The importance of competition among insect herbivores has been controversial, with some studies finding negative effects, others facilitation, and others asymmetric or delayed effects. Because plants may respond to herbivory with a variety of physiological changes, plant-mediated herbivore interactions may vary over time. Therefore, observed interaction may be a function of the time scale of the observations. To evaluate how herbivore interactions shifted across temporal scales, we studied a leaf beetle (Paria aterrima), a grasshopper (Hesperotettix floridensis), and an aphid (Uroleucon ambrosiae) that all feed on a high marsh plant (Iva fructescens). To measure concurrent interactions, two doses (either the same species or two different species to measure intra- or inter-specific interactions) of insects were put into a mesh cage with one potted plant for 7 days. To measure delayed interactions, one dose of insect was put in the cage for 7 days, then removed and replaced with another dose. To study longer-term patterns in the field, we visually sampled the insect populations on 38 patches of Iva every 3 days over two months, and constructed a structural equation model (SEM) to evaluate interactions among these three herbivores while also accounting for effects of environmental variables.
In the concurrent experiment, Hesperotettix had a negative effect on itself and on Paria. In the delayed experiment, Uroleucon had a positive effect on Hesperotettix, and Paria had a positive effect on itself. An SEM based on the sampling data found some results that were novel and some that were consistent with either the concurrent or the delayed effects experiments. The SEM indicated a negative effect of Uroleucon on Paria (novel), a negative effect of Hesperotettix on Paria (as in the concurrent experiment), and a positive effect of Uroleucon on Hesperotettix (as in the delayed experiment). In our experiments, interactions among herbivores shifted from negative to positive over time, presumably due to changes that herbivores induced in the shared host plants. If this result is general, it suggests that the study approach used may strongly determine our understanding of interactions among insect herbivores. In particular, studies focused on concurrent interactions, which are most typical in the literature, could be systematically biased. We encourage the use of multiple approaches to gain a broader understanding of how interactions among insects vary over time as their shared host plants respond to herbivory in multiple and complex ways.