Complex interplay between facilitation and competition in two invasive fruit pests

When one invasive species facilitates the invasion of another species, it can lead to invasional meltdown. However, when the two species depend on the same finite resource, facilitation and competition can occur simultaneously and impact population dynamics. Furthermore, factors like resource availability and population density could alter the balance between facilitation and competition. Drosophila suzukii and Zaprionus indianus are two invasive insect pests that infest small fruit crops. The co-localization of these species in the same fields, coupled with the fact that Z. indianus lacks a hardened ovipositor that can penetrate the skin of fruit, has led to the hypothesis that D. suzukii facilitates the invasion of Z. indianus in most crops. Here, we test the hypothesis that facilitation and competition can occur simultaneously by investigating oviposition behavior, egg-to-adult viability, and fitness metrics in three crops. Overall, we found that the nature of these interactions depended on the type of fruit. In blueberry, D. suzukii facilitated egg laying and development of Z. indianus, while D. suzukii had reduced body size (a proxy for fitness) in response to interspecific competition from Z. indianus. However, in blackberries and raspberries, we found no evidence of facilitation, and competition was primarily density dependent regardless of the species composition. Thus, in these species, the nature of interspecific interactions varies across host plants. These results have important implications for managing these invasive pests and further our understanding of how interactions between invasive species may change depending on resource composition.