Managing Spotted Wing Drosophila in Florida Blueberries

Spotted wing drosophila (SWD), Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), is an invasive pest that was introduced into Florida in 2009. This pest originated in Southeast Asia and resembled other common Drosophila flies that accumulate on over-ripe fruit beginning to decay.

Since its first detection in 2009, SWD has been established in all of Florida’s fruit-producing counties. Surveys indicate that SWD are active year-round in Florida’s warm climate, with peak activity from April to May when the blueberry harvest is at its peak. Although SWD is a significant pest of blueberries in Florida, with good cultural practices, proper monitoring, and appropriate insecticide applications it can be successfully managed. This publication is intended to be used as a resource for Florida blueberry growers to successfully identify and manage SWD.

Damage

Adult female SWD lay their eggs primarily in ripe fruit, and the larvae hatch and consume the fruit from the inside, ultimately causing the fruit to collapse. Punctured holes in the fruit from egg-laying females can lead to secondary infections by fungal and/or bacterial pathogens, either in the field or post-harvest. The presence of SWD larvae and the resulting damage is unacceptable for the fresh berry market. Visible larvae or damage on the fruit will often cause entire loads of blueberries to be rejected by packing houses or distributors. However, infested fruit can sometimes go unnoticed until the fruit is in consumers’ hands.

Management

The low tolerance for SWD in fruits demands appropriate management tactics to protect Florida blueberry crops. To date, there are no action thresholds for SWD; therefore, correct identification of the pest is essential. Integrated pest management (IPM) techniques should be used to manage SWD, including identification and monitoring, good cultural practices, and the application of appropriate labeled insecticides.

Identification

Adult SWD are small (1/8 in or 3–4 mm), with a yellowish-brown body, black stripes across the abdomen, and prominent red eyes. The male SWD can be recognized by a single dark spot on the front margin of their wings that is lacking in most other Drosophila flies (Figure 1). The female SWD lacks the dark wing spots and has a large, serrated ovipositor (egg-laying organ) that is used to cut into ripe fruit to lay eggs underneath the fruit skin (Figure 2). A single adult female can produce 300–600 eggs during its lifetime. The eggs have breathing tubes that extend through the berry skin (Figure 3). Spotted wing drosophila eggs produce white cylindrical larvae (maggots) with black mouthparts (Figure 4) that feed on soft fruit tissues before the berries are harvested, causing the fruit to degrade. Each generation can take as little as 8–10 days from egg to adult at optimal temperatures ranging from 22°C to 28°C (71.6°F–82.4°F), and there can be multiple overlapping generations present during the blueberry harvest window.

Spotted wing drosophila belongs to a different family from fruit flies, which belong to the family Tephritidae and include Mediterranean fruit fly, blueberry maggot fly, Caribbean fruit fly, Oriental fruit fly, and Mexican fruit fly, as well as others often reported in the news media. Drosophilid flies are not closely related to tephritid flies, and management of the two groups is different. For instance, rare outbreaks of Mediterranean fruit flies in Florida have been managed in part with mass releases of sterilized male Mediterranean fruit flies, but so far there has been no research on SWD in Florida that has been focused on releasing sterilized insects.

Monitoring

Producers can determine whether SWD are present on their farms by monitoring strategically placed traps for captured adults. Place traps and begin monitoring them during the early stages of fruit development, and continue through the end of harvest. Hang traps throughout the field and along the field border. Place traps in the center of the bushes, in the shade, and away from morning sun; and check the traps at least weekly.

Spotted wing drosophila traps are commercially available from different suppliers. Two of the most commonly used traps with lures are the Scentry and Trécé traps. Both have lures that are intended to mimic the SWD’s food source. The Scentry (Figure 5) and Trécé (Figure 6) traps are a wet trap and a sticky card, respectively. One study has shown that a red sticky trap using the Trécé lure was more effective at trapping the initial emergence of SWD (i.e., it trapped SWD earlier than the other traps included in that study). The same study showed that the Scentry wet trap captured more SWD overall, although it also captured many beneficial insects (Panthi et al. 2022).

Monitor for larvae in fruit by placing a sample of at least 15 berries in a strong salt solution (one cup of salt to one gallon of water). If any larvae are present, they will begin to emerge after approximately 30 minutes. A sugar solution or boiling can also be used, which involves submerging fruit that is possibly infested with SWD maggots into a sugar solution or into boiling water to induce any maggots to emerge from the fruit.

If you detect any adults or larvae, take action immediately to manage the infestation.

Cultural Practices

Pruning

Spotted wing drosophila adults are sensitive to temperatures higher than 30°C (86°F) and conditions dryer than 20%–33% RH, so they tend to prefer the inside and lower parts of the plant canopy, where it is cooler and more humid (Tochen et al. 2015; Enriquez and Colinet 2017). Pruning to open the plant canopy, in addition to helping with disease management, can also make the canopy less hospitable to SWD. A less dense canopy can also improve pesticide spray coverage.

Pesticides

Apply registered insecticides immediately upon detecting SWD. Again, as of the date of this publication, no action thresholds have been established for this pest. Some blueberry growers make preventative applications of insecticides, especially if SWD are seen in traps adjacent to production areas or have been present during previous years. Table 1 lists registered insecticides for use against SWD. These insecticides target adult flies; there are no insecticides available for egg or larval control inside fruit. Spray applications should be made in the early morning or late evening, when SWD adults are most active, and good spray coverage is essential. If flies are absent as indicated by a rigorous monitoring program, pesticide sprays for SWD may be delayed until a presence is detected. A rotation program using insecticides with different modes of action should be used to minimize the development of resistance. All label instructions for insecticides must be followed. If you are exporting fruit, carefully check the maximum residue limits for the importing jurisdiction.

References

Enriquez, T., and H. Colinet. 2017. “Basal Tolerance to Heat and Cold Exposure of the Spotted Wing Drosophila, Drosophila suzukii.” PeerJ Mar 23:5:e3112. https://doi.org/10.7717/peerj.3112

Iglesias, L., J. Price, C. Roubos, J. Renkema, and O. Liburd. 2016. “Spotted Wing Drosophila in Florida Berry Culture: ENY-861/IN839, rev. 1/2016.” EDIS 2016 (3). Gainesville, Fl. https://doi.org/10.32473/edis-in839-2016

Panthi, B., K. R. Cloonan, C. Rodriquez-Saona, et al. 2022. “Using Red Panel Traps to Detect Spotted-Wing Drosophila and Its Infestation in US Berry and Cherry Crops.” Journal of Economic Entomology 115:1995–2003. https://doi.org/10.1093/jee/toac134

Sial, A., B. Little, C. Roubos, et al. 2023. “Spotted Wing Drosophila in Organic Berry Crops.” UGA Cooperative Extension Bulletin 1497.

Tochen, S., J. M. Woltz, D. T. Dalton, J. C. Lee, N. G. Wilman, and V. M. Walton. 2015. “Humidity affects populations of Drosophila suzukii (Diptera: Drosophilidae) in blueberry.” Journal of Applied Entomology 140:47–57. https://doi.org/10.1111/jen.12247