To protect crops from SWD infestation, once susceptible fruit is ripe and SWD is in your area, manage them aggressively. Using a combination of tactics is better than relying on one; and is the foundation of integrated pest management (IPM). Talk to your local Extension office about how to monitor for SWD, read Extension newsletters and alerts, and know your crops’ growth stages. When adult SWD are present on your farm AND fruit are ripening, it is time to protect fruit and reduce SWD population growth with insecticides, unless exclusion netting is in place.
Raspberries, blackberries, and blueberries are at high risk of SWD infestation. Fall-bearing and late maturing varieties are at far greater risk than early maturing ones, because SWD populations build exponentially to very high levels in late summer and early fall. June-bearing strawberries may escape injury, whereas late summer fruit or day-neutral varieties may suffer damage. Cherries, both tart and sweet, elderberries, peaches and plums are also susceptible, but harvests may occur before SWD populations buildup. Thin-skinned grapes can be infested directly, though cracked or damaged berries are more susceptible.
Female SWD can lay eggs directly into sound fruit. They prefer ripe fruit, but can lay eggs in fruit even as ripening begins. Therefore, keep an eye on fruit development in your fields. Egg laying activity is greater under conditions of low light, such as dawn and dusk or in dense plant canopies, weed-shaded areas in a planting, or parts of the planting shaded by adjacent woods or buildings. Adult SWD, in general, are most active during cool, humid times of the day. We have had significantly cool and moist weather this spring and first catch of SWD has occurred in a few areas in NY already.
Examine your plantings for conditions that promote SWD infestation and take steps to eliminate them. Although we cannot change the weather, we can alter conditions in the planting to reduce the cool, dark, humid areas preferred by SWD. Canopy, weed and irrigation management will make the environment less favorable. If your fruit planting lends itself to full enclosure, consider exclusion netting to keep SWD out.
- Canopy - Pruning and training systems must maintain an open canopy to increase sunlight and reduce humidity. This will make plantings less attractive to SWD, will reduce SWD activity and will improve spray penetration and coverage. Added benefits include improved fruit color and flavor promoted by sunlight, easier picking by workers and customers, and easier weed management. Pruning tactics have been developed to achieve excellent fruit yield and open the canopy. I will detail these in a later blog. Although the best time to prune is over, knowing different strategies now will help you in the future.
- Weeds - Mow row middles and field edges routinely to reduce preferred habitat for SWD within and around the planting. Eliminate weeds within rows to increase sunlight penetration into the canopy, reduce preferred habitat, and improve spray penetration into and deposition on the canopy.
- Irrigation - Repair leaking drip lines and avoid overhead irrigation when possible. Allow the ground and mulch surface to dry before irrigating. Eliminate problem areas where water puddles are slow to dry out. Raised beds are essential for raspberry production to reduce Phytophthora root and crown rot and will also help maintain a dry environment under the planting.
As fruit begins to ripen, know if SWD has been found in your area. If you are monitoring SWD with your own traps, check them routinely. If feasible, check them daily. It is easier to sort through a small number of vinegar flies caught in traps to look for SWD than it is to sort through 40-400. Females usually arrive first, but males are soon to follow and often caught along with females.
- The blog - Keep up with notices on the SWD blog.
- Newsletters - Read your local Extension newsletters and alerts.
- Map - Review the New York monitoring network’s SWD distribution map.
If SWD is in your area and susceptible fruit is just about ripe, insecticide treatments could begin. This will be especially true in years when SWD arrives early, because SWD populations will build to high levels placing even summer-maturing fruits at risk, particularly when weather conditions are ideal for SWD activity—cloudy, cool, moist. When SWD populations are high, treatments should be applied every five to seven days and repeated in the event of rain. Choose the most effective insecticides with pre harvest intervals that work for your picking schedule. Rotate insecticides according to their modes of action to prevent the development of insecticide resistance. Insecticide sprays will kill or suppress SWD adults, thereby reducing egg laying and slowing population buildup.
- Insecticides – Based on the Cornell Pest Management Guidelines for tree fruit, grapes and berry crops we’ve put together quick guides to the latest list of approved insecticides for use against SWD on berries and on tree fruit & grapes. Always read and follow the pesticide label instructions.
