Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
I have gotten three reports in just the last few days and have confirmed two of them as Spotted Wing Drosophila (SWD, Drosophila suzukii) infestations in blueberries in South Central and Southern Maryland. These infestations started out just like they did last year in blackberries and raspberries. Growers noticed that berries were starting to rot prematurely on the plant and after a short time the berries fell to the ground (Fig. 1). If you look closely at some berries you can see tiny puncture marks in the fruit where the female SWD fly used her ovipositor to saw into the ripening fruit and place her egg inside the berry (Fig. 2). This egg then hatches and the maggot feeds in the berry. The maggots will feed for about one week and then pupate either in the berry or just outside of it. On one farm there is probably going to be about a 20-25% fruit loss and on the others it could be somewhere between 35-60%. The question then becomes what can be done now? Unfortunately there is not much that can be done other than try to reduce the amount of berries that become infested by spraying every 5-7 days. The infestation will be slowed, but the fly population will be very difficult to control because there will be so many other sources of rotting fruit for the adults to lay their eggs and the larvae to develop in.
What needed to be done was for growers to use SWD traps to try to detect the presence of the adults BEFORE they laid eggs in the fruit. Detecting the larvae in the infested fruit is too late to implement an effective management program. If the adults are found early enough insecticide applications can be timed better and can prevent or at least slow an infestation. I can’t emphasis enough that growers of small fruit anywhere in Maryland or the Mid-Atlantic need to have the SWD traps out NOW in their small fruit and they need to check them twice per week for the adult males (Fig. 3). We are not sure why these particular farms have these bad infestations; the growers did not do anything to bring about the problem. Some insecticides that have been shown to work include: Pyrethroids: fenpropathrin, zeta-cypermethrin, and lambda-cyhalothrin; Neonicotinoids: acetamiprid and imidacloprid; Spinosyns: Radiant and spinetoram and the organophosphate Malathion, which has a short postharvest interval (PHI), making it useful to use during harvest (fenpropathrin (Danitol) also has a short PHI). Be sure to READ THE LABEL before applying any insecticide to your crop as some chemicals can be used on some fruit, but not others and postharvest intervals can also vary by fruit crop. Pesticide applications should be rotated to reduce the chance of resistance developing. A more detailed description of the SWD fly, its biology and how to monitor and manage it can be found at the UME fact sheet: http://www.agnr.umd.edu/Extension/agriculture/mdvegetables/files/spottedwingedfly.pdf
Figure 1. Blueberries on ground from SWD damage
Figure 2. Blueberry with SWD punctures (arrows)
Figure 3. SWD adult male