By Ed Ricciuti
It is still too early to say for sure, but North American fruit growers may have caught a big break in their daunting battle against the invasive fruit fly known as spotted-wing drosophila (Drosophila suzukii), which has cost more than $700 million in crop damage annually since its arrival in 2008. The stroke of luck is that two small wasps that parasitize the fly’s larvae in its native Asia have established a beachhead astride the border of British Columbia and Washington state and could serve as natural allies for embattled growers.
A change in scene from its original home has not helped the fly escape the lethal attention of the wasps, Leptopilina japonica and Ganaspis brasiliensisaccording to a new study published in May in Environmental Entomology. Within the area of southwestern British Columbia where the researchers carried out their study, both wasps parasitize the fly at the same level as in their natural Asian range.
A team of scientists from Agriculture and Agri-Food Canada, the British Columbia Ministry of Agriculture, the University of British Columbia, and the US Department of Agriculture collaborated on the study, which explored the associations between spotted-wing drosophila (SWD), its host plants, and the wasps in several habitats. The researchers say “the close association between the two larval parasitoids and D. suzukii that exists in Asia has evidentially been reconstructed in North America, resulting in the highest parasitism levels of D. suzukii yet recorded outside of its area of origin.” Their findings suggest the wasps potentially are an effective biological of SWD around the globe.
“Remarkable,” is the way the researchers describe how the wasps have fit in to the ecology of their introduced range in North America. “They have evidently re-formed a close association with D. suzukii across a wide range of host plants including cultivated and wild shrubs, trees, and low-growing plants in a wide variety of habitats and seem to be co-existing with each other in a manner very similar to their native range,” they write in their journal article.
For an entire growing season, the research team carried out their work in a wide range of habitats, ranging from farmland to forest. Plants used by SWD included both cultivars such as raspberries and wild types such as salmonberry. The study suggests that wild plants serve as a key reservoir for populations of SWD that then disperse to fruit crops so control likely must extend beyond farm fields.
The United States Department of Agriculture recently approved use of G. brasiliensis as a control agent, with releases planned for this year. L. japonica is under consideration as well. Both wasps have been introduced in a few other parts of the world where SWD also has shown up, but not extensively.
Although it all bodes well for fruit growers, there could be a hitch because the majority of the parasitism observed occurred after SWD infested fruit, says one of the researchers, Paul Abram, PhD., of the federal Agriculture and Agri-Food Canada.
“We do not yet know exactly how this delay between infestation and parasitism will play out in terms of its effect on biological control efficacy, and we still need to verify that this is a consistent feature of the system in multiple years,” he says. “Theoretically, it can allow the pest to stay one step ahead of the biological control agent as they move between different fruiting plants over the course of the season and start reproducing in a new time and place before the parasitoids’ impact sets in. We are planning experiments for the upcoming field season where we directly test the population dynamics consequences of delayed parasitoid arrival on pest suppression.”
Pest managers must act fast to control SWD, which strikes fruit just as it is about to ripen, unlike other fruit flies that prefer damaged fruit and fruit well past ripening. Worse, SWD hits fruit with a one-two punch: first when the female’s serrated ovipositor penetrates the skin of the fruit and again when the white larvae hatch and start feeding while hidden inside the fruit, impervious to treatment.
Spotted-wing drosophila was introduced in North America in 2008. The parasitoids arrived, presumably by accident, in British Columbia within the past five years or so. Ganaspis brasiliensis was discovered last year in Washington State just over the Canadian border in wild blackberries. Leptopilina japonica showed up in Washington the year before in a trap set to catch the much-ballyhooed hornet Vespa mandarinia. Both probably belong to the same populations north of the border.
Whether the wasps were interacting with SWD in North America as they did in their original home was not previously clear, which was the motivation behind the study. More work is necessary before the full impact of the wasps can be determined. Control of mobile pests like SWD requires management over a large landscape against a known ecological background. Researchers need to learn more about the level of parasitism in and out of cultivated areas and how the wasps interact with the fly’s seasonal ecological relationships and its population dynamics. Beyond that, the ability of the wasps to tolerate climatic conditions outside of the study area is an open question, requiring years of additional sampling to answer.
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book his is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.