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Crop, Stock and Ledger
Time to Monitor for Spotted-winged Drosophila
June 18, 2019
Dave Shiley, Extension Educator – Local Food Systems and Small Farms
The commercial fruit growers in Illinois are well aware of an insect known as the Spotted-winged drosophila or SWD. It is a type of fruit fly, first detected in the United States in 2008, and first recorded in Illinois in 2012. Since then, it has spread throughout Illinois. Adult male SWD flies have a prominent spot on each wing, but the wings of females are not spotted. Females of this species differ from other Drosophila species, fruit flies, by having an ovipositor that is serrated or saw-like. The serrated ovipositor enables this insect to cut open the "skin" of thin-skinned fruits and lay eggs into them as they begin to ripen.
Larvae or maggots develop as the fruits ripen and can be present at harvest. Oftentimes the fruit looks fine when picked, but already contains developing larvae. If you don't immediately refrigerate the fruit after harvesting, within 36 to 48 hours, the fruit begins to "melt down" and collapse and larvae become clearly visible. SWD infests a wide range of common soft-skinned fruit crops including blueberries, raspberries, blackberries, peaches, strawberries, cherries, and grapes.
If you grow any of these soft-skinned fruits, you MUST manage this insect unless you plan to eat infested fruit. Strawberries that are harvested before mid- to late June, and early blueberries will likely escape infestation, because SWD that survive the winter are relatively low, but become active in mid-June to early July. Populations build up rapidly through the summer, and the likelihood of heavy infestations increases through the fall. Researchers in the Midwest have captured adult flies in traps into December.
A key step in managing SWD is monitoring, for adult flies. To monitor for adult SWD flies, you can make traps or buy them ready-to-use. Some simple instructions for making traps and how to use them can be found on the University of Illinois Extension website.
To determine whether fruit is infested, immerse a sample of harvested fruit in a sugar-water solution containing, 1 cup granulated white sugar per 1 quart water. Within one-half hour, larvae will float to the surface.
To prevent infestations or at least limit losses to SWD, a combination of cultural and chemical approaches will be necessary. Clean picking and frequent picking, removing damaged fruit as well, can reduce population buildups within plantings. Exclusion by use of screening or fine-mesh netting has been shown to reduce infestations as well, although difficult to use.
Insecticides are needed for effective control of SWD. Apply insecticides to blueberries, raspberries, blackberries, and similar small fruit crops beginning at the onset of fruit coloring and ripening. Pre-harvest intervals (PHIs) and recommended application intervals must be followed on the product label and varies depending on the product. Alternating insecticides to ones with different modes of action is also recommended. Remember that all insecticides have some toxicity to bees, so brambles that have blooms and fruit may need to be sprayed in the evening after bees have stopped foraging.
Some selected insecticides for SWD control include (active ingredient names) bifenthrin, spinosad, pyrethrins, and malathion. Not all of these insecticides are labelled for all soft skinned fruit, so read and follow the instructions on the label. If you have questions or need additional information, contact the University of Illinois Extension office in Champaign at 217-333-7672. You can also reach me at the Extension office in Arthur at 217-543-3755.