Adventive larval parasitoids reconstruct their close association with spotted-wing drosophila in the invaded North American range

Abstract

Two species of larval parasitoids of the globally invasive fruit pest, Drosophila suzukii (Diptera: Drosophilidae), Leptopilina japonica and Ganaspis brasiliensis (both Hymenoptera: Figitidae), were detected in British Columbia, Canada in 2019. Both are presumed to have been unintentionally introduced from Asia, however; the extent of their establishment across different habitats with diverse host plants used by D. suzukii was unclear. In addition, there was no knowledge of the temporal dynamics of parasitism of D. suzukii by these two parasitoids. We repeatedly sampled the fruits of known host plants of D. suzukii over the entire 2020 growing season in British Columbia. We documented the presence of L. japonica and G. brasiliensis and estimated the apparent percentage of D. suzukii parasitized. Across a large region of southwestern British Columbia, both L. japonica and G. brasiliensis were found to be very common across a variety of mostly unmanaged habitats over the entire course of the season (May-October) in the fruits of most host plants known to host D. suzukii larvae. The two parasitoids were responsible for more than 98% of D. suzukii larval parasitism and usually co-existed. Parasitism of D. suzukii was variable among hosts plants and sites (0-66% percent parasitism) and appeared to be time-structured. Our study demonstrates that the close association between the two larval parasitoids and D. suzukii that exists in Asia has evidently been reconstructed in North America, resulting in the highest parasitism levels of D. suzukii yet recorded outside of its area of origin.

First records of adventive populations of the parasitoids Ganaspis brasiliensis and Leptopilina japonica in the United States

“…There are indications that the two species co-occur regularly, as was reported in coastal British Columbia (Abram et al 2020(Abram et al , 2022a. No species of Asobara Förster (Hymenoptera: Braconidae), which are common parasitoids of Drosophilidae, were found in Washington.…”

“…All finds of G. brasiliensis and L. japonica in North America to date have been in moist, mild-winter coastal regions of the Pacific Northwest (Abram et al 2022a). However, the primary tree fruit growing regions in Washington and British Columbia, as well as a growing blueberry industry in Washington, are in the semiarid regions in the rain shadow of the Cascade mountains.…”

“…However, the two Figitidae species were the targets of this limited survey, and more extensive surveys may reveal the presence of Asobara species; for example, Asobara cf. rufescens (Förster), was found associated with D. suzukii in nearby British Columbia (Abram et al 2020(Abram et al , 2022a.…”

“…The small sample size leaves open the possibility that additional haplotypes could be present at other times of year on other host plants or locations. To test this possibility, we sequenced the COI region (using the same methods as described above) of an additional 36 G. brasiliensis specimens from collections made in 2020, details of which are reported in Abram et al (2022a). These were from three different host plants and six different sites representing a variety of habitats from June to October (Table 1).…”

“…These adventive populations of larval parasitoids of D. suzukii on both continents are probably the result of unintentional introductions. Adventive populations of G. brasiliensis and L. japonica in British Columbia appear to be well established, co-existing and parasitizing D. suzukii larvae in diverse habitats on a wide variety of different host plants over the course of the growing season (Abram et al 2022a). However, exotic parasitoids collected in Canada cannot be intentionally released into the US.…”