Spotted-wing drosophila (Drosophila suzukii) arrived in North America in 2008 and has been been the No. 1 pest problem for soft-skinned fruits like berries for the last decade. Detecting the presence of these invasive flies could soon get easier, though, with a new molecular test for analyzing DNA in bulk trapping samples—sensitive enough to sniff out just one target D. suzukii in a sample of 2,000. And the best liquid for preserving trapped flies’ DNA comes from the dental-care shelf. (Photo by Justin Renkema, Ph.D.)

By Melissa Mayer

Melissa Mayer

When it comes to fighting bad breath and tooth decay, mouthwash is just the thing to step up a basic oral hygiene game—but could it help trap invasive insects for identification?

That’s what a team of researchers from Ontario, Canada recommends in an article published in February in the Journal of Economic Entomology. They say that the novel quantitative polymerase chain reaction (qPCR) assay they designed to efficiently identify the fruit fly known as spotted-wing drosophila (Drosophila suzukii) could help monitor for this invasive pest—and using mouthwash as a trapping liquid could make a difference at higher temperatures.

A Berry Expensive Problem

Justin Renkema, Ph.D.

Spotted-wing drosophila (SWD) arrived in North America in 2008 and is a particular problem for soft-skinned fruits like berries. In fact, it’s been the No. 1 pest issue for those crops for the last decade, according to Justin Renkema, Ph.D., researcher at Agriculture and Agri-Food Canada and lead author on the study.

The pest has a specialized ovipositor (egg-laying appendage), which is serrated so it can saw into ripening berries to deposit eggs. The resulting maggots go to town on the fruits, rendering the berries mushy and unfit for sale. That adds up to substantial economic losses for farmers, who must also invest management costs into monitoring and treating for the pest.

While many opt for scheduled insecticide applications, models make it clear that an integrated pest management (IPM) approach works better and decreases the downsides of regular insecticide use, such as negative effects on nontarget insects or the development of resistance. IPM requires monitoring, usually with bottle traps that feature a lure above a drowning liquid. These traps also attract other insects, especially other fruit flies, and sorting through the bycatch to manually identify spotted-wing drosophila is labor-intensive.

“One of the big issues in monitoring these flies in order to make pest management decisions is that the traps and lures that attract them also attract a lot of other Drosophila species,” Renkema says. “And because they are fairly similar, it takes a lot of work to go through all the samples to count the number of SWD. … What we wanted to do is try to use a molecular test to replace looking at the flies under a microscope and counting them.”

That’s where qPCR comes in. This method takes a bulk sample—like environmental DNA or the contents of a trap—and amplifies the target DNA for sequencing. Renkema’s coauthor Shu Chen, Ph.D., senior research scientist at the University of Guelph, put together an assay using primers and probes specific to spotted-wing drosophila. In the lab, the assay was sensitive enough to detect the DNA of a single spotted-wing drosophila specimen in a sample of up to 2,000 mixed-species fruit flies.

“One of the big issues in monitoring these flies [spotted-wing drosophila] in order to make pest management decisions is that the traps and lures that attract them also attract a lot of other Drosophila species,” says Justin Renkema, Ph.D. “And because they are fairly similar, it takes a lot of work to go through all the samples to count the number of SWD. … What we wanted to do is try to use a molecular test to replace looking at the flies under a microscope and counting them.” (Photo by Justin Renkema, Ph.D.)

The Secret’s in the Sauce

The team also looked at how to optimize those bottle traps for use with qPCR. In many cases, water works just fine as a drowning liquid, but the team found that their assay was less sensitive as the water temperature and time the insects spent in the water increased—for example, when held at 33 degrees Celsius for seven days. It turns out that bacteria colonize the water more effectively at higher temps and over longer timeframes, and some bacteria essentially eat DNA, breaking down the flies to the point that their DNA becomes undetectable.

One option is to instead use alcohol as a drowning liquid, and the team reported that traps filled with ethanol performed just fine even at the upper temps and time frames. But alcohol has its own problems: It evaporates quickly, and it’s both flammable and expensive (especially if it’s shipped).

The solution? Mouthwash. The readily-available dental liquid contains alcohol, so it boasts similar antimicrobial capabilities at those higher parameters—but it’s also easy to source and won’t catch fire.

The team plans to trial their assay in the field this summer. Renkema says the molecular-detection tool could make it possible to increase the number of traps set out for monitoring since the contents could be combined and assayed together.

“One of our goals for this project was to give farmers a tool so that they could potentially reduce insecticide use, especially at the beginning of seasons and at the end of seasons,” says Renkema.

For berry growers and scouts, that could be sweet news.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.