What to watch for: When treating for SWD

Female SWD on raspberry at the Upper Mountain Research Station, Summer 2010. Photo: HJB

As spotted wing drosophila (SWD) is detected in more and more locations throughout the southeast (Georgia, Florida, 4 locations in South Carolina, 13 locations in NC), many small fruit growers are treating their crops. See here for lists of registered pesticides in blueberries, grapes, strawberries, and caneberries. For several reasons (efficacy, cost, and preharvest interval), organophosphate and pyrtheroid insecticides are among the most commonly used materials against SWD. The current management recommendations for SWD are weekly pesticide applications, rotating modes of action, beginning when fruit are ripening and stopping at the end of harvest and increased sanitation (frequent & thorough harvest, cull removal & destruction). These management recommendations represents a potentially large increase in pesticide use, at least in the short term, in these small fruit crops.

While these classes of insecticides are effective against SWD, they come with risks as well. The key risk, from a pest management stand point, is non target impacts on other pests and beneficial insects. While we we cannot anticipate all of the non target impacts, some are clearly likely.

The most likely non target effect of SWD treatments is the possibility to flare spider mites.

Twospotted spider mite female and eggs. Photo: HJB

Spider mites are economically significant pests of strawberries, grapes, and caneberries (raspberries moreso than blackberries). Both organophosphate and pyrethroid insecticides have been documented as flaring spider mites.

Strawberry, grape, and caneberry growers should scout their plantings for spider mites prior to beginning SWD treatments. A good rule of thumb is to observe at least 10 leaves or leaflets per acre or per variety block, if they are smaller than an acre. Spider mites can be observed and counted with a 10x hand lens. If spider mites are present, planting should be treated with a miticide before beginning organophosphate or pyrethroid treatments. North Carolina blueberry growers more commonly use these insecticides are rarely, if ever, have issues with spider mites, and I do not anticipate a need to manage mites in blueberries.

Spider mites may not be the only non target (unintentional) pest made worse by SWD treatments. Organophosphates and pyrethroids are broad spectrum materials, meaning they kill many different types of insects, including beneficial predators. The insects these predators may control could increase in their absence, but we cannot necessarily predict which insects these may be. Growers treating for SWD should be vigilant and scout their fields at least weekly to assess whether any new or unexpected insect or damage is present.