Spotted-wing Drosophila: Management in Home Plantings

Fact Sheet No. 5.596 Insect Series|Home & Garden

Whitney Cranshaw1 it develops within ripening fruit rather than Spotted-wing drosophila (SWD)2 is an on yeasts. Quick Facts insect only recently found in Colorado that Adults of the spotted-wing drosophila has proven to be very damaging to several are tiny flies (ca 1/0-inch length, 1/5-inch • Spotted-wing drosophila kinds of fruit crops. Small fruits, notably wingspan), light brown and in general ap- is a small fly that develops late bearing raspberries and strawberries, pearance, fairly typical of most other small within many kinds of fruits. are at particular risk of damage. Damage fruit flies. The males are distinguishable It is particularly damaging is caused by the developing larvae, which from other species of Drosophila by having to late fruiting plantings of feed within fruit causing it to rapidly soften a dark spot towards the tip of the wing, a raspberries, blackberries, and (Figure 1). “spotted wing” (Figure 2). Females are less strawberries. Spotted-wing drosophila is native to easily distinguished, requiring some mag- Japan and was first observed in North nification. They possess a structure used to • The young stages are tiny America (California) in 2008. It then spread lay eggs (ovipositor) that is relatively large maggots that feed within rapidly throughout much of the United and uniquely serrated, which allows them berries and cause them to States and Canada. First detected in Colo- rapidly soften. rado (Fort Collins) in 2012, it has since been found to occur in numerous sites in both • Regular picking of all ripe eastern and western Colorado. fruit at least twice a week is useful in reducing damage by spotted-wing drosophila.

• Insecticides are available to help control spotted-wing drosophila but they can only be applied during times of the day when pollinating insects Figure 2: Adults of a “vinegar fly” or “small fruit fly” on an overripe peach. These can be common are not actively visiting the insects in homes or areas where fermenting crop. materials are present, but do not attack intact fruit as does the spotted-wing drosophila. Figure 1: Larvae of spotted-wing drosophila in a raspberry.

Life History and Habits Spotted-wing drosophila (Drosophila suzukii)2 is a member of the “small fruit fly” or “vinegar fly” genusDrosophila . The small fruit flies are familiar insects to many peo- ple, sometimes found abundantly indoors, where they feed on yeasts associated with overripe fruit or the sediment of beverage containers. An important difference in the habit of the spotted-wing drosophila is that ©Colorado State University Extension. 3/17. 1 Whitney Cranshaw is a Professor and Extension Figure 3: Comparison of a female (left) and male extension.colostate.edu Specialist of entomology at Colorado State University (right) spotted-wing drosophila. Photograph 3/2017 by Eric LaGrasa, courtesy of the Washington Department of Agriculture. 2 Drosphila suzukii Diptera: Drosophilidae to insert eggs into thin-skinned fruits sheltered location. In 4-15 days, adults (Figure 3). emerge from the pupae to mate and lay Winter is spent in the adult stage with eggs continuing the cycle (Figure 6). the flies remaining in a semi-dormant Optimal conditions for development condition (diapause) during the cold are around 680-770F (200-250C) and sev- months. During this time they may use eral generations are produced annually. many different sites for winter protection, Populations typically start to increase but areas within the skins and husks of rapidly in early July. The highest numbers decaying fruit can be particularly impor- of insects are usually present in August tant places where they will survive winter. and September. With the return of warm weather in spring Spotted-wing drosophila can breed the adults will emerge and seek out hon- and develop within a wide variety of eydew, nectar and oozing sap to sustain Figure 6: Adult male (left) and female (right) ripening thin-skinned fruits, including spotted-wing drosophila. Photograph courtesy of themselves. Beverly Gerdeman, Washington State University. various berries, tree fruits, and fleshy fruits As ripening fruits become available in produced by various trees and shrubs late spring and early summer, egg laying (Table 1). They can also develop in ad- resumes. When laying eggs, the female Table 1. The relative importance of ditional fruits, such as apples and crabap- creates a small slit in fruit skin and inserts various fruits for spotted-wing dro- ples, after they have become overripe. 1 to 3 eggs. This is repeated over a period sophila (SWD), based on Colorado surveys. of weeks and a single female can lay up to 350 eggs in her lifetime (Figure 4). Most Highly Favored Host Plants Management of Spotted- Raspberry Strawberry wing Drosophila Blackberry

