it effectively mimics mealybugs to avoid disturbance by ants.4. Ant tending increased densities of the grape mealybug, Pseudococcus maritimus , by increasing the number of surviving first-instar mealybugs. Parasitoids were nearly absent from the vineyard infested with P. maritimus . Therefore, ants improved either mealybug habitat or fitness.5. There was no difference in mealybug distribution or seasonal development patterns on ant-tended and ant-excluded vines, indicating that ants did not move mealybugs to better feeding locations or create a spatial refuge from natural enemies.6. Results showed that while Argentine ants were clearly associated with increased mealybug densities, it is not a simple matter of disrupting natural enemies. Instead, ant tending includes benefits independent of the effect on natural enemies. Moreover, the effects on different natural enemy species varied, as some species thrive in the presence of ants.

It has long been assumed that the North American pipevine swallowtail, Battus philenor (L.) (Papilionidae, Troidini), is protected from natural enemies by aristolochic acids sequestered from its Aristolochia food plants. This study confirmed that populations of B. philenor from Virginia and east Texas sequester these compounds. A comparison of the aristolochic acid profiles of the Virginia butterflies and their A. macrophylla food plants revealed several differences. The aristolochic acid fraction of the foliage was dominated by aristolochic acids I and II, whereas the insects had a much lower proportion of aristolochic acid II and contained, in addition, substantial amounts of aristolochic acids Ia and IVa, which were not detected in the plants. The eggs, larval integument, osmeterial glands, pupal cuticle, and adults (wings and bodies) all contained aristolochic acids. These findings help explain the abundant ecological data indicating that both immature and adult B. philenor are unpalatable and protected from natural enemies.

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