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 2016 and 2019, respectively. 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. To address these gaps, 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 among host plant species. 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. Parasitism of D. suzukii was variable (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.

Aromatic border plants in early season berries do not increase parasitism of spotted wing drosophila, Drosophila suzukii

“…Indeed, it has been found that parasitism of D. suzukii by L. japonica and G. brasiliensis may not occur until 1-3 weeks after the first infestation. 21 This is reflected in our data by the first parasitoids not emerging from incubated fruit until 3 weeks post-infestation (Figs 2 and 3(A)). Additionally, the peak of Leptopilina sp.…”

“…Like the ACV traps, all Leptopilina samples were labelled as Leptopilina species. We expect that the Leptopilina emerging from fresh berries were more likely to be L. japonica rather than L. heterotoma because L. heterotoma is not often able to overcome the strong immune defence system of D. suzukii 21,27 …”

“…To determine the number of D. suzukii and associated parasitoids in the strawberries, we used a slightly modified protocol developed for standardized surveys across the lower mainland of BC in 2020. 21 We used Medium Square Ziploc containers (96 × 16 × 14 cm) with a 7.5 × 7.5 cm hole cut in the lid for ventilation for berry incubations. We stretched a nude, knee-high stocking with the toes cut off over the top of the container and tied it off to allow air flow but prevent insect escape.…”

“…19,20 However, to accurately assess the potential efficacy of these parasitoids for control of D. suzukii, information about their population dynamics will be particularly important. 21 If they are to be effective, the populations must reach adequate levels for control during the period where D. suzukii is causing damage or else the phenological mismatch could render them ineffective for damage control. 22 Outside of specialist parasitoid species, intercropped systems have been demonstrated to support higher populations of more generalist natural enemies that could further reduce D. suzukii numbers.…”

“…Various studies have demonstrated the ability for supplementary floral resources to enhance the survival and activity of parasitic hymenopterans like L. japonica and G. brasiliensis 19,20 . However, to accurately assess the potential efficacy of these parasitoids for control of D. suzukii , information about their population dynamics will be particularly important 21 . If they are to be effective, the populations must reach adequate levels for control during the period where D. suzukii is causing damage or else the phenological mismatch could render them ineffective for damage control 22 .…”