Spotted wing drosophila (SWD) is one of the most persistent, invasive pests in Canadian soft fruit production. This small but prolific fly first arrived in Canada from Asia in 2009. SWD is infamous for laying eggs inside ripening fruit where its larvae later feed, causing premature rot. For fruit producers, SWD requires intensive management often including regular insecticide applications, strict sanitation regimes and frequent monitoring. Fortunately, help has arrived in the form of two tiny parasitic wasps.
SWD typically overwinters in natural habitats, such as forests and hedgerows bordering crop fields. In spring and early summer, SWD reproduces in early-season, wild fruiting plants (e.g., salmonberry in BC). SWD then moves into cultivated raspberry, cherry and blueberry fields as fruit ripens. Later, SWD takes advantage of crop and non-crop plants, like wild and cultivated blackberries, before the adults move to overwintering sites.
Unfortunately, SWD can’t be easily managed in natural habitats. Insecticide sprays only kill the flies that are in crop fields during spraying. Later, other flies can invade fields. Tackling SWD requires management across the landscape, within crop fields and surrounding overwintering natural habitats.
This is where biological control can play a role. Natural enemies that kill SWD – such as predators, pathogens and parasitic wasps – may help to slow the growth of SWD populations in natural habitats and, in turn, reduce pressure on crops. However, when SWD invaded North America from Asia, its specific natural enemies were left behind. Most predators and parasitoids native to Canada have only very modest impacts on SWD. For example, native parasitic wasps, typically the most effective biological control agents for insect pests, killed less than 3% of SWD in most studies.
Researchers in the USA and Europe looked at what organisms attack SWD in Asia to see if effective, natural enemies specific to SWD could be introduced to North America. These could provide long-term management in natural habitats, with the goal of reducing the number of SWD that move into crop fields. To receive permits to release foreign biological control agents requires years of research to demonstrate that the agents would be effective and pose no adverse risk to native organisms. Thorough research found two potential candidates from China, Japan and Korea: the samba wasp (Ganaspis brasiliensis) and ronin wasp (Leptopilina japonica). These tiny (~3 mm), black, parasitic wasps specialize in laying eggs in SWD larvae inside fresh fruit. The wasp offspring feed on and kill SWD larva, then develop into adult parasitic wasps which will then attack SWD.
Researchers have concluded that neither ronin nor samba wasps lay eggs in any insect (or animal) other than vinegar files (in the genus Drosophila). The samba wasp was found to be a particularly promising candidate as it attacked very few species of vinegar flies other than SWD. Fortunately, like most parasitic wasps, neither species stings humans.
The regulatory process to request an intentional release of the samba wasp in the USA has begun. Our Canadian team initiated an OSC3 project to develop a suite of ecological pest management strategies for SWD. Research on the safety and efficacy of the wasps to provide long-term, self-sustaining SWD control could support a petition to release one or both in Canada.
Our researchers made a surprising discovery when conducting routine SWD natural enemy surveys in the BC’s Fraser Valley, east of Vancouver. Both samba and ronin wasps were attacking SWD even though there had been no intentional releases. In 2020, we found these wasps are very common across most of south-coastal BC and routinely killed at least 8-20% of SWD larvae in natural habitats. The two wasps usually acted together – killing SWD on at least 13 species of crop and non-crop fruiting plants. However, surveys in British Columbia (Kelowna area), southern Ontario and Nova Scotia in 2020 did not find these wasps.
It is unknown how the wasps arrived in Canada. Genetic relationships among SWD populations reveal a history of multiple accidental SWD introductions to North America. It’s possible that some introductions from Asia contained SWD individuals that were parasitized by samba and ronin wasps.
The consequences of these parasitic wasps establishing in BC won’t be known for a few years. But, there are a few things we can say with some confidence about the role they may play in organic SWD management in Canada.
First, these wasps will not be a silver bullet that eliminates SWD as a pest. While they clearly have potential to have a self-sustaining impact on SWD populations, they will not kill all the SWD in an area. Hopefully, combined with other natural enemies having smaller impacts, the wasps will reduce SWD populations enough that producers will be able to spray insecticides less frequently or delay initial sprays until later in the season.
Second, these wasps kill SWD larvae only when they have damaged fruit; unlike an insecticide, they don’t provide immediate crop protection. Rather, they kill SWD that would later cause damage to other fruit. Because the greatest benefits of these wasps will be achieved from their activity in natural areas and they are highly susceptible to even organic insecticides, there is no anticipated benefit of releasing wasps into crop fields. Rather, we can just enjoy the “free” service they provide in reducing overall SWD pressure.
The establishment of samba and ronin wasps means that, for the first time, natural enemies are specifically targeting SWD across the landscape, at least in south-coastal BC. We anticipate that these natural enemies could become important components of integrated management strategies against SWD. For example, we might make simple changes to habitats surrounding crop fields to enhance these natural enemies. This will be a research focus in years to come.
We have a unique opportunity to study how these parasitic wasps disperse across the landscape and their impact on SWD over time. Ultimately, the results observed in the Fraser Valley will lay the groundwork to consider releases of these wasps in other regions of Canada. To achieve this, we continue our work to forecast the safety and efficacy of the wasps in other areas. Our goal is to develop and submit a petition to the Canadian Food Inspection Agency to consider releasing these biological control agents into other Canadian regions to provide long-term, self-sustaining control of SWD. In the meantime, our researchers will monitor other fruit-growing regions to determine if these parasitic wasps arrive on their own.
(OSC3) Originally published in Organic Science Canada magazine, Issue #3, Spring 2021
