Fallen Fruit Breeds Invasive Flies

spotted-wing drosophila closeup

The invasive fruit fly spotted-wing drosophila (Drosophila suzukii) is a significant pest of a wide variety of fruit and berries in North America. A new study in the Journal of Economic Entomology suggests growers should take care to remove fallen or discarded fruit in their grounds and facilities to reduce potential breeding sites for D. suzukii. (Photo credit: Flickr/Martin Cooper, CC BY 2.0)

By Andrew Porterfield

The spotted-wing drosophila (Drosophila suzukii) made its way from Asia to Hawaii benignly enough: Its Hawaii arrival was discovered in 1980 but didn’t trigger aggressive attempts to eradicate it. The fruit fly’s 2008 North American arrival in Santa Cruz, California, however, ushered in a far more destructive era for the fly—it’s now in nearly every U.S. state and Canadian province, including colder climates like Michigan, and is responsible for yield losses in berries and other fruits of up to 80 percent.

Andrew Porterfield

Applications of chemical insecticides are now the only method that appears to hold back infestations of D. suzukii, but they present problems:

  • Weekly applications also kill natural enemies of D. suzukii, which could actually enhance pest populations.
  • Sprays that continue into harvest periods can violate maximum residue limits, which can affect international trade of fruits.

Researchers from Michigan State University have looked at a nonchemical alternative that could reduce the rather high breeding ability of the fruit fly from Asia. Matthew Grieshop, Ph.D., associate professor of entomology, and his colleagues at MSU looked at the effects of removing fallen fruit on D. suzukii breeding patterns. By removing potential breeding sites such as fallen fruit, the research suggests, population levels of the fly could be reduced. Their study, “Evaluation of Off-season Potential Breeding Sources for Spotted Wing Drosophila (Drosophila suzukii Matsumara) in Michigan,” is believed to be one of the first to evaluate whether removing fruit wastes could reduce breeding flies. Their study was published last week in the Journal of Economic Entomology.

Grieshop’s team collected fruit wastes from 15 farms in southern Michigan. Fruits included apples, pears, grapes, peaches, plums, raspberries, and cherries, as well as fruit pomace (remaining fruit after grinding processes), and the team looked at fruits from cider- and wine-making facilities as well as orchards. The fruits were evaluated for the presence of the spotted-wing drosophila, other drosophila relatives’ emergence, and host suitability.

The researchers found that 100 percent of dropped apples, pears, grapes, grape pomace, and raspberries contained spotted-wing drosophila; the highest number of flies were found in grapes and pears. Forty percent of dropped apple pomace contained the pest. Higher concentrations of the fly were found in fruit wastes in cidery and wine-making sites. Multiple-fruit cultivation also appeared to boost the presence of the fly. Peaches, plums, pumpkins, and cherries were not heavily infested with the fly, if at all.

The findings have major implications for vineyards and farmers growing susceptible fruits near vineyards. Grape wastes were found to be a popular reproductive site for the fruit fly, a finding that surprised the researchers. “Grapes are not typically considered to be especially threatened by the pest,” Grieshop says. “The spotted-wing fruit fly does not reproduce well on intact grapes—likely due to think skins.” But, for rotting or fallen grapes, the fly shows more attraction.

The fact that “we were able to find spotted-wing drosophila at all the apple orchards we sampled” was also a surprise, Grieshop says. “We expected to find spotted-wing drosophila at apple sites that had proximity to berry and cherry plantings but did not know if we’d find it in orchards that were not adjacent to these crops,” he adds.

For vineyards and cider makers, the research presents a warning that the spotted-wing drosophila is not “just another vinegar fly.” Because fallen grapes and apples are fertile ground for reproduction and the number of flies were greatest at these facilities, any fruit waste at those facilities needs to be disposed of as quickly as possible, Grieshop warns. Incineration is one current and effective method, as is burial at least four inches deep. Grieshop says his laboratory is evaluating various composting methods but hasn’t developed any firm conclusions on optimal strategies. For instance, adding animal manures might make the fruit less attractive to flies, but the research is still ongoing.

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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