Florida berry growers learn smarter ways to manage spotted wing drosophila

by Rosemary Hallberg, Southern IPM Center

A recent project in Florida has helped strawberry and blueberry growers see spotted wing drosophila control in a new light. Extended monitoring, insecticide trials and grower workshops have led to a change in the way growers treat their fields.

SWD on strawberrySpotted wing drosophila is a vinegar fly pest of soft-skinned fruits such as brambles, blueberries, strawberries, peaches, apples and grapes. Unlike other vinegar flies, which infest dead or decaying fruit, SWD deposits its eggs into ripening fruit. The eggs develop inside the fruit and render the fruit unmarketable. In fact, if USDA inspections reveal SWD-infested berries, the grower must destroy the entire shipment, along with the entire field of unpicked fruit. It’s no wonder that SWD is one of the highest priority pests in the South.

Because of the high cost of possible SWD infestation, most blueberry growers prophylactically treat their fields every 10 days. Strawberries, on the other hand, have not traditionally been considered a high-risk crop for SWD, so many strawberry growers have not been concerned about the pest.

University of Florida entomologists Dr. Oscar Liburd, Dr. Gurpreet Brar and Dr. Teresia Nyoike, along with plant pathologist Dr. Amanda Hodges and masters student Lindsy Iglesias used a $29,895 IPM Enhancement Grant to explore SWD presence in strawberry and extend monitoring in blueberries to southern Florida. In addition, they evaluated several conventional and reduced-risk insecticides and launched an SWD education program for growers.

The Florida strawberry season begins in October and lasts through February. Since strawberry growers have not been on the alert for SWD, most scouted for other vinegar flies, which pose much less of a threat to the ripening crop, and few growers treated their fields for SWD.

“This is the first survey that recorded an abundance of SWD in strawberry,” says Liburd. “Because growers didn’t expect it in strawberry, a lot of them didn’t spray. Our study found that it was widespread in strawberry.”

Traps proved that SWD was present on more than 60 percent of the strawberry farms monitored for SWD. After learning about results from the first two trap counts, growers began a monitoring and rotational spray program. No SWD were found in the third trap count immediately after spraying commenced.

Many blueberry growers in southwest Florida, on the other hand, learned that SWD was not a problem in their area. Most of them had been spraying every 10 days to prevent the pest from attacking the crop.

“We showed growers in the southwest that it didn’t make sense to spray because we didn’t find any SWD in the traps,” Liburd says. “By monitoring and then spraying, growers can reduce their residues by 50 to 60 percent.”

Thirty percent of those growers actually ceased regular treatments and began regularly monitoring for SWD. As a result, their treatments decreased by 50 percent. Some growers didn’t need to spray at all for the entire season.

In areas where SWD is a problem, however, blueberry growers have to treat their crop frequently during the harvest period from March until May. Because only a few insecticides are registered for use in Florida blueberries with a short post-harvest interval, growers have few options to rotate, increasing the chances for insecticide resistance. Strawberry growers have even fewer options, and organic growers have only two organic insecticides available to them.

Female SWD can lay up to 600 eggs at one time and produce up to 10 generations a year. Warmer temperatures often encourage even more generations, as they shorten the insect’s life cycle.

On a farm where SWD is a problem, growers must protect their crop during the harvest season and be able to harvest the fruit daily if needed. However, many of the effective insecticides labeled for SWD require a few days before anyone can reenter the field. Liburd and his colleagues therefore needed to find reduced-risk options that would have shorter reentry periods, in addition to testing the effectiveness of the products already available.

Lab and field trials of several insecticides returned positive results for three reduced-risk insecticides and one conventional insecticide. In lab studies, captured SWD were exposed in a sealed container to each insecticide. The field studies involved spraying plants in the field, then taking leaves back to the lab to expose them to SWD.

“Obviously we can’t release SWD in the field,” Liburd says. “But we needed to do field trials because often the lab studies show efficacy, but in the field, it’s a different story.”

In fact, day 5 of the field study proved how different the trials could be, when all insecticides being tested showed lower efficacy after being washed off of the plants after a rain storm.

The organophosphate malathion, already registered for use for berries in Florida, proved to be effective at controlling SWD. Three reduced risk products, one of which is a spinosad (Entrust) and is registered for organic use, also proved over 90 percent effective. One new insecticide currently being reviewed for registration, and which offers a good rotational possibility because it differs from the other insecticides registered for use on blueberries, also was highly effective at controlling SWD.

Two pyrethroids, Hero and Mustang, also provided full control of SWD; however, they are not registered for SWD in strawberries.

Entrust, a spinosad, and Pyganic, a pyrethrin, are the only products currently available to organic growers. In tests, Entrust was far superior to Pyganic, but Liburd counseled growers to continue using Pyganic as a rotational insecticide. One organic grower who had been relying heavily on Pyganic switched to Entrust after learning the results.

The hands-on trainings focused on identification and scouting and trapping tools, introducing growers to new products that have fewer impacts on the environment and human health. Most attendees came from small farms or backyard orchards, along with some large growers and Extension faculty. Each of the three workshops had over 100 attendees. By the end of the workshops, at least half of the participants were able to identify SWD females. Each grower participating in the training took home an extension publication explaining how to monitor for and manage SWD.

Liburd plans to monitor the progress of EPA registration for the new product being labeled for berries and for another that will have strawberries added to its current label. As he keeps growers apprised of the progress, he will also continue stressing the importance of monitoring for the pest and rotating insecticides to prevent SWD from developing insecticide resistance.