Above: Spotted wing drosophila male.
This article was written by Mary Roduner, former SDSU Extension Consumer Horticulture Field Specialist.
In 2013 a new fruit fly pest was identified in South Dakota, the spotted wing drosophila or two spotted fruit fly, Drosophila suzukii. These fruit flies differ from the standard “vinegar fruit fly”, Drosophila melanogaster; that we commonly find on overripe fruit.
Vinegar fruit fly females lay eggs on the surface of overripe fruit. They are attracted by the ethylene gas given off during the ripening process. These flies are controlled mainly by removing any infested or overripe fruit.
The spotted wing drosophila is another creature all together. These flies came from Asia and were found in California for the first time during 2008. Since then they have spread throughout most of the U.S. and southern Canada causing severe financial losses. The estimated raspberry and blackberry production loss during 2012 in the states with large production fields was approximately $6.7 million. Strawberry losses exceeded $207 million. Production costs go up because of the need for increased sprays and higher scrutiny during harvest, adding to the loss taken. They have the potential to do serious damage to fruit and wine production throughout our entire region.
This pest does not only affect production fields. It will attack any soft fruits available. Favorite fruits include blackberries, raspberries, blueberries, cherries, strawberries, peaches, apricots, tomatoes, apples, crabapples, currants, chokecherries and sand cherries. Homeowners are also at risk of losing their entire crops. June bearing strawberries seem to be affected less than the day neutral types because they ripen very early in the season before the first adult flies emerge.
Above: Spotted wing drosophila female with curved ovipositor.
Female fruit flies are attracted to fruit that is almost ripe. They have what is called a “saw-like ovipositor”. This is a hard curved appendage at the end of the abdomen. The fly uses it to punch a hole into the fruit and lays her eggs inside the fruit. Once the egg is inside the fruit, insecticides cannot reach the larvae or maggot. Life cycle length depends on outdoor temperatures. During hot weather they can go from egg to adult in 8 days. Cool weather slows development. Generations overlap because the female can live for two weeks and lay up to 300 eggs during her life.
Above: Spotted wing drosophila eggs.
Eggs hatch within 24-36 hours. About 48 hours after eggs are laid the fruit begins to soften and quickly collapses into a jelly-like mass. Clear to white larvae approximately ¼” long can be seen inside the fruit. Red-brown pupae with two “horn-like” projections develop within 4-5 days. Once the fruit is infested it becomes inedible in approximately 72-96 hours.
Above: Spotted wing drosophila larvae in raspberry.
At this time control is difficult. Insecticide sprays are used to control only adult flies. Organic insecticides are not very effective and have almost no effect on the flies. Chemical sprays need to be sprayed routinely and the insecticide type rotated to prevent resistance. Care must be taken to use sprays with the shortest pre-harvest interval available. Pre-harvest interval is the time from spraying until the fruit can be harvested.
The best control method is to remove any fruit that is ripe or overripe. Pick all fallen fruit off the ground and destroy by freezing or for small amounts run them down the garbage disposal. Do not put affected fruit it in the compost pile or they may complete their lifecycle there and re-infest fruit.
Vigilance is the most important part of control. Surveying for presence or absence of the fruit flies is being done throughout the state. Bait traps with a yeast and sugar mix are placed inside fruit trees or bushes to catch adults and checked weekly. Insects from the traps are placed in vinegar and sent to the Rapid City Regional Extension Center for identification.