An introduced parasitoid facilitates host range expansion of a resident parasitoid

Invasive species can cause maladaptive behaviours in native consumers, with evolution of behavioural or physiological traits thought to be the main way out of these so-called evolutionary traps. However, shorter-term ecological processes resulting from biological invasions could provide other ways to escape evolutionary traps. Asobara cf. rufescens, a resident (i.e., possibly native or previously introduced) parasitoid of drosophilid fly larvae in North America, can rarely produce offspring on the invasive fruit pest Drosophila suzukii in the laboratory, but in often emerges from field-collected D. suzukii. We hypothesized that the successful development of A. cf. rufescens in D. suzukii in the field is facilitated by Leptopilina japonica, a recently introduced parasitoid of D. suzukii. In laboratory experiments, we found that A. cf. rufescens had more than 30-fold higher offspring emergence from D. suzukii when L. japonica was also present. This facilitation occurred most frequently when A. cf. rufescens parasitized D. suzukii after L. japonica, possibly because parasitism by L. japonica destroyed the hosts cellular immunity that would otherwise prevent A. cf. rufescens development. The recent arrival of L. japonica may have partially rescued A. cf. rufescens from the evolutionary trap set by D. suzukii and, consequently, resulted in an expansion of its host range.