by Jessica Cole, Patty Lucas, and Ric Bessin
Extension Entomology, University of Kentucky College of Agriculture
Spotted wing Drosophila (SWD), Drosophila suzukii, is a serious pest of soft-skinned fruit and was detected in two locations in 2012 and confirmed by the USDA APHIS. SWD is a new fruit fly pest of soft fruits and berries. Unlike other fruit flies, SWD causes damage when the female flies cut and lay their eggs in healthy fruit. This results in small maggots in the fruits at harvest. A survey sponsored by Kentucky Horticulture Society in 2013 found it to be widespread in Kentucky. There have been reports of infested blueberries, raspberries and blackberries.
SWD attacks a number of different fruiting crops and weeds, but raspberries, blackberries, and blueberries are among the most susceptible crops that we grow commercially in Kentucky. It has also been reported to attack strawberries, grapes, cherries, tomatoes, and peaches in other states. SWD will also lay eggs in and complete development in several weedy or wild hosts including poke weed and honeysuckle.
Figure 1. SWD larvae in a blueberry.
Fruit flies are often associated with damaged, overripe, or rotting fruits and vegetables. This is not the case with SWD. What makes the SWD different is that the female has an enlarged, toothed ovipositor (egg layer) that enables her to lay eggs under the skin of ripening fruits that are otherwise appear sound. Generally, soft-skinned fruit become vulnerable to attack as they begin to soften and turn color during the ripening process. This is in the final 7 to 10 days before harvest. The larvae tunnel and feed under the skin of the fruit and can reach a length of 4 mm. There is often a sunken area at the site of egg laying and damaged fruit may appear to collapse from the internal damage and rots. Fruit that may appear sound at harvest can show signs of infestation within 1 to 2 days. Small maggots can be observed in the fruit or crawling out of harvest containers.
Figure 2. Tip of the female SWD abdomen with the arrow pointing out the enlarged and hardened ovipositor.
Adult SWD emerging in the fall will overwinter and reemerge in the spring to mate. They will lay their eggs in the summer on early ripening fruit. The female SWD will insert their saw-like ovipositor into ripening fruit and deposit 7 to 16 eggs a day and greater than 300 eggs in their lifetime. Depending on the weather the life cycle can be 8 to 14 days with a mid-season life span lasting 3 to 9 weeks. SWD are most active at temperatures above 68°F but will decrease laying eggs in temperatures above 86°F. Eggs deposited in the fruit will hatch in 2 to 72 hours; pupation can take place both inside and outside the fruit and last 3 to 15 days. There may be 10 or more generations per year in Kentucky. This combined with the female’s ability to lay hundreds of eggs may result in very high populations by the end of the season. Early season crops like strawberries may escape SWD injury or be exposed to smaller populations, while fall ripening crops like some raspberries may ripen when SWD populations peak. SWD overwinter as adults.
Adult SWD have a body length of 2 to 3.5 mm, an amber body color with distinct dark banding of the abdomen, red eyes, and clear wings. The adult male has a dark spot on each wing near the tip and two small but distinct dark ‘bands’ on each front leg. The female is harder to identify as it is her ovipositor, banding of the abdomen, and appearance of a cross vein in the wing that distinguishes her from other fruit flies. The dark bands with each abdominal segment are unbroken with the female and the cross vein is clear and distinct.
Figure 3. Male spotted wing Drosophila with the characteristic wing spot.
Figure 4. Spotted wing drosophila female displaying enlarged
ovipositor, characteristic body color and abdominal banding..
Trapping for SWD
Trapping is used to detect SWD in commercial plantings. We recommend placing the traps 2 to 3 weeks before the start of harvest. The traps are placed in a dense part of the canopy of the crop as the female SWD prefer to rest during the day in dark, dense locations. The trap is made of a one-quart deli container with about one inch of apple cider vinegar to which one drop of dish soap has been added (otherwise the SWD may be able to walk on the surface of the vinegar). Sixteen ¼ inch holes are punched below the rim to allow the SWD to enter the traps. Use 2 traps on each farm, they can be in the same crop or different crops. It is possible to move the traps between different fields as the season progress, say from strawberries to blackberries to grapes for example. Traps need to be checked weekly and vinegar is changed weekly. Be sure to dispose of the vinegar outside of the planting.
This trap will capture a large number of fly species and several unimportant fruit flies. It is critical to accurately identify the collected specimens with a powerful hand lens or dissecting microscope. The male SWD are recognized by the single black spot on each wing and the two dark combs on the front legs. The females are recognized by their amber color, and the enlarged ovipositor which is hardened and pointed.
If any SWD are confirmed in the traps, we recommend treating the crop on 5 to 7 day intervals as fruit begin to color and soften through the end of harvest with a recommended insecticide (it may be necessary to respray after a heavy rain). Producers need to follow pre-harvest interval restrictions for the respective insecticides carefully.
Fruit that appear undamaged can be sampled for SWD. To sample fruit for SWD larvae a simple floatation method can be used. Place a small number of ripe, apparently undamaged fruit into a gallon bag. Add sugar syrup (mixture of ½ cup sugar mixed into 1 quart of water) to the bag and seal the bag. Mash the berries, then let the berries settle to the bottom of the bag, any small, white larvae present should float to the top.
This method does not work with damaged, rotten or overripe fruit as these are likely already infested with other non-economic fruit fly larvae. The larvae will appear the same.
CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.
Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!