Priority resource access mediates competitive intensity between an invasive weevil and native floral herbivores

Authors: LOUDA, S. - University Of Nebraska; Rand, Tatyana; KULA, A.A. - University Of Maryland; ARNETT, A. - University Of Nebraska; WEST, N. - University Of Nebraska; TENHUMBERG, B. - University Of Nebraska

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/27/2011
Publication Date: 10/1/2011
Citation: Louda, S.M., Rand, T.A., Kula, A.A.R., Arnett, A.E., West, N.M., Tenhumberg, B. 2011. Priority resource access mediates competitive intensity between an invasive weevil and native floral herbivores. Biological Invasions. 13(10): 2233–2248.

Interpretive Summary: Mechanisms underlying invasive species impacts remain incompletely known. We tested the hypothesis that priority resource access by an invasive biocontrol beetle, Rhinocyllus conicus, alters the outcome and intensity of competition with native flower-feeding insects, in particular a native fly, Paracantha culta, on a native host plant, Cirsium canescens. Three main results emerged. First, we found clear evidence of competition between R. conicus and the native fly. Competition was asymmetric, with R. conicus out-competing P. culta. Second, priority access to resources by R. conicus accelerated the competitive suppression of the native P. culta. Evidence for competitive suppression included decreases in both the numbers and total biomass of the native flies, plus decreases in individual fly mass and, so, potential fitness. Third, we found evidence for three concurrent mechanisms underlying the competitive effects. This study provides the first experimental evidence for negative, non-target competitive effects of a weed biological control agent on native insect herbivores. These experimental results expand on past studies by demonstrating unequivocally that not only native plants, but also their associated native herbivores, can be significantly impacted by a weed biological control insect. This research, thus, re-enforces recommendations that urge increased caution in the release of agents with any evidence that they can use native plants as secondary host plants.

Technical Abstract: Mechanisms underlying invasive species impacts remain incompletely known. We tested the hypothesis that priority resource access by an invasive biocontrol weevil, Rhinocyllus conicus, alters the outcome and intensity of competition with native floral herbivores over flower head resources of the non-target, native host plant Cirsium canescens, especially with the predominant, synchronous tephritid fly Paracantha culta. Three main results emerged. First, we found clear evidence of competition between R. conicus and the native fly. Competition was asymmetric, with R. conicus out-competing P. culta. Second, priority access to floral resources by R. conicus accelerated the competitive suppression of the native P. culta. Evidence for competitive suppression included decreases in both the numbers and total biomass of the native flies, plus decreases in individual fly mass and, so, potential fitness. Third, we found evidence for three concurrent mechanisms underlying the competitive effects. The invasive R. conicus interfered with P. culta pre-oviposition behavior. The weevil also reduced fly post-oviposition performance by both interference, shown by a shift of P. culta away from the preferred receptacle resources as R. conicus priority increased, and preemptive resource exploitation, suggested by the importance of flower head size in determining the number of insects and the magnitude of R. conicus effects on P. culta. Thus, priority resource access by an invasive insect increased competitive interaction strength and impact on native insects in an invaded floral guild. Further, this study provides the first experimental evidence for negative, non-target competitive effects of a weed biological control agent on native insect herbivores.