Spotted Wing Drosophila: A New Threat To Tender Fruit And Berry Crops
Table of Contents
Spotted wing drosophila (SWD) is an invasive vinegar fly of Asian origin that has the potential to cause extensive damage to many fruit crops. It was first identified in North America in 2008 (California). Subsequent field surveys in 2009 have resulted in additional finds in Florida, Oregon, Washington, North and South Carolina, Utah, and British Columbia. National surveys conducted by the Canadian Food Inspection Agency (CFIA) detected SWD in traps collected from Quebec, Manitoba and Alberta this past year.
In early November (2010), the CFIA collected SWD flies from a trap located in a residential area of the Niagara region, Ontario. SWD may have been introduced through the fresh fruit pathway on imported fruit from infested areas. Because of the distribution and potential for movement in infested fruit, it is not possible to regulate SWD. Surveys coordinated by OMAFRA in 2011 indicate SWD has spread to most fruit producing regions.
So far, SWD has not caused economic injury to commercial fruit crops in Ontario, but based on patterns of introduction, establishment and spread observed in other affected areas, growers should be prepared for its appearance in the field again in 2012. Early detection and rapid response are critical for successful management of this pest.
Growers are accustomed to the vinegar flies that are commonly associated with overripe, damaged and dropped fruit. What makes the SWD different is that the female has a heavily serrated ovipositor that allows her to saw through intact fruit and lay her eggs under the skin. Larvae hatch and feed on the fruit, rendering it unmarketable. Disease pathogens and other insect pests can also enter through the egg-laying holes, causing further deterioration of the fruit.
The SWD will attack thin-skinned fruit such as raspberry, blackberry, blueberry, strawberry, apricot, cherry, plum, peach, nectarine, hardy kiwi, elderberry, currant, dogwood, mulberry and sometimes grape.
The SWD adults and maggots are very similar in appearance to the common vinegar fly normally associated with over-ripe, decaying or damaged fruit. They are not closely related to the much larger fruit fly maggots found in fruit, such as the apple maggot, the blueberry maggot, and the cherry fruit fly.
Adults: small (2-3 mm), red-eyed fly with a pale yellow or brown abdomen marked by dark brown complete unbroken bands or stripes.
Figure 1: Adult male SWD are small (2-3 mm) light yellow or brown flies. Wings have a single spot on the leading edge of the wing, close to the tip, and centered around the first wing vein. The antennae are short and stubby. The front feet have dark combs on the first and second segments. Photo Credit: Sheila Fitzpatrick, Agriculture & Agri-Food Canada, Pacific Agri-Food Research Centre, Agassiz.
Figure 2: Wings of females lack spots. As with males, the dark bands at the edge of each abdominal segment are unbroken when viewed from above. Photo credit: E. Beers, Washington State University.
Figure 3; Females have a well-developed saw-like ovipositor. The serrated edge is much darker than the rest of the ovipositor. Note the egg that is still attached. Photo credit: E. Beers, Washington State University.
Eggs: laid under the skin; translucent white, elongate, and about 0.6 mm long X 0.18 mm wide; have a pair of thread-like "breathing tubes" that protrude from the oviposition puncture and are visible on the fruit surface (Figure 4).
Figure 4: The egg's thread-like breathing tubes protrude from the oviposition puncture and are often visible on the fruit surface. Photo credit: E. Beers, Washington State University.
Larvae: up to 6 mm, white, cylindrical, with dark mouth hooks at the front end and a pair of tan caudal spiracles at the rear end; a single fruit may be infested with multiple maggots. After maturing, the larvae partially or completely exit the fruit to pupate (Figure 5). Larvae must be reared to adults for positive identification.
Figure 5: Larvae are up to 6 mm at maturity with dark mouth hooks (left) and tan caudal spiracles (for breathing) (right). The caudal spiracles sometimes protrude from infested fruit. Photo credit: E. Beers, Washington State University.
Pupae: 3 mm in length, brown; two stalks with small finger-like
projections on one end protrude from infested fruit (Figure 6).
Figure 6: Pupae: 3 mm long, brown, football-shaped, and have two stalks with small finger-like projections on one end. Photo credit: BC Ministry of Agriculture.
While it is not possible to distinguish SWD larvae from those of other common vinegar flies, the presence of larvae in intact fruit prior to harvest should be viewed as suspicious. Signs of infestation by SWD may be confused with normal aging of mature fruit. Most uninfested ripe fruit will show mild overall softening after several days. When fruit has been attacked by SWD, early mould, wrinkling and softening can be seen at 2-3 days following egg-laying, and most fruit show obvious damage after 3-4 days (Figure 7, 8, 9). Pin-prick sized scars or holes (Figure 10) from which sap exudes may also be evident. Small holes created by larvae for breathing may also been seen, sometimes with larval breathing tubes visible (Figure 11). Eventually, fruit softens in spots and the skin collapses, with a wrinkled appearance that becomes very obvious after about 5 days. Fruit left hanging following harvest may show signs of heavy infestation (Figure 12).
Figure 7: Raspberries show damage quickly, with scarring and collapse appearing as soon as 1-2 days after egg-laying.
