Host–parasitoid interactions on urban roofs: an experimental evaluation to determine plant patch colonisation and resource exploitation

In cities, green roofs may favour an increase in the local diversity, although the role they play on host–parasitoid interactions is still unclear. Here, we tested the hypothesis that the specialist leaf‐miner Liriomyza commelinae and its parasitoid assemblage are able to colonise and exploit patches located on roofs, but with a lower efficiency than at ground level. We also predicted that parasitoid species may differ in their abilities to colonise patches on roofs due to traits related to their life strategy (idio/koinobionts) and morphology (body size). Through experimental exposure of non‐mined and mined host plants located at ground level and on the bare roofs of houses, we determined colonisation levels for the leaf‐miner and its parasitoids respectively. Changes in leaf‐miner abundance, parasitism rates and parasitoid species richness were also investigated. L. commelinae was able to locate host plants on rooftops, but revealed a lower rate of colonisation and abundance than on the ground. A few species of the parasitoid assemblage were associated with the leaf‐miner in plants on roofs, resulting in decreasing colonisation rates, less species richness and lower parasitism. Interestingly, parasitoid species categorised as ‘small’ did not colonise plants on roofs. Our results provide evidence on how habitats at height influence host–parasitoid interactions in cities, representing one of the first approaches to the biological role that green roofs might have on them.