A Little Bug Could Cause Big Problems

Spotted Wing Drosophila
By Vern Grubinger

A nasty new pest of ripe fruit has arrived in your area, or is heading your way. Spotted wing drosophila (SWD), also known as the Asian fruit fly, has been buzzing across the country over the past few years. SWD was found in California is 2008; then Oregon, Washington and Florida in 2009; Michigan and North Carolina in 2010; and finally arriving in New England in 2011.

Description of the fly

SWD is native to Asia, and it's a little bigger than a common fruit fly. What makes it different is that the females have saw-like egg-laying appendages (serrated ovipositors) that let them cut into ripening (not rotting) fruit to deposit their brood. The eggs hatch into larvae that feed on the fruit. The larvae then go through three stages, or instars, before they pupate and emerge as adults, which live for 20 to 30 days. Females can lay 100 eggs a day, so the SWD population has potential to quickly increase, exploding in number much like spider mites can. The male adults are easy to identify and give the species its common name because they have a single dark spot on the edge of each of their wings. SWD survives in northern Japan, so clearly it can overwinter in cold climates here.

This yellow sticky card has both SWD and non-SWD flies on it. The adult male SWD, circled in red, has the characteristic dark spot on the leading edge of each of its wings. Growers must be able to properly identify SWD in order to monitor the population.
Photo courtesy of Vern Grubinger.

Host range

SWD attacks many plants, both cultivated and wild. The latter is of concern since that means there are a lot of reservoirs for the pest in the natural landscape, including wild raspberries, pokeweed and autumn olive. SWD has been reported in most berry crops, grapes, cherries and other tree fruits. It has a preference for softer-fleshed fruit. Putting these facts together, the Eastern seaboard appears to be a good environment for this pest, given our diversified farms and landscape, which could allow SWD to go from one host to the next as they ripen.

Fall raspberries, grapes, blueberries and day-neutral strawberries may be among the most vulnerable crops. One problem is that damage is not always immediately visible when fruit is attacked. Raspberries can look perfectly ripe and firm but in the cup of fruit will be small maggots wandering around, then fruit collapses as disease sets in. This has also been seen in grapes and blueberries. SWD leaves only a small pinprick on fruit from egg laying. This wound creates an entry for microbes, so within a few days fruit flesh starts to break down, leading to discolored regions and eventual collapse. At this point, the small, white larvae can be easily detected.

Monitoring for SWD

Simple traps work well for detection, but not once fruit is ripe because they will catch other fruit fly species, and it may be difficult to distinguish between the species. It is critical to put out traps before fruit starts ripening. You may trap some SWD on any day much above freezing, even in winter you.

Traps can be made using a plastic 32-ounce cup with several 3/16 to 3/8-inch holes around the sides of the cup, leaving a 3 to 4-inch section without holes so you can pour out liquid bait. The holes can be drilled or burned with a hot wire. Place 1 to 2 inches of pure apple cider vinegar into the trap as bait. To help attract flies and ensure that trapped flies don't escape, place a small yellow sticky card in the cup. Hang or stake traps in the shade near ripening fruit. A "super bait" formula may be a more effective attractant; it is described in the links below.

In addition to monitoring, a similar system may be used to "trap out" lots of SWD by placing an inch or 2 of liquid bait in the bottom of many 5-gallon pails. Be aware that liquids in a large container can pose a drowning hazard to toddlers.

Managing SWD populations

Physical control (exclusion) might work, using a screen with mesh less than 1 mm, but you may need to bring pollinators inside mesh to service the crop. If applying insecticide, start when fruit are present and just starting to change color. Pyrethroids, spinosad, malathion have worked, and they simply need to make contact with flies. However, using these broad-spectrum sprays every five to seven days is not a good IPM approach, and some materials have long days-to-harvest limitations. There could also be rapid selection for resistance, especially to spinosad on organic farms, since other organic materials like pyrethrins have not shown efficacy.

Researchers are studying other strategies such as baiting insecticides with sugar and salt, since SWD has automatic response to these flavors when they encounter it. In the future we may be able to use them in very small quantities in traps or perimeter sprays, but this is not yet known. In the lab kaolin clay (Surround) is a feeding deterrent and will also kill SWD in a few hours, but applying it to fruit and removing the residue is problematic. For now, management tactics include: eliminate wild hosts (pokeweed and brambles especially); monitor with traps to know if SWD is present; spray crops as they're ripening if flies are detected.

Research and extension personnel met this past winter to collect more information and to plan for coordination of monitoring for SWD in 2012. At The Connecticut Agricultural Experiment Station, extensive laboratory studies are underway to better understand the behavior of SWD and how to optimize trapping and use of insecticides.

Current advice to growers

While there is much we do not yet know about this pest and how to manage it, here are some actions you can take this season:

1. Familiarize yourself with this pest; you must be able to distinguish it from other fruit flies. It arrived in New England last year, it attacks many types of firm, ripe fruit, is winter hardy, and the population can quickly increase.

2. Set traps to monitor for the arrival of SWD. Traps with various baits including apple cider vinegar are easy to make and will be more effective when not competing with ripe fruit, so set them up before fruit crops start to turn color. Other regions of the country found SWD populations did not build up until early summer, and then it was abundant into fall. Fall raspberries, day-neutral strawberries, grapes and blueberries may be our most vulnerable crops. I think it's worth monitoring in June bearing strawberries, too.

3. Know what and when to spray. Materials with the best efficacy appear to be: spinosad (Entrust), spinetoram (Delegate), malathion, advanced generation pyrethroids (Warrior II) and Lannate. There will be considerable selection for resistance if materials like Entrust are used too often. It is important to time the first application of insecticide to when SWD are known to be present and fruits are just starting to ripen.

4. Consider netting on a small scale. With high-value, high risk-plantings (e.g. fall raspberries) it may be possible to exclude SWD by covering with netting that has a mesh opening less than 1 mm.

5. Use postharvest practices to reduce overwintering populations. Clean up and remove as much unharvested fruit from the field as possible. Once all fruit has been harvested the flies are active into late fall. This is the time to set up monitoring traps or pails to try and trap out adults before they can overwinter. There may be many flies caught when no other food is available for them. The bait will need to be changed frequently to maintain trap efficacy.

The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.