Apr 3, 2017

Monitor, use controls to protect fruit from SWD

Spotted wing drosophila (SWD) is now considered generally distributed in Virginia. Its hosts include apples, blackberries, blueberries, cherries, nectarines, peaches, pears, plums, grapes, raspberries and strawberries. While cherries are a preferred early season host, grapes are a preferred late-season host. Berry crops, grapes and cherries are at greatest risk.

Monitoring. Traps using bait of yeast or apple cider vinegar may be used for monitoring. Traps using apple cider vinegar alone are attractive and less odorous to work with than with yeast added; traps with added yeast may be somewhat more sensitive, but fluid should be replaced with each service of the traps. Traps should be checked at least weekly. Most of the drosophila flies collected will not be SWD, so the flies collected must be checked carefully. Male SWD have a characteristic black spot at the tip of the wings. Females lack this spot, but are slightly larger than other females and have a large, serrated ovipositor, unusual among drosophilids.

Chemical control. Control measures are directed against adults; there are no effective controls for larvae in the fruit. As vulnerable fruit approach ripeness, weekly spray applications should be made. When using organic materials, shorter spray intervals will be needed because of the shorter residual life of botanical insecticides. Insecticides with different modes of action should be rotated in order to delay the development of pesticide resistance. Organophosphates (malathion and phosmet) are effective (check labels for registrations on specific crops), as are pyrethroids (be wary of induction of secondary pests). Spinosyns offer an additional mode of action class, with spinetoram having greater efficacy than spinosad.

If SWD needs to be controlled in a vineyard setting, it may be helpful to make an application just before berries close in clusters, since many oviposition points are in the protected inner surfaces of the cluster. So far, Surround has been used for this purpose experimentally; research is planned on additional options.

When designing a chemical control program for SWD, it will be critical to rotate among different mode of action classes, but also to consider the maximum number of applications per season of each insecticide. The latter issue includes applications applied before SWD becomes active or the host fruit become susceptible. The number of organic tools is limited, and management in organic berries will be a challenge.

Three materials of differing modes of action are spinetoram (Delegate and Radiant), malathion and zeta-cypermethrin (Mustang Max). Malathion is registered on grape, caneberries, peach and nectarine, but not on cherry or apple. The flowable formulation of malathion is safer than the EC formulation, but the flowable formulation may be in shorter supply. When using the EC (oil-based) formulation, use caution if also applying the fungicide captan. The oil can act as a penetrant, potentially causing harm to the vine.

In 2011, apparent tolerance to pyrethrin developed after repeated application in Oregon. It will be important to rotate insecticides of differing modes of action to control this insect. The two main organic options are spinosad, pyrethrin and kaolin. Use cultural controls to help minimize the need for insecticides, such as open pruning to improve air circulation.

Cultural control. Harvest fruit promptly to eliminate breeding sites. This issue should be kept in mind once SWD establishes in an area, since at times grape growers may leave berries in the vine to allow greater development of some harvest parameters. Any overripe or rotten fruit nearby should be destroyed. In vineyards, pomace produced during the crushing process should not be dumped near the producing vineyard block. This can become a source for many SWD.

– Douglas Pfeiffer, Virginia Tech