For a study of trapping invasive fruit flies in cherry orchards, researchers at Michigan State University marked thousands of spotted-wing drosophila (Drosophila suzukii) fruit flies with fluorescent dust, such as this male marked in blue. The flies were released in an orchard with a monitoring trap, and the recaptured flies were then examined and counted under ultraviolet light. (Photo credit: Danielle Kirkpatrick)

By Ed Ricciuti

While working on her Ph.D. at Michigan State University’s Department of Entomology, Danielle Kirkpatrick has spent many a day knocking out thousands of flies half the size of your typical red ant, sorting them by gender, separating them into batches, and squirting each group with its own color of flashy fluorescent powder. Then she and her colleagues released the flies in tart cherry orchards and tried to catch them again.

Ed Ricciuti

What may seem like a rather esoteric exercise actually was a first-of-its kind research project with an eminently practical aim, reducing depredations of the fruit fly known as spotted-wing drosophila (Drosophila suzukii) on fruits and berries, reducing costs for growers and, ultimately, for consumers at the checkout counter.

The study, published last week in the Journal of Economic Entomology, was the first attempt, in an orchard setting, to estimate the density of the spotted-wing drosophila (SWD) based on the catch in a monitoring trap. Such estimates are critical because pest-management decisions depend on the density of a pest within the area to be treated.

Conducted with MSU colleagues Larry Gut, Ph.D., and James Miller, Ph.D, Kirkpatrick’s research in two Michigan tart cherry orchards over two growing seasons shows that the catch in a single monitoring trap can be used to estimate SWD’s density among cherries. She cautions, however, that the information provided by the study may not be directly translated for use in the field with other fruits but is, at least, a model for controlling the species in a variety of crops.

Michigan State University’s Danielle Kirkpatrick views a red panel trap with recaptured spotted-wing drosophila (Drosophila suzukii) flies, previously marked with fluorescent dusts, under a microscope with an ultraviolet light. (Photo credit: Danielle Kirkpatrick)

Michigan State researchers used medical insufflators filled with various colors of fluorescent dusts to mark thousands of spotted-wing drosophila (Drosophila suzukii) flies in the laboratory before releasing them in the field and analyzing the patterns of recapturing via monitoring traps. (Photo credit: Danielle Kirkpatrick)

Since it was discovered in North America in 2008, this invasive SWD, native to Asia, has caused hundreds of millions of dollars in damage to cherries, peaches, nectarines, grapes, strawberries, blueberries, and many other fruits and berries. SWD can give growers fits because it strikes fruit just as it is about to ripen, and it damages fruit twice: 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. Growers may unknowingly harvest fruit with internal damage. Thus, once the eggs are laid, all bets are off, so an infestation of adults must be confronted immediately. Over the past two years, infestations in Michigan cherry orchards have peaked in mid-July.

SWD can blitz fruit crops quickly because it is difficult to detect and has a high rate of reproduction. While adults live for three to nine weeks, the fly’s life cycle from egg to adult can be completed in a week or two. That means a handful of flies can become millions in a few generations, all within a single growing season. All things considered, growers need an early warning system for SWD, Kirkpatrick says, because just one fly caught in a trap can mean a full-blown infestation is underway and control measures must be implemented immediately.

Kirkpatrick’s research involved releasing batches of flies at various distances—up to 200 meters but mostly less than that—from a trap hanging in a cherry tree and analyzing the catch, if any. The experiment was replicated many times over the two seasons. To match flies to their release points, they had to be marked. One standby method is to dust anesthetized insects with eye-catching colors. Each batch of flies was dusted while in a glass beaker with two puffs of colored powder from a squeeze bulb. Colors picked are hard to miss: DayGLO’s Horizon Blue, Signal Green, Aurora Pink, Blaze Orange and Saturn Yellow. As for separating males from females, “It does take some time to sort them,” says Kirkpatrick. How does one tell the difference between a he-fly and she-fly? “The males have a black spot on the leading edge of their wings and the females do not,” she explains.

After marking thousands of spotted-wing drosophila (Drosophila suzukii) flies with fluorescent dust and recapturing a small percentage via monitoring traps, researchers at Michigan State University returned to the lab to count the caught flies on the trap panels under ultraviolet light. (Photo credit: Danielle Kirkpatrick)

A female spotted-wing drosophila (Drosophila suzukii) fly marked with green fluorescent dust and later recaptured and viewed under ultraviolet light. (Photo credit: Danielle Kirkpatrick)

Female female spotted-wing drosophila (Drosophila suzukii) fly marked with orange fluorescent dust and later recaptured and viewed under ultraviolet light. (Photo credit: Danielle Kirkpatrick)

Monitoring insects is not merely a matter of stationing a trap and hoping it will catch something. The technology and methodology of trapping are quite precise. The trap used in Kirkpatrick’s study emitted an odoriferous attractant and used the color red as a visual clue. The odor plume of attractant that arose from it had a reach, or the extent at which insects respond to it, of about three meters, the researchers found. They also figured out its trapping area, or the area around the trap through which the target insects are dispersed and from which they might reach the trap. Kirkpatrick and her colleagues determined the trapping area of the trap used in the study had a radius of 93 meters, for an area of about 6.7 acres (2.7 hectares).

Her research will help growers figure out how to station their monitoring traps most effectively, placing enough of them out without overlapping. “We now have an indication of over what areas the trap covers so growers can do best trap placement,” says Kirkpatrick.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book 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.