Article written by Madeline Wimmer, UMN Extension Fruit Production Educator
Images: Exclusion netting is one method to manage spotted wing drosophila (SWD) from blueberries and other fruit crops (left). A map provided by the Vegetable Disease and Insect Forecasting Network can give insight for SWD population pressure in various regions in the Upper Midwest (right).
Right now, spotted wing drosophila (SWD) populations are forecasted to be at lower levels in southern Minnesota, and very low for regions that are north of St. Cloud. In general SWD populations can vary at different farm sites based on the existence of wild berries within forested edges, for example, other susceptible crops grown on site that have already ripened, along with overall management practices, and other environmental conditions.
SWD has been a major disruptor to vulnerable crops like raspberries, which were previously grown with less requirement for insecticide inputs before SWD’s arrival. There has been an abundance of research done on SWD management, looking at cultural practices like pruning and irrigation choices, along with product recommendations and approaches using trap and kill methods, biological management practices from nematode ground applications to parasitoid impact on SWD, as well as physical management practices, which includes practices like exclusion netting.
For the practice of exclusion netting, timing and product choice matters. Netting should not be applied before bloom to allow for pollinator activity, but should be fully installed before SWD is on site. SWD is a very small pest and mesh size matters. Typically mesh that is 1.0 x 0.6 mm in size, or 80 gram netting, works well. Careful attention to designing the method for applying netting is important. Ben and Natalie McCavoy of Blue Fruit Farm have recently invested in this practice and started by applying the netting over well piping to give it some structure, which did not end up working out.
They have switched to a simpler over-the-row approach using sandbags to weigh the netting down, a resource that is easy for them to utilize since they live close to a source of sand that is available to the public. The sandbags prevent the netting from blowing over and weigh it down to prevent any gaps where SWD could enter. Because the netting is directly on the plants, they have found it’s important to wait until plants are fully dry to apply it in order to prevent excess wetness and humidity.
Other methods can work for insect netting. For those who already have a high tunnel, netting can be applied at the high tunnel ends with a zipper, and on the sides for when the side plastic is rolled up. Vestibules can be created as well to create a decontamination zone and further prevent SWD from coming in.
Other benefits to netting include protection from birds and hail. While I was recently visiting Blue Fruit Farm, I saw a ground squirrel running between the rows along the netting–it too was excluded.
Storing netting over the winter is another important consideration. For farms with different sized rows, netting should be labeled to keep it organized. Nets should also be free of debris and dry upon storing. Costs for netting will vary depending on inflation and distributor pricing, but a 2020 article from Cornell estimated the cost to be around $10,000 per acre.
Keep in mind that, no matter which management method, or combination of methods used, early management timing is key with SWD. It’s a fruit fly with a short life cycle, that can develop very quickly with warm temperatures, and because of this, infestations can get out of control very quickly. Invest in early detection, followed by management now to prevent a disastrous situation later.
Many non-chemical management methods like exclusion netting should be set up before SWD is on site. For chemical management there is a low tolerance threshold for action, which means management should begin when the first fly is trapped on site. To learn more about conventional and organic chemical options for SWD management, check out the Midwest Fruit Pest Management Guide section on bush berries, starting on page 174, Blueberry Third to Summer Covers - Insects.
Resources:
Thinking exclusion? (Cornell University)
Spotted wing drosophila (University of Minnesota Extension)
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