The multitrophic consequences of concurrent insect invasions: a range-expanding herbivore and its associated parasitoid affect native tritrophic interactions

Authors: CARRASCO, DAVID - Swedish University Of Agricultural Sciences; DESURMONT, GAYLORD - European Biological Control Laboratory (EBCL); LAPLANCHE, DIANE - Neuchatel University - Switzerland; PROFFIT, MAGALI - Swedish University Of Agricultural Sciences; GOLS, RIETA - University Of Wageningen; BECHER, PAUL - Swedish University Of Agricultural Sciences; LARSSON, MATTIAS - Swedish University Of Agricultural Sciences; TURLINGS, TED - Neuchatel University - Switzerland; ANDERSON, PETER - Swedish University Of Agricultural Sciences

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2017
Publication Date: 7/21/2017
Citation: Carrasco, D., Desurmont, G., Laplanche, D., Proffit, M., Gols, R., Becher, P.G., Larsson, M.C., Turlings, T.C., Anderson, P. 2017. The multitrophic consequences of concurrent insect invasions: a range-expanding herbivore and its associated parasitoid affect native tritrophic interactions. Global Change Biology. DOI: 10.1111/gcb.13836.

Interpretive Summary: It is well established that invasive species of insects can have direct negative effects on native ecosystems. Global climatic changes may lead to the arrival of multiple range-expanding insect species into new environments (concurrent invasions). In this study we investigated the consequences of concurrent invasions for the trophic interactions between native species. Using an exotic caterpillar (Spodoptera littoralis) and its specialist natural enemy (the exotic parasitoid Microplitis rufiventris) as model species of invaders, we showed that the ecological consequences of the introductions of these invaders depend on whether the two species are introduced separately or concurrently. Separately, on the one hand, both species had a negative effect on the fitness of native insects, but the exotic parasitoid could not survive without the exotic caterpillar. Concurrently, on the other hand, the exotic parasitoid could persist and both invaders could impact the native insect species. This study illustrates the complexity of invasion scenarios and supports the idea that the overall impact of multiple invaders is greater than the addition of their individual impacts.

Technical Abstract: Global climatic changes may lead to the arrival of range-expanding species into new environments. Species from different trophic levels sharing the same climatic niche may invade new habitats simultaneously or in quick succession, causing the formation of multiple novel interactions into native food webs. Concurrent invasions have been hypothesized a critical factor for understanding community ecology under climate change, but experimental evidence is lacking. Here we explored the effect of the concurrent arrival of a range-expanding herbivore in Europe, Spodoptera littoralis, and its associated parasitoid Microplitis rufiventris, on the native herbivore Mamestra brassicae, and its associated parasitoid M. mediator, when co-occurring on a native plant, Brassica rapa. Overall, interactions between the herbivores resulted in a positive effect for the exotic herbivore (higher pupal weight) and a negative effect on the native herbivore (higher mortality). At the third trophic level, the presence of the exotic parasitoid negatively affected the native herbivore (increased mortality) and the native parasitoid (decreased parasitism rate). On the other hand, the exotic herbivore and its parasitoid were unaffected by the presence of the native parasitoid. Our results suggest that the arrival of S. littoralis and its parasitoid would result in new competitive interactions at the second and third trophic level, which would be detrimental for the investigated native species. Because the exotic parasitoid is unable to parasitize the native herbivore, the negative effects associated with its presence would only occur if the exotic herbivore is also present. Furthermore, the presence of the exotic parasitoid may attenuate the negative effect of S. littoralis on Ma. brassicae. These findings illustrate the complexity and interconnectedness of multitrophic changes resulting from concurrent species arrival to new environments, and the need for integrating the ecological effects of such arrivals in the general framework of global invasion patterns driven by climatic changes.