Spotted Wing Drosophila Update
The spotted wing drosophila, Drosophila suzukii.
Photo by Hannah Burrack, North Carolina State University, Bugwood.org.
If you're a fruit grower, I hope by now you know that there's a small insect causing big damage to soft fruits that ripen in late summer and fall. It's called spotted wing drosophila (SWD), and it has been spreading quickly over the past few years.
SWD was first found in California in 2008. Then in 2009 it spread to Florida, Oregon and Washington. From Florida it moved up the East Coast to arrive in New England in 2011, and last year it was found across most of the Northeast, but not all over the northernmost part of the region. If it hasn't arrived in your area yet, this year it probably will.
SWD is related to the common fruit fly (more properly called a vinegar fly), which is known for laying its eggs in rotting fruit. What makes this new fly different is its ability to lay eggs in firm, ripening fruit. The females have a saw-like egg-laying appendage (a serrated ovipositor) that lets them cut small holes in the skin of fruit while depositing an egg in each hole. The eggs hatch into tiny white larvae that feed on the fruit's flesh. The larvae develop for about 10 to 14 days before emerging as adults, ready to mate and lay more eggs. Females can lay 300 eggs in their lifetime, so the SWD population can rapidly increase when conditions are right.
The male adults give this species of fly its common name; they have a single dark spot on the front edge of their wings. Females lack the spots on their wings.
Crops most susceptible to SWD damage are fall raspberries, cherries, blueberries and day-neutral strawberries. Peaches and grapes are also among the vulnerable cultivated crops. Once attacked, damage to fruit is not immediately visible. It can look perfectly ripe and firm, but it will rot just before or soon after harvest. This is because SWD makes only a small pinprick on the fruit during egg-laying. By itself, the hole does no harm, but egg-laying introduces microbes into the fruit, so within a few days the fruit's flesh starts to break down, leading to discoloration and eventual collapse.
If you look closely, the small white SWD larvae can often be seen inside the fruit at this point. They are easy to see in raspberries, but in some crops, such as blueberries, they are less obvious. That's why I got several calls from growers last year in late August asking: "What disease is making all my blueberries rot?"
SWD also attacks many wild plants and cultivated ornamentals. This means there are a lot of hosts for this pest in the landscape. Wild raspberries, pokeweed, autumn olive and Korean dogwood are a few. The Northeast appears to be a favorable environment for SWD, given our diversified farms and landscape. The pest can go from one host to the next as they ripen. Alas, our cold winters don't seem to offer much control of SWD, as it hails from Asia, where it survives in northern Japan and other climates just as cold as ours.
However, all is not lost when it comes to growing fruit. SWD has a preference for softer-fleshed fruit with thin skins, so crops like apples, pears and thick-skinned grape varieties do not appear to be at risk. In theory, SWD can also attack some vegetables, like tomatoes, but their skins seem to be thick enough to prevent attack, except when a fruit has cracks.
Last year, SWD was not found in large numbers until about mid-August to mid-September, depending on the location. The first SWD were caught in traps anywhere from mid-June to mid-July, so it seems to take a while for the population to build up. Therefore, earlier-ripening varieties of fruits, like summer-bearing raspberries and strawberries, appear to be at less risk than their relatives that ripen in the late summer and fall. Early-ripening blueberry varieties have also been observed to have less SWD damage than those that ripen at the end of the blueberry season, even though the difference in timing may be just a few weeks.
This small planting of blueberries was covered with netting to protect against SWD damage. The netting mesh must be smaller than 1 millimeter square to exclude the flies, and it has to be placed over the crop before any fruit begins to ripen.
Photo by Vern Grubinger.
Timely and complete harvesting is one way to minimize SWD damage. Picking crops as soon as they start to ripen and keeping them picked clean may avoid the buildup of SWD in a planting. Prompt refrigeration or freezing of fruit upon harvest will reduce losses to disease and slow the development of any eggs or larvae present. Note that there is no known risk to human health posed by ingesting SWD, so as gross as it sounds, eating a few eggs or larvae is not hazardous, and they are so small you won't even know they are there.
Netting may help to exclude SWD from vulnerable fruit plantings. Insect netting with a mesh opening smaller than 1 millimeter square may keep the flies from getting to the crop. Obviously, to be effective, the netting needs to be placed over the crop before the fruit starts to ripen, and it must be securely sealed along the ground and at all entry points to prevent flies from getting in. Netting offers an additional benefit, because it can also keep birds from eating ripe fruit. Care must be taken to avoid ripping the netting, and in larger plantings a structure may be needed to hold the netting up off the plants so workers can easily get in to pick the fruit. Prompt removal of netting when it is no longer needed may extend its life by reducing the length of time it is exposed to sunlight.
