Indirect species interactions can profoundly effect community structure. When herbivores share a food source, they impact one another indirectly through resource competition and plant trait-mediated interactions. Using Russian knapweed Acroptilon repens (Asteraceae) as a study system, I explore plant-mediated interactions between two herbivores: a gall midge Jaapiella ivannikovi (Cecidomyiidae) and a gall wasp Aulacidea acroptilonica (Cynipidae). Russian knapweed is a noxious weed found throughout the western United States. Weed management provides an opportunity to manipulate community member presence at a large scale. Experiments conducted across spatial scales, from individual herbivores co-occuring on a single host plant to observations of populations across Colorado, allow us to determine how findings from greenhouse experiments can help us understand competitive interactions in the field.
In the greenhouse, we exposed plants to midges alone, wasps alone, or both insects. The resulting insect offspring and plant traits were measured to determine how these two herbivores interact with one another. Additionally, we visited sites across Wyoming and Colorado to observe presence of these two insect species in knapweed populations. At the three sites where both species co-occur, we took detailed transect observations. We asked if interaction outcomes in the greenhouse matched population dynamics in the field.
Results/Conclusions
Greenhouse experiments indicate interspecific facilitation occurring between midges, J. ivannikovi, and wasps, A. acroptilonica, sharing a host plant. When midges are present on host plants, wasps produce more offspring. Wasps appear to have minimal impacts on midges indicating that this facilitation is asymmetric. However, when midge and wasp galls occur within four centimeters of one another, they exhibit more typical competition outcomes with negative effects on resulting offspring counts of both species. Facilitation and competition appear to play important roles in these species interactions.
Across Colorado, wasps are more likely to establish, when released as biological control agents, where midges already occur. Field observations suggest that the two insect species regularly co-occur on the same plants when sharing a knapweed population more often than expected by chance. However, insects do not tend to aggregate at high levels, as individuals from both species spread out across a knapweed host population. We suspect that both facilitation and competition are acting in tandem to lead to both greenhouse and field observations. Plant trait-mediated impacts may explain interaction outcomes and co-occurrence patterns.