Moderately Important SWD Host Plants Trapping/Monitoring Chokeberry This insect can easily be captured in Elderberry Sweet cherry traps baited with apple cider vinegar and Nanking cherry other attractants. A typical trap design Tart cherry uses a red plastic cup with a clear plastic Plums Redtwig dogwood lid to keep out water (Figure 7). The flies Currant (particularly Red Lake) enter the trap through a series of holes Gooseberry (3/16” to ¼” diameter) punched along the Apples, Crabapples (after fallen on the ground and thoroughly softened sides near the top of the cup. The trap is hung from the plants using a string or Figure 4: Eggs of spotted-wing drosophila laid on Other Fruiting Plants that Support SWD twist tie that threads through two of the a strawberry fruit. Photograph courtesy of Hannah Regent serviceberry Burrack, North Carolina State University. European cotoneaster holes. Scarlet Hawthorn, Arnold Hawthorn Yew Depending on temperature, the eggs Grapes (after fully ripening) can hatch in 12-72 hours. The larvae, tiny Tatarian honeysuckle Viburnum 'Emerald Triumph' maggots pale in color, will then feed in- Blueberry side the fruit (Figure 5). They become full grown in approximately 5 to 7 days after Fruiting Plants Grown in Colorado that Have Not Been Observed as SWD Hosts ) which they transform into pupae. Larvae Asparagus* may either remain inside the fruit to pu- Barberry pate or crawl away to pupate in another Cactus Cotoneaster (except European) Bittersweet Chokecherry* Euonymus 'Recap European' Hawthorn (except Arnold, Scarlet) Figure 7: Life cycle of the spotted-wing drosophila. Junipers Figure courtesy of Cornell University/NYS IPM Mock strawberry Program. Mountain-ash Cheyenne privet (Ligustrum) Rose* A small amount (one or two ounces) Russian olive Skunkbush of attractant is then put in the bottom of Sugar hackberry the cup. Apple cider vinegar is probably Snowberry the most commonly used attractant and Viburnum (except Emerald Triumph) Virginia creeper is quite easy to use. But there are many other effective lures, typically involving Figure 5: Larvae of spotted-wing drosophila in a * These plants have been reported elsewhere red wine or various fermenting materials, strawberry fruit. Photograph courtesy of Hannah as being capable of supporting development Burrack, North Carolina State University. of spotted-wing drosophila alone or in combination. Adding a drop or two of detergent can improve capture. The trap should then be placed in a the fruit. Refrigerated to 350F (20C) will shaded area, hung within the crop near prevent any further development of in- the fruit. Traps should then be checked sects and kill those that are present within on a regular basis, at least once a week. about 3 days. When checking a sample dump the Any damaged fruit that is not har- contents into a shallow light colored tray. vested for consumption should also be To then identify spotted-wing drosophila removed and destroyed. Placing culled one must closely examine all the fruit flies fruit in a plastic bag and allowing it to in the sample. A bit of magnification is heat in the sun can reliably kill develop- very useful for this, as it is necessary to ing insects. Putting infested fruit in a observe the small spot on the wing of compost pile is not a reliable way to kill the males to determine if SWD is pres- developing SWD. ent. (These traps will collect other species of small fruit flies but they will not have Sanitation a spot on the wing. Female SWD also Many kinds of trees and shrubs pro- lack the spot on the wing.) Record the duce fruits that can support the develop- number of male spotted-wing drosophila ment of spotted-wing drosophila (Table each time the traps are examined. Add 1). Some of these can be particularly fresh bait and rehang the trap after each important for this insect and it is useful to sampling. remove these alternate foods. Dropped Monitoring spotted-wing drosophila apples and crabapples that soften on with these traps is useful for three rea- the ground should be cleared out. Fallen sons. For one, it can let one know if the peaches and plums are also important insect is present at the site, as many areas food sources that are relatively easy to have not yet become infested. Where remove. the insect is known to be present, traps can indicate when the adult flies begin Crop Canopy Management to move into susceptible crops. And, if Spotted-wing drosophila prefers traps are regularly checked and captures dense shade and cooler areas in the recorded, it is possible to detect if there canopy. Pruning crops in ways that open are increases – or decreases – in the up the crop canopy can deter insects from numbers of insects present. This is useful colonizing the crop. information when deciding what type of management might be needed. Cultivar selection Although these traps are very effective in capturing spotted-wing drosophila, The crops most badly damaged by