This article was first published in the Spring 2014 edition of the Tilth Producers Quarterly Journal.
Spotted wing drosophila (SWD, Drosophila suzukii) is an invasive vinegar fly first discovered in Washington in 2009. It has rapidly spread into all important fruit-producing areas of the United States, and represents a serious threat to fruit marketability.
SWD differs from native vinegar flies in that it will attack sound ripening fruit in addition to overripe, soft, or rotting fruit. SWD adult females have a serrated ovipositor, which allows them to cut into intact fruit and lay eggs. When female SWD lay eggs (oviposit) in ripe, undamaged fruit, the point of oviposition becomes an opportunistic opening for decomposition organisms and native fruit flies, and more importantly, creates an enlarging area of damage as larvae hatch and begin feeding. Although this pest will infest raspberries and blackberries preferentially, other fruits produced in the region are at risk for attack, including cherries, late season strawberries, blueberries, and black raspberries (Dreves and Langellotto, 2013). Tree fruit such as peaches and pears, though not at high risk before harvest, can become damaged if SWD have access to the fruit as it ripens postharvest.
Good SWD management begins with knowing when SWD is present in fields and when a given fruit crop is susceptible to attack. Regional trap reports, such as those available free from the Small Fruit Update and the WSU Tree Fruit Research and Extension Center, give excellent early and season-long indications of pest populations (Peerbolt Crop Management, 2014; Washington State University, 2014). Producers can consult these reports for specific locations and crops to estimate building pest pressure.
Farmers wishing to monitor and maintain their own traps will find information in “Monitoring, Identifying and Fruit Sampling for Spotted Wing Drosophila (SWD)” (Gerdeman et al., 2011). To scout successfully, producers must be able to differentiate SWD from native Drosophila species. Traps should be hung in the field or orchard as the fruit begins to ripen or as soon thereafter as possible. It is important to check traps and replenish fly bait weekly.
There are limitations of trapping: as yet, there are no reliable correlations between adult flies trapped and levels of infested fruit, and sometimes fruit will be infested in the field before traps catch adult flies. Because of these limitations, growers do best to simultaneously monitor the regional reports along with fruit ripening in their fields and begin sprays when activity is indicated and fruit is ripening. Recent research has shown that yeast solutions, wine, or proprietary lures catch greater numbers of SWD and earlier in the season than apple cider vinegar (Burrack et al., 2014).
Fruit sampling can gauge SWD pressure in a fruit crop, and is useful as a way to determine the effectiveness of pest management practices, but not as an indicator of when to start spraying. Fruit sampling is another way to gauge the SWD pressure in a fruit crop. A saline solution is added to a random sample of harvested fruit to force larvae out of the fruit tissues so they can be seen and counted (Gerdeman et al., 2011). It is important that the sampled fruit appear undamaged so as not to inadvertently mistake native vinegar fly larvae in already damaged fruit. If no larvae emerge, fruit can be lightly crushed to promote larval escape into the solution. Fruit sampling can be frustrating for first-time samplers, as tiny larvae are difficult to see or distinguish from fruit fibers.
Following a careful regime of cultural management will decrease SWD pressure in a field, but will not bring it to zero.
Eradicate unmanaged alternate hosts near production fields. As far as possible, producers should take out wild Himalayan and evergreen blackberries, wild cherry trees and other wild hosts, and abandoned orchards that are most likely to serve as habitat and breeding grounds for SWD.
Harvest frequently and thoroughly. SWD can attack green or pink fruit, but the likelihood of infestation rises dramatically as ripening fruit gains color and softens. The longer nearly ripe or ripe fruit remains in the field, the greater the opportunity for it to become infested. Short harvest windows and clean picks will capture fruit as soon as it ripens. All unmarketable or diseased fruit, culls, and drops should also be removed from the field and disposed of by solarization, landfill, burial, or potentially as livestock feed if it can be handled efficiently and without opportunity for further larval development or infestation. Composting the culls is not recommended. U-Pick operations will require extra vigilance to ensure clean picks and field sanitation. Growers must also be mindful of clean-up of fields at the end of the season, removing all fruit that could harbor SWD.
Ensure prompt cold storage. Producers must make sure their cold storage chain is efficient and timely, with no major gaps prior to reaching market. Prompt cold storage can slow or halt the development of SWD eggs and tiny larvae before they cause perceptible damage to the fruit (Bolda, 2010; Burrack et al., 2014). Forced air cooling is more effective in lowering the fruit temperature than putting the containers in coolers.
Manage canopies to reduce cool humid microhabitats. Viability of SWD eggs is lower under dry, warm conditions (Burrack et al., 2014). Pruning or using wider plant spacing to open up and increase airflow to the plant canopy can reduce pockets of humidity and shading that are amenable to SWD survival, and will also aid in more thorough coverage for chemical controls. The use of mulches that reduce standing water and canopy humidity is also helpful.
