Spotted wing drosophila management in 2017
Spotted wing drosophila (SWD) is continuing to be a challenging pest for berry growers in 2017. We have found SWD earlier and at higher pressures than in previous years, surprising some berry growers with unmarketable fruit. Michigan has seen unusually early activity as well- trap catches are increasing about 2-3 weeks earlier than they have seen in past years. The week ending July 14th Michigan saw a sharp increase in SWD catches, with catches three times higher than in the previous week. The population increases expected in late July-early August are being seen now. In Ontario we are seeing the same trend. This week numbers are continuing to increase; there are multiple sites with more than 10 flies per trap. What does that mean for Ontario berry growers?
Commercial berry growers will need to protect fruit earlier and regularly. Strawberry growers who are still picking late varieties such as Malwina may find SWD in their fruit and will need to manage for SWD (Figure 1). Control will be difficult with the thick canopy. Summer raspberries, which usually avoid serious SWD infestation in early fruit, are also at risk. Keep SWD managed in these crops to protect near-by fruit.
Figure 1. SWD damage to strawberries
With the wet weather we have been experiencing it may be difficult to spray regularly, so it is important to keep your fields as clean as possible. Pick the fruit clean every other day if possible, and remove the unmarketable berries from the field. Bury this unmarketable fruit 30cm deep or more each day, or solarize the fruit by covering with a plastic bag, sealing the edges. Deal with culls quickly- do not leave culls exposed for more than one day. Consider renovating strawberries early to protect your early crops. Plan your spray program now; check the product registrations for 2017 on our website. Plan now to prepare for a good rotation with multiple classes. Use enough water and pressure to ensure adequate coverage, and use spray equipment that will allow effective coverage.
Monitoring for SWD
We are conducting a small monitoring program this year at 10 sites across the province. To confirm SWD in your own crops, and to determine the effectiveness of your spray program, you can monitor SWD on your own by conducting a salt water test or a plastic baggie test. For a salt water test you will need to dissolve 1 part salt in 16 parts water (1/4 cup salt in 4 cups water). Place approximately 100 healthy-looking berries into a shallow dish, and pour the salt water over the fruit until it is completely covered. Mash the fruit lightly with a potato masher. Wait for 10-15 minutes, then check to see if any larva has floated to the surface. SWD larvae are small, white, and tapered at both ends, and can be 1-4 mm long. You may need a hand lens to detect the very small larvae. It is impossible to distinguish SWD from other drosophila larva, however, if you are using healthy, sound, ripe fruit in the tests, not overripe fruit, it is probably SWD larvae emerging from the berries. Sometimes other pests, such as cherry maggot or blueberry maggot, can be found in fruit. These larvae are larger than SWD and blunt at one end
An alternative to the salt water test for finding larvae is the plastic baggie test, ideal for busy growers. Simply place healthy fruit into a plastic baggie, seal it and place it in a warm place, such as a windowsill, for a few hours. The heat will drive larvae out of the fruit. The larvae will look like small white threads on the surface of the fruit or on the inside of the bag. Use these two methods for your own monitoring program and stay ahead of this fly!
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