Berries Facing New Threat

FAE Extended Coverage: Berries Facing New Threat

Editor’s Note: This article is the first in a series of six special features on research presentations from the 2010 Florida Ag Expo. Special thanks to DuPont Crop Protection for sponsoring this series.

A year ago in August, the spotted wing drosophila (Drosophila suzukii) was discovered in the strawberry producing area of Hillsborough County after having been known about one year in California and for less time in Washington. Since arriving in Florida, the fly has expanded its range to include at least the Carolinas, Michigan, Kentucky, and Louisiana. It is probably infesting most states among those, but has not been detected by folks that know its damage. The fly also is expanding its range in Europe. No commercially significant damage was reported from Florida berries during the winter and spring 2010 strawberry and blueberry seasons, probably a blessing of the record-breaking cold weather we experienced then.
This fly that originated in the Orient resembles the common Drosophila spp. flies that accumulate on over-ripe bananas, berries left without refrigeration, rotting fallen citrus, and other fruit beginning to spoil. In fact, both are small, have prominent red eyes and, indeed, are closely related.Wing tips of spotted wing drosophila males contain a dark spot that is lacking in our common drosophilids.

Female spotted wing drosophilas possess serrations on their egg laying organ that can cut soft surfaces of sound fruit to lay eggs inside. Common drosophilid flies are without that modified ovipositor. Spotted wing drosophila eggs that hatch inside fruit become white maggots, which can soften and ruin fruit in the field or can accompany harvested fruit undiscovered until the fruit are in consumers’ hands.
This group of small flies often is called the pomace, vinegar, or fruit flies, but “fruit flies” in this case is confusing since that common name applies to larger flies, the Tephritidae, often problematic and reported in the news media when outbreaks occur. Tephritids include banded winged flies such as Mediterranean fruit fly, Caribbean fruit fly, Oriental fruit fly, Mexican fruit fly, and others. Drosophilid flies are not closely related to tephritid flies and management of the two groups can be vastly different. For instance, rare outbreaks of Mediterranean fruit flies in Florida are managed in part with mass releases of sterilized male Mediterranean fruit flies. This technique has not been developed for drosophilids and is impractical to consider in most cases.

On The Fly

The spotted wing drosophila is expected to survive in Florida’s climate, and given the swift colonization on the U.S. West Coast, berry growers should be prepared to encounter this fly. The degree of interference to production is clearly unknown. However, management plans are surfacing. There are tactics that can be applied as conditions warrant. Presently, there are no action thresholds established or even farm-level scouting protocols established.

Management practices immediately available in Florida for spotted wing drosophila are those used to manage our common drosophilids. Additional techniques of adapting tephritid fruit fly baits with toxicants are being considered and developed for Florida berries, but some problems exist in transferring the procedure to the spotted wing drosophila/berry system.
The most important progress in managing the new pest will be achieved by implementing cultural practices that deny spotted wing drosophila their breeding sites and kill immature spotted wing drosophila inside infested fruit. This can be accomplished in a strawberry field by removing marketable berries quickly, before they are infested, and removing and properly disposing unmarketable fruit and the immature insects they may harbor. Any fruit not to be sold should be collected and buried or collected, covered, and sent to municipal disposal sites. It would be a big benefit to blueberry farms if strawberry growers destroyed their plants immediately after final harvests to reduce carryover into that crop.

More Management Methods

Additionally, applications of appropriate insecticides should be made as spotted wing drosophila appear. Insecticides useful to control adults where permitted include malathion, diazinon, fenpropathrin, bifenthrin, zeta-cypermethrin, spinosad, and spinetoram. There are no insecticides available for egg or maggot control inside fruit.
Recurring applications at close intervals may be required under heavy pressure, for populations of mixed life stages, or when flies move from outside sources into fruiting crops. When these conditions are absent, applications could be held to one lifecycle or longer, probably 10 days to two weeks or longer during much of Florida’s fruit production period.
A component of tephritid management often includes large droplet applications of protein-based bait like Nu-Lure (Miller Chemical & Fertilizer Corp.) mixed with an insecticide or GF-120 (Dow AgroSciences) bait manufactured with spinosad insecticide. It is uncertain if such tools can be effective for spotted wing dosophila under any circumstances. However, problems maintaining adequate moisture likely will exist in the bait residues used in Florida’s spotted wing drosophila/berry systems. And it may be problematic to deliver sufficient quantities of effective bait and toxicant mixture in an environment of heavy feeding pressure by common drosophilid flies.
Production by vigilant and responsive berry growers in Florida probably will not be reduced by this new pest, so long as the present management tools remain effective and available and growers cooperate to manage spotted wing drosophila throughout the area. New management measures must be developed, though, to help assure long-term control of the pest and to reduce the impacts that presently available insecticides can bring to bear on Orius spp., Phytoseiulus persimilis, and other naturally occurring or applied beneficials useful in Florida berry pest management.