Chemical Control of Spotted Wing Drosophila

This past week spotted wing drosophila (SWD) was detected in two additional counties (Webster and Daviess) following the previous detection in McCracken County. Producers of blueberries, blackberries, red and black raspberries are advised to begin monitoring for SWD if they haven’t already begun. This article addresses some of the issues producers need to consider when spraying during the harvest period. While we address chemical control of SWD, producers are also strongly encouraged to use an integrated approach to SWD management that includes SWD sanitation through clean harvest, post harvest refrigeration, and canopy management.

Figure 1. SWD larvae removed from blackberries using the fruit floatation method. (Photo: Ric Bessin, UK)

Figure 1. SWD larvae removed from blackberries using the fruit floatation method. (Photo: Ric Bessin, UK)

If SWD is detected on a farm with susceptible crops during the harvest period, producers must manage this invasive insect to avoid larvae in the fruit. For many producers, this may mean spraying during the harvest period. Only insecticides recommended for SWD on their respective crops should be used. Other products may not provide satisfactory control. See ENTFACT 230 for a list of recommended insecticides for the different small fruit crops.

Observing PHIs
Because we are spraying periodically during the harvest period, pre harvest intervals (PHIs) are critically important to ensure label compliance and safety. The pre harvest interval is the minimum amount of time that must pass between the end of spraying and the start of picking. The PHI allows for residues to dissipate to acceptable levels. Because small fruit crops are often picked very frequently, it may not be practical to use materials with PHIs longer than 3 days. Insecticides with PHIs of 1 day are more accommodating of picking schedules and U-pick operations.

Preventing Resistance
SWD is able to complete a generation in just over a week with favorable summer temperatures. A general rule of thumb is to switch insecticide modes of action with each new pest generation. That is, we try not to control consecutive pest generations with insecticides that share the same mode of action. Insecticides sharing the same mode of action share the same IRAC group number (these are usually displayed on the product label). So each week we must switch to a recommended insecticide in a different IRAC group. The insecticides in the ‘Very Effective’ and ‘Effective’ groups belong to IRAC groups 1B (organophosphate), 3A (pyrethroid), 5 (spinosyns), and 28 (anthrilic diamides). This provides producers with enough alternatives to switch modes of action.

Obtaining Good Coverage
SWD prefers to hide in shaded places in the canopy during much of the day, so sprays must be able to penetrate into the dense canopy of caneberries to be effective. High pressure and relatively high volume sprays are needed to ensure adequate coverage. Producers should thin dense canopies to allow for good spray coverage.

Reapplication of Sprays
Generally, it is not recommended that spray intervals be extended longer than 7 days with SWD. Spray intervals of longer than 7 days have been associated with poor control. Following a significant rain, I recommend reapplying SWD sprays.

By Ric Bessin, Extension Entomologist

Posted in Fruit