- Resistance management – Insects treated with the same pesticide repeatedly may develop resistance to that pesticide’s mode of action. The Insecticide Resistance Action Committee (IRAC) has developed groupings for modes of action. When materials in one IRAC group are used exclusively over an entire growing season and over years, they are at high risk of becoming worthless as a control measure due to resistance development. Always rotate between IRAC groups, as described on the label.
- Protecting pollinators – If your crop is flowering, that means there are pollinators visiting flowers. Therefore, spraying insecticides will place pollinators at risk of non-target exposure to insecticides, unless these materials are applied when pollinators aren’t active, such as during dawn or dusk, or when the crop is no longer flowering. This can be particularly challenging for raspberries and blackberries, which may have a long bloom period that spans fruit ripening. Organic-approved products with the active ingredients spinosad, azadirachtin, and pyrethrum are toxic to pollinators. No matter which insecticide you choose, always read the label and keep pollinators safe from insecticide exposure.
Regularly inspect fruit in the planting for symptoms and signs, paying close attention to fruit ripening in areas prone to SWD activity—near woods, shaded or wet areas—span a random transect of the planting. Sample ripe fruit and examine it microscopically for egg breathing tubes or check for larvae with salt flotation. Get infested fruit out of the planting so SWD populations don't have a chance to buildup.
- Symptoms - Fruit can be inspected for evidence of larval feeding. Small holes in berries where the eggs were laid may leak juice when the berry is gently squeezed; this is especially diagnostic on blueberry, cherry, and plum. Infested red raspberry fruit may leave a red juice stain on the berry receptacle when the fruit is picked. Fruit with small indents or bruises where the berry surface appears to have flattened or deflated may be damaged. Help with identifying symptoms is found in the fact sheet, Recognize Fruit Damage from Spotted Wing Drosophila (SWD), from Oregon State University.
- Breathing tubes - Egg breathing tubes are two tiny, evanescent, white hairs attached to the egg laid just below the fruit skin. In blackberry, grape, blueberry and cherry it may be relatively easy, though tedious, to find these on fruit in which eggs have been laid, but magnification is essential. A 20x hand lens or loop or a dissecting microscope is needed, and patience. Examine the entire fruit surface. Fuzzy fruit, such as raspberry, are harder to examine because of the difficulty distinguishing breathing tubes from normal plant hairs. To confound the inspection, once eggs hatch, the breathing tubes fall off. You may be able to train your eye to see SWD egg breathing tubes.
- Sample fruit - Salt flotation can be used effectively to keep records of infestation levels in your harvests. At least 100 fruit per block per harvest should be observed for infestation. Immerse fruit in a solution of 1 Tbsp. (14.8 cc) table salt per 1 cup (236.6 ml) water. The salt solution causes larvae to move out of fruit and float into the salt solution. Suggested methods were adapted for NY growers by Laura McDermott in Guidelines for Checking Fruit for SWD Larvae in the Field (pdf).
- Sanitation - Excellent sanitation will reduce SWD populations. Fruit should be harvested frequently and completely to prevent the buildup of ripe and over-ripe fruit. Unmarketable fruit should be removed from the field and either frozen, "baked" in clear plastic bags placed in the sun, or disposed of in bags off-site. This will kill larvae, remove them from your crop, and prevent them from emerging as adults.
Protect your harvests and customer base. Pick only the best and perhaps still slightly firm fruit to help them last longer in your markets. Chilling fruit after harvest is an essential step in prolonging shelf life. Picking crews can pick overripe or suspect fruit into a separate container to get them out of the field. Although the larvae of SWD are safe to eat, most people won’t want to do so. Informing customers about SWD and making sure they refrigerate fruit once home will help them understand how to deal with this invasive insect, and still benefit from eating nutritious and delicious fresh fruit.
- Cool berries - Chilling berries immediately after harvest to 32° - 33° F will slow or stop the development of larvae and eggs inside the fruit. U-Pick customers should be encouraged to follow this strategy to improve fruit quality at home.
- Proactive – Be proactive with your customers. Let them know that you are doing everything you can to manage SWD in your fruit crops. Inform them about refrigerating or freezing fruit as soon as they get home. Highest quality of preserves, jams and jellies will be achieved if prepared soon after purchase.
The take home message for SWD management—use a combination of tactics, choose IPM.