Figure 8:Strawberries deteriorate quickly following egg-laying. Fruit wrinkles and fruit softens, with mould appearing after only 3 days.
Figure 9:Blueberries start to soften after about 3 days. Photo credit: V. Walton, Oregon State University
Figure 10: Infested cherry showing pin-pricks and damage directly under the skin. In this shot, it is possible to see larvae and pupae which have exited the fruit. Photo credit: V. Walton, Oregon State University
Figure 11: Larval breathing holes on blueberries. Photo credit: BC Ministry of Agriculture
Figure 12: Fruit left on the tree following harvest is often heavily infested. Note presence of pupae on the cherry clusters. Photo credit: V. Walton, Oregon State University
We currently have limited field experience with SWD in Ontario, and so must rely on information from other areas where this pest exists. In temperate climates, the SWD overwinters as an adult in protected areas, including heated buildings and heated foundations. They move back into host crops in the spring.
Mild winters such as that experienced in 2012 may result in increased overwintering survival. Monitoring in 2011-2012 showed that SWD adults continued to be active in unheated tunnels until February of this year. Climate models indicate that the SWD will survive Ontario winters, but without further study in our fruit production areas, we won't know for sure how significant or consistent a pest it will be. Hot, dry conditions may have an adverse impact on SWD populations.
Adults are relatively long-lived, so there can be multiple, overlapping generations each year. A female may lay almost 400 eggs during her lifetime. Adults become active in the spring, with damage occurring to fruits as they begin to ripen throughout the growing season.
Last year, we did not detect SWD in the field until the middle of August, and numbers did not increase until late September. In 2012, we had our first field capture on June 25th, at a farm that was previously identified as positive for SWD in 2011. We do not know what the implications are for earlier field detections, but it is possible this may lead to a population build up earlier than that observed last year, potentially placing summer crops at risk.
Note that no infested fruit have been reported to date. However, it is important to control SWD if there have been captures in your area and fruit is turning colour.
Research on the local biology and optimized management of SWD is being conducted by OMAFRA, the University of Guelph, the Vineland Research and Innovation Centre and Agriculture & Agri-Food Canada. Management of SWD involves a combination of monitoring, cultural controls (sanitation / timely harvest) and the use of registered insecticides.
Monitoring for SWD should include the use of baited traps to collect adults and examination of susceptible fruit near harvest for the presence of larvae.
Commercial vinegar fly traps are effective in trapping SWD adults (Figure 13). Simple traps can be constructed of any 250-750 ml plastic containers with lids. Perforate the containers with 4-6 small holes (3-5 mm each) to allow entry of SWD flies and prevent trapping of larger species. Bait traps with 2-3 cm of apple cider vinegar, and check them regularly. Place traps when the temperature is consistently over 10°C, and/or when fruit starts to form, at least one month prior to fruit ripening. Once fruit begins to ripen, flies are less likely to be attracted to traps. The apple cider vinegar must be replaced weekly.
Figure 13: Commercially available Contech Vinegar Fly traps may be used to monitor SWD, though homemade versions are also effective. Use only apple cider vinegar, which must be replaced weekly. Photo credit: E. Beers, Washington State University
The best locations for traps are sheltered areas near field edges
or in hedgerows, particularly if wild hosts are present. Use a minimum
of two traps per block. It is important to look for both males and
females, as females may outnumber males, particularly early in the
season. In addition, suspect infested fruit can be collected and
larvae reared to adults for identification purposes
Cultural controls are important components of management. Removal of over-ripe or dropped fruit, timely harvest, and removal of wild hosts will help reduce populations. Composting is not a reliable way to destroy eggs and larvae in fruit. Cull fruit may be disposed of in a sealed container to prevent the emergence of new adults. Research conducted in the US indicates that solarization of cull fruit for 7 days kills 99-100% of SWD. Solarization consists of covering fruit with 1-2 mm clear plastic, and tightly sealing the edge of the plastic with soil. The same research indicates mechanical crushing of fruit resulted in 57% mortality as opposed to 55% mortality in uncrushed fruit left on the ground (Washington State University, Oregon State University, University of California - Davis, 2011). SWD adults require sources of moisture. Repair leaking drip irrigation lines and minimize overhead irrigation, where feasible.
When flies are detected in traps and if fruit is in a susceptible stage (beginning to colour), an insecticide should be applied. Fruit must be protected from the time they start to colour until harvest is completed. Re-application may be required depending on the residual activity of the product. Several insecticides have Emergency Use registrations for SWD control in 2012: Malathion 85E and Malathion 25W (malathion, Group 1B), Ripcord 400EC (cypermethrin, Group 3), Delegate WG (spinetoram, Group 5), Entrust.80W (spinosad, Group 5) and Pyganic (pyrethrins, Group 3). For more information on these products, see www.omafra.gov.on.ca/english/crops/facts/swd-registrations.htm
Surveys coordinated by OMAFRA in 2011 indicated the pest was widespread in Ontario. We are tracking the incidence of SWD again in 2012. Early detection is required to help reduce the potential for damage caused by this pest. If you suspect SWD, it is important to have samples identified as soon as possible. Collect samples of infested fruit or trapped adults and submit them to the Pest Diagnostic Clinic (University of Guelph), or contact OMAFRA.
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