The number of spotted wing drosophila found in traps this season continues to increase. Spotted wing drosophila was first found in the United States in 2008 on the West Coast. It was first found in Minnesota in 2012.
What is spotted wing drosophila?
Drosophila suzukii is a small brown fly with a striped abdomen, red eyes and, yes, the males have a single dark spot near the tip of each wing. The females lack the latter characteristic but can be identified by the large serrated ovipositor, the part that deposits eggs into the fruit. It is an invasive fruit fly native to east Asia. The larvae or maggot is white with a cylindrical, tapering body only an eighth of an inch long.
The larvae leave the fruit to pupate and eventually the adult emerges. SWD can complete its life cycle, egg to adult, in seven days, allowing multiple generations to occur each year. Females start laying eggs one day after emergence. Populations increase rapidly, usually peaking in August.
In Minnesota, raspberries have been hardest hit by SWD. Blackberries, blueberries, strawberries, grapes, plums and cherries are also susceptible, as well as wild fruits such as elderberry.
Start monitoring for SWD as soon as fruit sets. Use an apple cider vinegar trap to monitor the garden. Hang the trap in the shade near the fruit crop. Change the vinegar often, disposing of it away from the trapping location.
Sanitation is important to prevent population build-up. Harvest fruit frequently to ensure ripe fruit is not in the garden for extended periods. Remove and destroy old fruit. Place this fruit in a sealed clear plastic bag and leave in the sun to kill any eggs or larvae present. Do not compost or bury old fruit, as both are unreliable in killing SWD.
Fine mesh insect netting can be used to keep the adults from reaching the fruit. The netting also will provide protection from birds. A disadvantage is the netting will need to be removed each time the fruit is harvested.
Insecticides target adults and will not control larvae in the fruit. Evening is the best time to apply insecticides. Adult SWD are not as active during the heat of the day. Evening also is the best time to apply insecticides to avoid injury to pollinators.
Spinosad and pyrethrin are approved organic options. Rotate between chemicals to avoid the development of resistance. Read and follow all label directions.
If infestation is suspected, place fruit in the refrigerator to slow development of larvae. Fruit infested with the larvae is safe to eat — not too appetizing, but it is safe.