New fruit pest poses threat to growers, gardeners

Times were when URI plant experts used to tell homeowners that the most care-free fruit to raise in their backyards were blueberries.

Source: MSU Extension Bulletin E-3140, Oct. 2010.

Blueberries were easy to grow, had great health benefits and required acidic soil, light pruning, sunlight and little else. Best of all, blueberries required no spraying, unlike other fruits, and the only pests that affected them were birds looking for breakfast. But those days are gone.

A new pest has arrived in the Northeast and it is posing a threat to backyard fruit growers as well as commercial ones.

The pest is called the spotted wing drosophila, a fruit fly, which has the nasty habit of laying its eggs in ripening small fruit such as blueberries, strawberries and raspberries. The larvae feed on the fruit and in the case of raspberries turn them into mush.

This fruit fly is different from other more common fruit flies. The female spotted wing drosophila has an ovipositor (an appendage insects use to deposit eggs) that actually has teeth and thus can “saw” through tougher tissue such as a berry that is not ripe. Common fruit flies on the other hand are attracted only to bruised or soft skinned fruit.

The spotted wing drosophila does have one thing in common with other fruit flies in that it is short-lived—a generation will last about 10 days. And that poses another problem for fruit growers: There are a number of insecticides that will combat fruit flies (only about two of them are registered for organic growers, however) but because the flies go through so many generations in so short a time, they can build up a resistance to an insecticide quickly, forcing growers to switch sprays periodically. That can become a real economic problem for commercial growers, says URI researcher Heather Faubert who is working on a monitoring project to detect the spotted wing critters in Rhode Island.

In this state and indeed in many New England states where high bush blueberry farms are of the pick-your-own variety, spraying poses yet another problem. All pesticides have something called “pre-harvest intervals” or PHI which is the time between the last application of the pesticide and the time the fruit can be safely harvested. If a certain pesticide has, for example, a three-day PHI, that means people are not allowed to harvest the fruit until three days have elapsed since the spraying was done. Depending on what pesticide they use, that delay could be another economic hurdle for pick-your-own farmers. Some pesticides have a very short PHI and one that can be used by organic farmers has no interval at all.

Researcher Heather Faubert has enlisted the help of students, staff and two URI Master Gardeners in her spotted wing drosophila monitoring project. Here Charlie Junod delivers the contents of one trap from Pippin Orchard which he checks on a weekly basis. Another master gardener, Hal Munroe, gathers trap contents from another orchard.

“I had a farmer tell me that he hasn’t sprayed his blueberry bushes in 12 years and he is hesitant to add another cost to his operation,” said Faubert who notes that pick-your-own operations are not generally high-profit in the first place. That farmer decided not to spray at all since his season is short and he figured the season would be over before the fruit flies could have any impact.

To monitor the influx of the fruit flies, Faubert has established monitoring traps at 12 locations throughout the state and is using students, URI staffers and URI Master Gardeners to help gather the critters on a weekly basis. Spotted wing drosophila flies have been caught at all 12 sites, including URI’s own East Farm which has hundreds of blueberry bushes. The other sites are commercial orchards and farms.

The monitoring traps are simple, low-cost affairs—plastic drinking cups with 1/8-inch holes poked in the sides, a plastic cover and a hanger. The bait in the cups is a mixture of white grape juice, apple cider vinegar, a tiny bit of ethanol and a drop of detergent. The flies are attracted to the mixture, enter the holes and drown. Once a week Faubert’s helpers empty the traps into plastic vials and deliver them to her lab where she looks at the flies with a microscope although a small hand lens can be used to spot them as well.

This is the first full growing season that the fruit fly has been widespread in the Northeast. Last year spotted wing drosophila were detected toward the end of the growing season but this year, they have been discovered much earlier.

Olivia Barsojian (left) and Carissa Koski are two Coastal Fellow students working at East Farm this summer and among their projects is monitoring traps for the spotted wind drosophila fruit fly which can infest blueberries and other small fruits. The traps contain a sweet mixture that attracts the flies which enter through tiny holes and then drown in the mixture. Both students are juniors. Koski is an environmental science major and Barsojian is majoring in wildlife conservation biology.

How much damage has been caused? Dr. Richard A. Coles of the Connecticut Agricultural Experiment Station says the full extent will not be known until the end of this growing season because this is the first year that growers were made aware of the situation.

The fly came to the U.S. from Southeast Asia and was first detected in Hawaii in 1980, says Cowles. It was detected in California in 2008 and by the following year it was up and down the West Coast. It then made a leap to Florida and from there it spread up the East Coast.

There are a number of chemical controls, notes Cowles who put together an extensive list of pesticides that can be used to control the pest (two of them, Entrust and Pyganic are available to organic growers).

Cowles recommends that certain pesticides would act a lot faster if mixed with sugar since the flies are “highly dependent on sugars to satisfy the demands of their flight muscles.” Cowles and others have a site that deals with the problem: cowles_spring_open_house_2012.pdf

Cowles said the growers of soft-skinned crops such as raspberries and day-neutral strawberries might have the biggest problem with the spotted wing drosophila. Other fruits susceptible to spotted wing drosophila attacks are grapes and stone fruit such as peaches and plums.

Development of a biocontrol for the fruit fly such as a parasitic wasp is not impossible, says Cowles, but would be very difficult because the eggs are enclosed in the fruit and the flies are on an extremely fast reproduction schedule. One avenue that might be possible, he said, would be the development of a pathogenic fungus, a project that he has in the talking stage in concert with Dr. Steven Alm, CELS entomologist.

For homeowners with just a few berry bushes in their backyards, Faubert suggests they might try making a yeast trap. The trap would consist of a tablespoon of baker’s yeast mixed with four tablespoons of sugar in 12 ounces of water. Perhaps, she suggests, the yeast trap will be more attractive to the flies than the berries.