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| The spotted wing drosophila |
By RUDI HEMPE/ecoRI.org News contributor
Times
were when University of Rhode Island plant experts used to tell homeowners that
the most carefree fruit to raise in their backyards were blueberries.
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’s posing a
threat to both backyard fruit growers and 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, according to URI researcher Heather Faubert, who is working on a
monitoring project to detect the spotted wing critters in Rhode Island.
Here and 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 aren’t 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.
“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, noting 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 these 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 at 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.
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 were
discovered much earlier.
How much damage has been caused? Richard Coles of the
Connecticut Agricultural Experiment Station said 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 United States from Southeast Asia and
was first detected in Hawaii in 1980, according to 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, said Cowles, who
has 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 (pdf)
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 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.
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.