Since the establishment of the brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) in North America and Europe, there has been a large, multi-group effort to characterize the composition and impact of the indigenous community of arthropod natural enemies attacking this invasive pest. In this review, we combine 98 indigenous natural enemy datasets spanning a variety of sampling methods, habitats, and geographic areas. To date, the vast majority of H. halys biological control research has focused on the egg stage, using sentinel egg masses to characterize indigenous parasitoid and predator communities and their contribution to H. halys egg mortality. Although egg parasitism and predation levels by indigenous natural enemies are low (typically <10% each) in most surveys, total egg mortality attributable to natural enemies can be higher (typically between 5 and 25%; up to 83%)—even though these values were likely underestimated in most cases because some mortality due to biological control was not recognized. In North America, where the most data are available, it appears that the relative prevalence of different indigenous parasitoid species varies among habitat types, particularly between crop and non-crop habitats. Predator species responsible for egg mortality are much less commonly identified, but appear to include a wide variety of generalist chewing and sucking predators. To date, studies of natural enemies attacking H. halys nymphs and adults are relatively rare. Based on our review, we identify a number of key research gaps and suggest several directions for future research.

The main modes of action of insect parasitoids are considered to be killing their hosts with egg laying followed by offspring development (reproductive mortality), and adults feeding on hosts directly (host feeding). However, parasitoids can also negatively affect their hosts in ways that do not contribute to current or future parasitoid reproduction (nonreproductive effects). Outcomes of nonreproductive effects for hosts can include death, altered behavior, altered reproduction, and altered development. On the basis of these outcomes and the variety of associated mechanisms, we categorize nonreproductive effects into ( a) nonconsumptive effects, ( b) mutilation, ( c) pseudoparasitism, ( d) immune defense costs, and ( e) aborted parasitism. These effects are widespread and can cause greater impacts on host populations than successful parasitism or host feeding. Nonreproductive effects constitute a hidden dimension of host–parasitoid trophic networks, with theoretical implications for community ecology as well as applied importance for the evaluation of ecosystem services provided by parasitoid biological control agents.

We report the presence of two Asian species of larval parasitoids of spotted wing Drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), in northwestern North America. Leptopilina japonica Novkovic & Kimura and Ganaspis brasiliensis (Ihering) (Hymenoptera: Figitidae) were found foraging near and emerging from fruits infested by D. suzukii at several locations across coastal British Columbia, Canada in the summer and fall of 2019. While G. brasiliensis was found in British Columbia for the first time in 2019, re-inspection of previously collected specimens suggests that L. japonica has been present since at least 2016. Additionally, we found a species of Asobara associated with D. suzukii in British Columbia that is possibly Asobara rufescens (Förster) (known only from the Palearctic Region) based on COI DNA barcode data. These findings add to the list of cases documenting adventive establishment of candidate classical biological control agents outside of their native ranges. The findings also illustrate the need for revisiting species concepts within Asobara, as well as host and geographic distribution data due to cryptic and/or misidentified species.

The basic biology, seasonal ecology, and hymenopteran parasitoid community of Plutella armoraciae Busck, 1912 (Lepidoptera: Plutellidae) was investigated. The study area was a dry grasslands region of interior British Columbia, Canada, where P. armoraciae feeds on the introduced mustard Sisymbrium loeselii L., 1755 (Brassicaceae) in partial sympatry with its congener, the diamondback moth Plutella xylostella (L., 1758) (Lepidoptera: Plutellidae). Sampling was conducted over five years, including one year of repeated, intensive sampling over a large region. The two Plutella species can be differentiated with morphology alone and we provide comparisons of features of the eggs, larvae, pupae and adults of each species. Plutella armoraciae was found to overwinter as eggs or early-instar larvae in S. loeselii rosettes, with adults emerging in April and May, and the first full generation each year taking place in May and June. Larvae fed within webbed flowers and buds of S. loeselii plants, giving rise to a second autumn generation of adults that laid eggs of the overwintering generation. The early summer generation of P. armoraciae larvae peaked earlier than, but partially overlapped with, the peak occurrence of P. xylostella larvae. Both Plutella species were relatively common across all surveyed sites, but not highly abundant at any site. At least two species of larval parasitoids, Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Diolcogaster claritibia (Papp) (Hymenoptera: Braconidae), were shared between the two Plutella species, and a community of eight other species of parasitoids and hyperparasitoids were reared from the pupae of one or both Plutella species. The observations of D. claritibia emerging from P. armoraciae are the first records of this parasitoid from a host species other than P. xylostella. It was also discovered that one of the most important parasitoids of P. xylostella worldwide, D. insulare, appears to use P. armoraciae as an overwintering host. We hypothesize that in areas where P. xylostella is a seasonal migrant, year-round resident Plutella species such as P. armoraciae could provide important overwintering hosts and reservoirs for shared parasitoids that help to reduce populations of P. xylostella.

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.