The invasive fruit fly spotted-wing drosophila (Drosophila suzukii) causes extensive damage to cherry and berry crops, and effective monitoring is vital to control efforts. A new study in Michigan found that spotted-wing drosophila consistently prefer red, glue-covered monitoring traps made of rectangles of plastic or spheres of plastic compared to the most commonly used clear deli-cup traps. The findings will advance the pursuit of more efficient and effective monitoring techniques for the pest. (Photo credit: Flickr/Katja Schulz, CC BY 2.0)

By John P. Roche, Ph.D.

Spotted-wing drosophila (Drosophila suzukii) is an invasive fly species that damages cherry and berry crops. Native to east Asia, it was first discovered in the continental United States in California in 2008, and it has spread through the U.S., Mexico, Canada, South America, and Europe. Whereas most fruit flies feed on damaged and overripened fruit, female spotted-wing drosophila have a serration on their ovipositor allowing them to deposit eggs under the skin of intact fruits. Because of this, their larvae can cause immense economic damage, sometimes wiping out entire fruit crops.

Management of spotted-wing drosophila populations is therefore vital, and successful management depends on effective sampling that can identify the presence of flies so that control measures can be implemented efficiently. A new study published in April in the Journal of Economic Entomology, by Danielle Kirkpatrick, Ph.D., and colleagues at Michigan State University, tested the efficacy of several trap designs. Their results will help inform improved monitoring and control of this voracious invasive fly.

Effective fly sampling traps have two benefits: One, they allow fruit growers to know when management efforts need to be applied, increasing effectiveness; and, two, they permit delays in applying insecticide applications until they are needed, reducing environmental impacts and lessening disruption of pest management for other insect species.

The most commonly used trap is currently a clear plastic deli cup with holes in the side and liquid at the bottom for trapping flies. Kirkpatrick—who conducted the study while pursuing her Ph.D. at Michigan State and recently began a postdoctoral role at the U.S. Department of Agriculture’s Agricultural Research Service in Kearneysville, West Virginia—and her colleagues were interested in testing how different designs of traps would perform compared with deli-cup traps. They compared deli-cup traps with three alternative trap designs:

1. rectangular panels of colored plastic covered in glue called Tanglefoot (panel traps);
2. glue-covered panels of plastic combined with a red sphere of glue-covered plastic (combination traps);
3. whiffle balls painted red and covered in glue (sphere traps).

They also tested two types of olfactory lures: a dry commercial spotted-wing drosophila lure called Scentry, and a wet yeast/sugar lure.

A female spotted wing drosophila (Drosophila suzukii) on a cherry. Spotted-wing drosophila cause extensive damage to cherry and berry crops, and effective monitoring is vital to control efforts. A new study in Michigan found that spotted-wing drosophila consistently prefer red, glue-covered monitoring traps made of rectangles of plastic or spheres of plastic compared to the most commonly used clear deli-cup traps. (Photo credit: Danielle Kirkpatrick, Ph.D.)

The objectives of the study were to test if a particular color of alternative trap was more effective and how the effectiveness of the alternative traps compared with deli-cup traps. The investigators placed fly traps with different colors, designs, and lures in cherry orchards and raspberry fields to test which trap types captured more flies.

In one experiment, the investigators placed red panel traps, green panel traps, white panel traps, yellow panel traps, combination traps, and deli-cup traps in a cherry orchard and baited all traps with Scentry lure. They found that significantly more spotted-wing drosophila were captured on red panel traps than on yellow panel traps, and significantly more were captured on combination traps than on yellow traps.

In another experiment, Kirkpatrick et al. placed red panel traps, green panel traps, white panel traps, yellow panel traps, deli-cup traps, and combination traps in raspberry fields and baited all traps with Scentry lure. The combination traps captured significantly more flies than both the deli-cup traps and the yellow panel traps.

In a third experiment, conducted in a cherry orchard, they placed three trap types baited with Scentry lure (red panel traps, red sphere traps, and deli-cup traps), and one trap type baited with the yeast/sugar solution (deli-cup traps). Significantly more flies were captured in a sphere trap with Scentry lure than in a deli-cup with Scentry lure or in a deli-cup with yeast/sugar. In addition, significantly more flies were captured in a red panel trap with Scentry lure than in a deli-cup with Scentry lure. Thus, there was a clear preference for red sphere traps and red panel traps over clear deli-cup traps.

In a final experiment, the investigators placed red panel traps baited with Scentry lure in a cherry orchard from June to August and determined the gender of all captured flies. They found no significant difference in the number of each gender captured.

As to the most important finding, Kirkpatrick says, “Traps integrating a red visual cue in combination with an olfactory cue consistently captured more spotted-wing drosophila than the clear deli-cup traps did.” This builds on the results of other studies that have observed that red sticky traps are more effective than clear deli-cup traps. And it makes sense evolutionarily—several preferred food types of D. suzukii, including cherries and raspberries, are red.

Regarding the lack of gender differences, Kirkpatrick says, “This finding could potentially reduce the time and effort it takes to check a trap in the field by identifying only males captured and could pave the way for real-time reporting of spotted-wing drosophila capture numbers to guide management decisions for growers.”

Making the trap more user-friendly will be the next improvement in the design, Kirkpatrick says. “Another important next step would be to further understand how captures relate to populations early in the season; this would help determine when chemical control programs should be initiated if numbers of spotted-wing drosophila captured in traps are low and fruit is at a stage vulnerable to infestation.”

The findings from this study provide insights into the sensory factors affecting foraging choices of spotted-wing drosophila, and the physical factors affecting capture. These discoveries could help shape improvements in the control of this economically damaging insect.

John P. Roche, Ph.D., is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has a Ph.D. in biology, has published 190 articles, and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.