New invasive pest

Spotted Wing Drosophila
An adult male Spotted Wind Drosophila fly.

Spotted Wing Drosophila in Western Nebraska

McKayla Stark, Extension Summer Horticultural Intern
Chrissy Peters, Extension Summer Horticultural Intern
Gary Stone, Extension Educator
Jim Schild, Extension Educator
Jeff Bradshaw, Extension Entomologist

The Spotted Wing Drosophila, Drosophila suzukii, (or SWD) was first discovered in the Western United States in 2008. It is a new, invasive pest that is attracted to soft, fleshy fruits, including most berries and pomes, such as cherries, grapes, blueberries, strawberries, raspberries, apples, and pears.

These flies are such a great risk because, unlike other fruit flies, they infest healthy instead of damaged fruit. Therefore, the SWD flies lay their eggs when the fruit begins to ripen. The damage caused by egg laying can expose the fruit to invading bacteria and fungal diseases such as sour rot.

Spotted Wing Drosophila larva
A Spotted Wing Drosophila larva.

Because of the SWD’s very small size (2-3 mm in length), it is likely to have been transported by humans through fruit production and distribution. The SWD can develop from egg to adult in just over one week! Many generations can develop over a growing season, making them very abundant and difficult to control.

The adult SWD has a round, light yellow-brown abdomen with no breaks in the abdominal lines, and red eyes. The color of the abdomen becomes darker later on in the season. The wing veins are very distinct and not blurred. On the first vein of the wings, the male SWD has two distinctive black dots, not patches. The male SWDs also have two, instead of one, dark bands, or combs, on their forelegs.

The only distinctive feature on the female SWD flies is their ovipositor. The ovipositor is longer than other vinegar flies and has two rows of serration, instead of one. The ovipositor is used to cut a slit in the flesh of the fruit to lay their eggs inside. SWD flies can lay seven to 16 eggs per day and up to 300 in their lifetime.

The indications of an infected fruit are not obvious. A hole from the ovipositor, the size of a pin prick, is the only visible damage before other indicators are visible. There may be some expulsion of liquid from the ovipositor hole. A few days after the eggs have been laid, the flesh of the fruit will begin to break down. Normally, the fruit begins to age around 4-5 days. The fruit breaks down by wrinkling, softening, collapsing, developing early mold, and becoming discolored. Keep in mind, some fruits, like raspberries, soften naturally, so they may be unaffected.

The best method to determine if raspberries are actually infected by the SWD flies is to set a trap. The basic structure of the trap starts with a cup with a lid. Small holes (3/16 – 3/8 inch) should be placed in the cup, leaving a section without holes to pour out the liquid inside. The holes should be equidistant and about 1 inch from the top of the cup (above the liquid). The holes need to be small so the trap is not letting in unwanted insects.

The liquid used for the trap can be any of the following: apple cider vinegar, vinegar with a drop of dish soap, or a yeast bait (1 tbsp. of active dry yeast, 4 tbsp. of sugar, and 12 oz. of water). Sticky traps should also be placed inside the cup to help catch the flies. The best time to hang the trap is when the average air temperature reaches 50 degrees Fahrenheit for several days, prior to the fruit ripening.

For fruit trees, the traps should be hung in the shade, near the fruit. For ground plants, such as strawberries, the trap should be hung in the plant, or securely placed on the ground. One trap per 5 acres is ideal. Check the trap weekly and replace the solution inside. Dispose of the solution in a location away from the traps. For identification purposes, insects can be separated from the liquid by using a coffee filter.

If you have correctly identified the presence of the SWD it is likely that larvae are present in nearby fruit. The larvae can be difficult to detect in fruit, because of their white/clear color. With close inspection one can observe the larvae moving within the fruit.

If unsuccessful, try using the Fruit Dunk Flotation method: Gather some berries and place them in a Ziploc bag. Gently crush the berries, then pour one of the following solutions into the bag: sugar and water (1/4 cup sugar to 1 quart water), brown sugar and water (1/4 cup brown sugar and 1 quart water), or salt and water (1 cup salt and 1 gallon water). The larvae will then float to the top. When using this method, it may be helpful to separate the fruit from the liquid and then filter out the water.

Prevention is key. Once the SWD has been discovered, management should be started immediately. Several insecticides are labeled on various crops for control of this insect.

For more information and a listing of available insecticides by crop, see NebGuide G2236, “The Spotted Wing Drosophila: An Invasive, Small Fruit Fly in Nebraska.” It is available on the Nebraska Extension Pubs website, extensionpubs.unl.edu. The direct link is http://extensionpublications.unl.edu/assets/pdf/g2236.pdf.

With insecticide use, be sure to rotate active ingredients in order to delay or prevent the development of resistant flies. Place traps away from the where the insecticide is sprayed.

Beneficial insects such as gall wasps, entomopathogenic fungus, bacteria, and parasitoids may be helping to eliminate the SWD. However, because the SWD is a new pest, it may take time for these beneficial organisms to establish.

With the use of any insecticide, always read the label. Stay tuned for more details regarding the presence of this insect pest in Nebraska as SWD surveys continue throughout the state.