Fruit IPM - Spotted Wing Drosophila Fruit Monitoring

Background

Unlike other vinegar (fruit) fly species (Drosophila spp.) that lay their eggs in over-ripe, previously damaged, rotting, and fermenting fruit, SWD will attack undamaged fruit as it ripens. Adults are small flies about 1/16-to 1/8-inch long with red eyes and an amber-colored body with black stripes on the abdomen.  Male flies have a black spot towards the tip of each wing. The females do not have spots on the wings, but they have a very prominent, saw-like ovipositor (egg-laying structure), larger than that of other vinegar flies. The female will penetrate the skin of soft-skinned fruit, laying the eggs just under the skin, leaving a small puncture ("sting") on the fruit surface (Figure 1). Eggs hatch and larvae develop and feed inside the fruit, and this damage can provide an entry site for other vinegar flies and secondary pathogens.

Fruit Sampling

Sampling for SWD in fruit is the best way to determine whether SWD is causing damage in your fields and to evaluate the effectiveness of management practices. Once SWD lays its eggs in a ripe fruit, the fruit begins to degrade, and other vinegar flies may also begin to infest it. Therefore, it is important to sample ripening fruit with no apparent damage rather than degraded or overripe fruit when determining SWD presence in fruit. The more fruit you sample, the more confident you can be in your results. We typically sample at least 30-40 fruit at market ripeness from various locations (including border rows) within the planting. Selecting fruit from the interior portions of the plant may increase your chances of detecting SWD. Recent research suggests that SWD density is higher in fruit in the central part of a plant. Most of the time, these berries also do not receive adequate spray coverage and are therefore not well protected from SWD.

There are a variety of methods to sample for larvae in the fruit. Perhaps the easiest is to look directly for larvae in the fruit. One of the earliest signs of larvae in raspberries is evidence of juice on the receptacle when the fruit is harvested. Individual fruit can be crushed or cut open, and you can look for larvae. A hand lens might help find some smaller larvae, but this method likely misses the smallest larval stages.

Larger larvae are visible to the naked eye.  SWD larvae are pointed at both ends and only a little longer than 1/8 inch when fully grown (Figure 2). SWD larvae cannot be visually distinguished from other vinegar flies, so selecting ripening good quality fruit rather than overripe fruit is important. Other flies that may be found within fruit, such as blueberry maggot, are larger at around 3/16 inch and are only pointed at one end (Figure 3). Other internal fruit-feeding pests such as plum curculio, raspberry fruitworm, cranberry fruitworm, and cherry fruitworm (Figure 4) may also be found with some of these methods.

• Figure 2. SWD larva in blueberry. Image: British Columbia Ministry of Agriculture, taken by T. Hueppelsheuser.
• Figure 3. Blueberry maggot in fruit. Image: Michigan State University, taken by Rufus Isaacs.
• Figure 4. Cherry fruitworm on blueberry. Image: North Carolina State University, taken by Hannah Burrack.

Larval Flotation

Larval flotation methods can be used to extract larvae from fruit and observe them floating on the top of a water solution. In this method, fruit should be gently crushed and/or cut into pieces (especially larger fruit, such as strawberries) in a container. This can be done in a thin layer in a plastic food storage container or in a sealed plastic bag, such as a gallon-sized resealable bag. Then a sugar (white sugar or light brown sugar works), water, or a salt (non-iodized seems to be better) water solution can be added. Usually, 1 cup of white sugar, table salt, or 2.5 cups of light brown sugar is used per gallon of water. Dissolving the salt or sugar in warm water a day or so beforehand can ensure it is fully dissolved and increase flotation. Sugar solutions tend to keep the larvae alive a bit longer, and live larvae are easier to see because they continue to move.

Agitate the fruit while it is in the solution, and allow 10–15 minutes for the larvae to emerge. SWD larvae (if present in the fruit) should float to the surface and be visible, other internal feeders, such as cranberry fruitworm that are larger tend to sink rather than float.

The only way to be 100% sure that you have SWD rather than other vinegar flies in your fruit samples is to rear the flies out of the fruit. Hold the fruit in a tightly sealed container (to prevent escape and secondary infestation by other vinegar flies) that still allows air to exchange. Making a hole in a plastic food storage container and then sealing it with fine mesh (like organdy) using caulk works well. Because the fruit will start to degrade, a lining like cotton, paper towels, or sand will be needed on the bottom to soak up the liquid and prevent the larvae from drowning.

Keep the containers at room temperature. You can then identify the emerging vinegar flies to see if they were SWD.

References

Dreves, A.J., A. Cave, and J. Lee. 2014. "A detailed guide for testing fruit for the presence of spotted wing drosophila (SWD) larvae." Oregon State University.

Hamby, K.A., M.P. Bolda, M.E. Sheehan, and F.G. Zalom. 2014. Seasonal monitoring for Drosophila suzukii (Diptera: Drosophilidae) in California commercial raspberries. Environmental Entomology 43: 1008-1018.