How Salt, a Coffee Filter, and a Microscope Could Aid in Managing Spotted-Wing Drosophila

filter method for finding spotted-wing drosophila

Follow these steps to sample fruit for Drosophila suzukii larvae: (A) Collect 8–16 oz (237–473 ml) of ripe berries; (B) place berries in a 1 gallon resealable plastic bag and lightly crush berries; (C) add salt water to berries, place bag upright in a plastic bin, and incubate for 1 h; (D) pour salt water and berries into course filter funnel and rinse berries off to wash larvae into reusable coffee filter; (E) use microscope or other magnifier to view larvae in the coffee filter; and (F) count the number of larvae in the bottom of the filter. (Images originally published in Van Timmeren et al. 2017, Journal of Integrated Pest Management)

By Andrew Porterfield

The red-eyed, spotted fly first appeared in the United States in strawberry, raspberry, and blackberry crops in Santa Cruz County, California, in 2008. Then the Southeast Asian invader Drosophila suzukii began showing up in crops in other areas of coastal California, eventually making its way to the far Pacific Northwest and then becoming a nationwide pest. D. suzukii’s biology is still not completely known, but its ability to destroy crops is, as farms can suffer yield losses of up to 80 percent when it arrives.

Andrew Porterfield

Also known as the spotted-wing drosophila (SWD), D. suzukii has been controlled chemically, using sprays when fruit is ripening. But control is challenging, since the fly has just a 10- to 14-day generation time, and females are highly fertile, which results in large and quick population increases. Consumer intolerance of infestation in fruit crops, as well as the fact that fly populations spike just as fruit ripens, have made farmers all but abandon non-chemical management methods.

Adding to these significant challenges is that monitoring for SWD using traditional baits and lures does not provide reliable predictions of infestation. Larval monitoring has been somewhat more effective, but visual assessments only pinpoint the largest larvae, missing entirely the smaller larvae that most often trigger an infestation.

So far, solutions to spot smaller larvae have proven expensive, labor-intensive, and too slow. However, a simple combination of salt, coffee filters, and microscopy may provide an answer to this fast growing, and apparently fast-migrating, pest.

Rufus Isaacs, Ph.D., professor of Entomology at Michigan State University’s Center for Integrated Plant Systems, and his colleagues at MSU and North Carolina State University found that mixing crushed berries in a salt solution, sifting the solution through a reusable coffee filter, and counting recovered SWD larvae with a 5-10X stereomicroscope can accurately identify numbers of larvae nearly two times faster than traditional tray methods. The researchers’ study is published today in the open-access Journal of Integrated Pest Management.

This simple method may help researchers better understand the life cycles and feeding habits of the invasive pest and help farmers reduce chemical use and better target their control efforts. Because the new method also allows for first instar larvae to be detected, growers would benefit from an earlier “jump” on the insect’s rapid growth.

spotted-wing drosophila larvae

An improved method for finding and identifying larvae of spotted-wing drosophila in fruit allows for even the earliest larval stages to be found. Here, a Drosophila suzukii egg and small, medium, and large larvae are seen under a microscope after sifting using a reusable coffee filter. Scale is in millimeters. (Image originally published in Van Timmeren et al. 2017, Journal of Integrated Pest Management)

“Growers can use this information to target insecticide sprays for curative control of small larvae, identify fields where the presence of larger larvae may indicate an unmarketable crop, or verify that no infestation exists,” the researchers write. “We expect this method to become an important component of rebuilding IPM [integrated pest management] programs in fruit crops affected by D. suzukii.”

The researchers compared the saltwater/filter method to visual observation (without filters) using salt and brown sugar solutions. Brown sugar and salt alone enhanced visual observations better than a tap water control (and there was no difference in effectiveness between sugar and salt). However, four times as many larvae could be seen using the saltwater-and-filter combination. Most important, first and second instar larvae were seen for the first time using the saltwater and filter. The saltwater/filter method was also faster, assessing fruits in 3.8 seconds versus tray-based methods, which took 6.5 seconds to assess. These times are significant for farmers and crop consultants who are tasked with determining the fate of fruit on very large farms and who could be facing an infestation that can ruin a crop in a manner of days. At agricultural scales, the seconds add up.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.

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