A spotted wing drosophila on a raspberry. They prefer soft-fleshed fruit with thin skins.
Photo by Hannah Burrack, North Carolina State University, Bugwood.org
ProtekNet Ultimate Plus (25 grams per square meter) or ProtekNet Standard Plus (80 grams per square meter) nettings should be effective for exclusion, but they are not inexpensive. These are available from Dubois Agrinovation (www.duboisag.com) in Quebec. Netting may make the most sense for small-scale organic plantings.
For many growers, insecticides will be part of an SWD management strategy. To be effective, you must have the equipment to get good coverage of all surfaces on the crop, especially leaf undersides. Spraying must start when SWD are in the area and fruit is present and just starting to change color. Insecticides need to be applied weekly as a crop starts to ripen, and pesticide types must be rotated to prevent SWD from developing resistance. Summaries of the different insecticides labeled for berry crops and tree fruit crops have been developed by the University of Connecticut Cooperative Extension; links to these and other helpful information are available at www.uvm.edu/vtvegandberry/SWDInfo.html.
In addition to making sure that a product is labeled for the crop, it's important to consider the relative efficacy of different materials and the preharvest intervals that determine how long you must wait to harvest after spraying. Research has found that the addition of 2 pounds of table sugar per 100 gallons of spray enhances the efficacy of insecticides for SWD control, except for pyrethroids.
Research and extension personnel have been using traps to monitor the arrival and population buildup of SWD. This pest is attracted to several kinds of bait, including vinegar, alcohol and yeast. Traps can be made from red plastic beverage cups that contain both bait and a drowning solution. The challenge is to develop a bait formulation for the traps that will be attractive enough to SWD to compete with ripening fruit and thus provide an early warning system. Very effective baits would offer the potential to "trap out" enough of the fly population to actually reduce crop damage.
Trap design and bait formulation are evolving based on field experience and laboratory trials. Current suggestions for SWD trap design and bait formulation are posted online; see the link above. Undergraduate integrated pest management (IPM) scouts at the University of Rhode Island developed what appears to be an effective trap design in which the fermenting bait (flour dough plus yeast) is placed in a small, screened cup that is then placed inside a larger container with a drowning solution (cider vinegar plus ethanol) to capture flies. Traps must be placed in a shady location within the crop.
Managing SWD requires multiple tactics, which include the following:
- Eliminate wild SWD hosts such as pokeweed and brambles to the fullest extent possible, though this may not be very effective since there are so many hosts in the landscape.
- Follow extension reports on SWD's arrival in the region.
- Cover small plantings with insect netting (less than 1-millimeter mesh) before fruit begins to ripen.
- Plan ahead to protect crops with a rotation of insecticides, conventional or organic.
- Set up traps to monitor the effectiveness of the insecticide program; intensive use of traps has the potential to attract SWD away from a crop and may complement insecticide use. Placing traps no more than 30 feet apart is suggested for "trapping out," but more research is needed.
- Harvest all fruit as soon as it is ripe; remove and destroy all rotten, infested fruit in an effort to limit SWD population buildup.
- Promptly place harvested fruit in a refrigerator or freezer to protect fruit quality and encourage your customers to do the same.
- After harvest, remove any remaining fruit from the field to reduce overwintering SWD populations.
- If you have not yet planted a vulnerable fruit crop, select early-ripening varieties.
- Prune plants to promote an open canopy, which will allow good insecticide coverage.
- If you have an SWD problem, consider informing your customers that there is a new insect pest and you are doing your best to manage it, but they may find some small larvae. Never use the word maggot.
Growers who have been hit with SWD in previous years are now taking many of the steps mentioned above to manage this pest. It is not something you can ignore. If you grow any vulnerable crops and haven't had serious SWD problems yet, don't be complacent - put a plan in place. The good news is that many growers, researchers and extension personnel across the country are working hard to learn what combination of methods will be sustainable for managing SWD over the long term, since it appears to be here to stay.
The author is a vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at firstname.lastname@example.org. Thanks to Dr. Richard Cowles at the Connecticut Agricultural Experiment Station for his research on SWD and for his contributions to this article.