In the summer of 2012, a newly arrived species of fruit fly, spotted-wing drosophila (SWD), was found in Minnesota, and has been giving fruit producers reason for concern ever since. SWD is an invasive species from Asia that is devastating to small, soft-skinned fruit such as berries and stone fruit. Unlike the common fruit fly, which can only lay its eggs in damaged and rotting fruit, SWD has the ability to lay its eggs in ripening fruit, allowing the larvae to develop when the fruit is at peak ripeness. Since it arrived here, growers and researchers have been scrambling to save some of our favorite fruits.
SWD is known to infest many cultivated and wild fruits including apricot, blackberry, blueberry, buckthorn, cherry, cranberry, currant, dogwood, fig, kiwi, peach, plum, raspberry, and strawberry. Such a wide variety of host crops makes it difficult to remove all host fruit in the area. Additionally, SWD grows from egg to adult in as little as 10 days. The short development period means there can be as many as 13 generations in a single growing season here in the Midwest. For certain crops like fall-bearing raspberries, 100% of the fruit can become infested with larvae.
Our research at the University of Minnesota focuses on understanding SWD’s behavior and devising new ways to control this devastating pest. Due to Minnesota’s cold and long winters, it is uncertain if SWD remains in the state during winter. The flies could be migrating long distances from warmer parts of the country every year. We suspect this could be the case because SWD is generally not found in Minnesota until the middle of June each year. However, there is some concern that it might be overwintering in greenhouses or other structures.
One of the most promising ways for growers to ensure the safety of their fruit is to adopt cultural control methods. Cultural control is the practice of modifying the environmental conditions to be less suitable for the growth of pests, whether they are insects, diseases, or weeds. In berry production this is accomplished through removal of all ripe and overripe fruit, physical barriers around the fruit crops, and cultivar selection. Growers in Minnesota have started using cultural control techniques such as installing plastic mulch and landscaping fabric under plants to prevent flies from growing in the fruit that falls to the ground. This practice also makes weed management and the removal of fallen and rotting fruit easier for fruit orchards. For some crops, like blueberries, the type of cultivar can help to protect fruit. Early season cultivars like Patriot and Polaris ripen before SWD populations are at their highest.
A reliable way to protect fruit from infestation is by using physical barriers. Growers have been using high tunnels for a variety of benefits such as season extension, increased control over environmental conditions, and lower pest and disease pressures. When retrofitted with fine mesh netting to seal off the opening, the tunnels have been found to decrease infestation to as low as 2%, as compared to 68% in open, insecticide sprayed crops.
Chemical control relies on the use of insecticide sprays, and is currently the main form of control for SWD. Several conventional insecticides have been shown to be effective against SWD, however, there is only one organically approved insecticide that is effective against SWD. The lack of alternatives is a problem for farmers that need to balance environmental and economic goals. Due to this concern, graduate student Matthew Gullickson conducted a laboratory experiment where he sprayed a variety of organic insecticides on raspberry fruit and placed them in containers with SWD to see how the flies responded to the insecticide. Two potential alternatives are a zero calorie sweetener and a product made from spider venom. The flies reflexively start feeding on sweet substances and when the sweetener is added to another insecticide, the effects can be greater than either of the chemicals alone. This summer, Matthew started an experiment to see how effective these products are in the field.
Although the impact of SWD is being felt by fruit producers across the country, the exact economic cost is uncertain. Gigi Digiacomo, a research fellow in the Department of Applied Economics at the University of Minnesota, is currently conducting an economic assessment of SWD in Minnesota. She is collecting data from growers about how much they are spending to manage this pest and the extent of the economic losses. Gigi is combining this information with the cost of the products and techniques in Matthew’s research to see if there are more economically sustainable alternatives to current practices. The combination of all of these research initiatives will hopefully allow Minnesota berry growers to continue to produce high quality, local fruit for all of us to enjoy.
Funding for this project was provided by the Minnesota Invasive Terrestrial Plants and Pests Center through the Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR). Special thanks to Steve Poppe, Dr. Mary Rogers, Eric Burkness, and Dr. William Hutchison.
More info on SWD:
Ripe raspberry; Matthew Gullickson
SWD larvae inside of a blueberry; Matthew Gullickson
Miniature high tunnels over raspberry plants in Morris, MN; Matthew Gullickson