Tri-trophic level interactions affect host plant development and abundance of insect herbivores

Understanding the interactions among plants, hemipterans, and ants has provided numerous insights into a range of ecological and evolutionary processes. In these systems, however, studies concerning the isolated direct and indirect effects of aphid colonies on host plant and other herbivores remain rare at best. The aphid Uroleucon erigeronensis forms dense colonies on the apical shoots of the host plant Baccharis dracunculilfolia (Asteraceae). The honeydew produced by these aphids attracts several species of ants that might interfere with other herbivores. Four hypotheses were tested in this system: (1) ants tending aphids reduce the abundance of other herbivores; (2) the effects of ants and aphids upon herbivores differ between chewing and fluid-sucking herbivores; (3) aphids alone reduce the abundance of other herbivores; and (4), the aphid presence negatively affects B. dracunculifolia shoot growth. The hypotheses were evaluated with ant and aphid exclusion experiments, on isolated plant shoots, along six consecutive months. We adjusted linear mixed-effects models for longitudinal data (repeated measures), with nested spatial random effect. The results showed that: (1) herbivore abundance was lower on shoots with aphids than on shoots without aphids, and even lower on shoots with aphids and ants; (2) both chewing and fluid-sucking insects responded similarly to the treatment, and (3) aphid presence affected negatively B. dracunculifolia shoot growth. Thus, since aphids alone changed plant growth and the abundance of insect herbivores, we suggest that the ant–aphid association is important to the organization of the system B. dracunculifolia-herbivorous insects.