Organic chemical controls are available but limited. Only two active ingredients, spinosad and pyrethrum, are available in organic formulations, and only Entrust (spinosad) has been shown to have good residual activity. Diligent, frequent spraying (sometimes on a 5-7 day schedule) is required for thorough control of SWD adults before eggs are laid in ripening fruit. Working around the pre-harvest interval (PHI) for these products requires careful planning. Organic sprays must be applied with an eye to preventing resistance, with strict adherence to the label, careful avoidance of spraying while bees are foraging, and appropriate rotations to a different chemical class. Postharvest sprays are recommended for recently harvested fields or orchards adjacent to un-harvested susceptible fruit. Conventional growers are also using spinosad products, so over-reliance on Entrust will promote resistance and potential loss of the only highly effective organic product available. Pyganic, while less effective, may be useful in rotation. Growers may consider using GF-120 (bait and kill system), even though Entrust and Pyganic are used as the primary controls. SWD adults that feed on the bait are killed. However, recommendations for using GF-120 are still in the development stage for Washington fruit growers. Many other products have been tried for organic SWD control, including neem-based products, but none have proved effective against SWD. With an aggressive organic spray program, it is important to monitor secondary pests like mites and aphids, as their populations may build.
Total SWD exclusion through the use of fine bird netting or no-see-um nets has been studied to some extent. This is of limited practicality for extensive plantings and the nets must be opened and closed regularly to balance harvester access with continued exclusion of the flies.
Researchers have also studied the possibility of applying sprays on alternate rows or on border rows only, to decrease applied pesticide and reduce the yield loss caused by equipment disturbance. Results from these studies indicate that in low pest pressure years, these alternate spray methods can approach the effectiveness of full field sprays while also reducing fruit drop, but in high pressure years, control is not adequate (Burrack et al., 2014).
Further research on SWD is focusing on biological control through the activity of parasitoid organisms, which are beneficial insects that parasitize larvae or pupae of developing SWD (Burrack et al., 2014). In Asia where SWD originated, it is not an economic pest, largely because native parasitoids keep SWD populations in check. Efforts continue on research objectives that include fully understanding the life cycle, population growth, and regional adaptation of SWD; establishing economic thresholds for the pest, and building long-term biological control (Oregon State University, 2014).
The authors would like to acknowledge Mark Sweeney, BC Ministry of Agriculture, for his technical review and input. Wendy Hoashi-Erhardt is a researcher with the Small Fruit Plant Breeding program at Washington State University-Puyallup. 253-445-4641, [email protected]. Andrea Bixby-Brosi is Post Doctoral Research and Extension Associate at Washington State University, Department of Entomology, Tree Fruit Research and Extension Center, Wenatchee. 509-663-8181, [email protected].
CITATIONS
Bolda, M. 2010. Length and Magnitude of Fruit Cooling and Spotted Wing Drosophila Mortality-Strawberries and Caneberries. University of California. http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=2474 (Accessed 24 Feb 2014).
Burrack, H.J., V. Walton, and R. Isaacs. 2014. Biologically Based Organic Management Strategies for Spotted Wing Drosophila [Video webcast]. eXtension. http://www.extension.org/pages/70121/biologically-based-organic-management-strategies-for-spotted-wing-drosophila#.Uwav3M5rZi0 (Accessed 20 Feb 2014).
Dreves, A.J and G. Langellotto. Emerging Pest: Spotted Wing Drosophila-A Berry and Stone Fruit Pest. In: Hollingsworth, C.S., editor. Pacific Northwest Insect Management Handbook [online]. Corvallis, OR: Oregon State University. http://pnwhandbooks.org/insect/pnw-insect-management-handbook/emerging-pest-spotted-wing-drosophila#sthash.tPoP9THY.dpuf (accessed 24 Feb 2014).
Gerdeman, B., L. Tanigoshi, and G.H. Spitler. 2011. Spotted Wing Drosophila (SWD) Monitoring, Identifying, and Fruit Sampling. FS049E. Washington State University Extension, Pullman, WA. http://cru.cahe.wsu.edu/CEPublications/FS049E/FS049E.pdf (Accessed 24 Feb 2014).
Oregon State University. 2014. Spotted Wing Drosophila. http://spottedwing.org/ (Accessed 24 Feb 2014).
Peerbolt Crop Management. 2014. Small Fruit Update. http://www.berriesnw.com/SFU.asp (Accessed 24 Feb 2014).
Washington State University.2014. Spotted Wing Drosophila. http://www.tfrec.wsu.edu/pages/swd/Home (Accessed 24 Feb 2014).