they only capture a fraction of the flies SWD are late season cultivars of raspber- that are attracted to the traps, perhaps a ries, blackberries and strawberries. Shift- ing to cultivars that bear early in the year Figure 8 a, b: Two trap designs for spotted-wing third. Furthermore, SWD is a fairly mobile drosophila. One (a) is a red plastic cup with a insect and new flies can be expected to (“June bearers”) can largely avoid damage clear top with entry holes punched near the top. continuously move into plantings. As a since SWD populations are low when The second (b) uses a clear container but has red crops ripen. tape to attract the insects and has holes in the top. result, when used alone, trapping has not Both designs can use lures such as vinegar, red proved an effective method to control wine, or similar fermenting material. Photographs this insect. Crop coverings courtesy of (a) Bob Hammon and (b) Deryn Nets of fine mesh (1 mm) can be used Davidson, Colorado State University. to exclude flies from plants. Such cover- Cultural and Mechanical ings need to be in place before ripening Controls fruit is present that will attract adult SWD. Chemical Control The netting should also be draped in a Insecticides applied to kill the adult Frequent harvest intervals way to prevent direct contact with the flies can be effective for control of spot- As fruit ripens it should be regularly fruit to avoid eggs being laid through the ted-wing drosophila. However, in home and thoroughly picked. If done every 3 or mesh. gardens options of available products are 4 days, this can prevent insects from suc- The use of these coverings will also quite limited (Table 2). Also, some SWD cessfully developing in the crop, reducing exclude other insects, including pollina- susceptible crops flower and produce fruit their numbers. tors. This can be an important consider- over an extended period, such as raspber- Harvested fruit should either be imme- ation when the crop is not self-fertile and ries. In these situations special care must diately consumed or stored in a refrigera- benefits from cross pollination. be taken to prevent killing bees and other tor. If left unrefrigerated any insects pres- pollinators visiting the crop. ent will continue to develop and damage Products containing the active ingredi- Table 2. A summary of insecticides ent spinosad (spinosyns) can provide con- useful for control of spotted-wing dro- trol for about 5 to 7 days. Less commonly sophila on raspberries, blackberries available is the insecticide acetamiprid, and strawberries in home gardens which is similarly effective. Insecticides Active Ingredient: Spinosad/Spinosyns that have very short residual activity, such Trade Names: Bonide Captain Jack’s Dead Bug Brew, Natural Guard Spinosad Land- as pyrethrins or insecticidal soaps, have scape & Garden Insecticide, ferti-lome Borer, not proven to be effective for control of Bagworm, Leafminer & Tent Caterpillar Spray, spotted-wing drosophila. others Preharvest Interval (caneberries, strawber- Thorough spray coverage is important ries): 1 day for effective control. Particular attention Minimum Interval Between Treatments: 5 should be given to cover the underside days Pollinator Protection Statement: “This of leaves and the interior of plantings product is toxic to bees exposed to treatment where SWD flies spend the most time. for 3 hours following treatment. Do not apply Adding a small amount of sugar to the this pesticide to blooming, pollen-shedding or nectar-producing parts of plants if bees may spray solution (2 tsp/gal) has been shown forage on the plants during this time period.” to improve control when using spinosad or acetamiprid. The adult flies feed on Active Ingredient: Acetamiprid Trade Names: Ortho Flower, Fruit and Veg- sugars and by adding sugar to the spray etable Insect Killer solution it increases its effectiveness by Preharvest Interval (caneberries, strawber- producing a bait in addition to a cover ries): 1 day Minimum Interval Between Treatments: 7 spray. days If plants continue flowering when sus- Pollinator Protection Statement: “This ceptible ripening fruit is present potential product is toxic to bees exposed to direct treatment. Do not apply this product while conflicts with pollinating insects can oc- bees are actively visiting the treated area.” cur. When this happens there are specific label instructions that must be followed to avoid killing these desirable species. No pesticides, of any type, may be applied to a flowering crop during periods of the day when bees are actively present and foraging in the crop. The insecticides list- ed in Table 2 can be safely applied if they are used during times of the day when bees are no longer on the crop, such as dusk or dawn. These periods of time may also be most appropriate for control since the adult flies are most active in the crop during these times.

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