Spotted-Wing Drosophila

(View male/female identification card)

Spotted-wing drosophila is established in all California counties where blueberries are grown but is of greatest concern in areas along the coast. Adults and larvae (maggots) closely resemble the common vinegar fly, Drosophila melanogaster, and other Drosophila species that primarily attack rotting or fermenting fruit. Spotted-wing drosophila, however, attacks undamaged fruit prior to harvest.

Adults are small (2–3 mm) flies with red eyes, a pale brown thorax, and abdomen with black stripes. The most distinguishable trait of the adult is that the males have a black spot near the tip of each wing. The female ovipositor is very large and serrated, so it is able to penetrate the skin of soft-skinned fruit and lay one to three eggs in each. The female will lay eggs in many fruit.

Eggs are laid within the fruit skin and can be recognized with a hand lens by two white breathing tubes that extend out of the surface of the fruit.

Larvae are tiny (up to 3.5 mm), white, cylindrical maggots that are found feeding in fruit. One or more larvae may be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.

Spotted-wing drosophila prefers mild, humid weather that usually occurs throughout the year in coastal areas of California and during the spring and fall in inland valleys. Peak activity occurs when temperatures are in the 60s and 70s (°F), which is typical during harvest seasons across all production regions of the state.

Spotted-wing drosophila has a life cycle that varies from one to several weeks depending on temperature and may have as many as ten generations per year. This rapid developmental rate allows it to quickly reach high numbers and inflict severe damage to a crop.

Unlike other vinegar flies that occur in California, spotted-wing drosophila is a significant pest that attacks healthy fruit prior to harvest. As fruit integrity is compromised by oviposition and larval feeding, common vinegar flies (i.e., Drosophila melanogaster) may also oviposit in the damaged fruit. Damage can also provide entry for infection by secondary fungal and bacterial pathogens, but this is not always the case. Often damage is not seen until the fruit is at the market; fruit is soft and has a reduced shelf-life.

Oviposition creates a small depression ("sting") on the fruit surface. Many larvae are possible within a single fruit because females can lay more than one egg in each fruit and multiple females will oviposit on the same fruit. Maggots develop and feed inside the fruit, causing the flesh to turn brown and soft. When infested fruit are gently squeezed, fluid exudes out of the hole in the fruit surface where the egg was laid.

Management of spotted-wing drosophila is required any time susceptible fruit and flies are present at the same time. Apply insecticides based on field history and trap catches, with the goal of preventing oviposition; insecticides do not control larvae within the fruit.

Spotted-wing drosophila natural enemies have not been observed.

Application of the Entrust formulation of spinosad is the only control method available to organic blueberry growers.

When applying pesticides, it is extremely important to consider resistance management. Drosophila flies have a history of rapidly developing resistance to insecticides that are used repeatedly and frequently. Additionally, insecticides such as spinosad and fenpropathrin are also two of the key insecticides used for citrus thrips control later in the season. If control of both pests within a season is needed, carefully plan the insecticide treatments in a way that reduces the risk of resistance development for both pests.

Monitor spotted-wing drosophila using homemade traps.

• Make a trap capable of holding liquid.
• Traps should be able to hang from a wire within the blueberry canopy.
• Traps need holes or other openings in them to release the scent of the bait, and for spotted-wing drosophila to enter the trap. Holes should not be too big or larger nontarget insects can enter.

The two most common versions of these traps are various sized bottles with 3/16-inch holes drilled in them or containers with mesh on top.

1. Bait traps with 1/2 to 1 cup of apple cider vinegar (depending on the size of the trap).
2. Add 1 teaspoon of a dishwashing detergent per gallon of apple cider vinegar to increase the number of flies that fall into the liquid bait and drown.
3. Place traps low in the blueberry canopy.
4. Check weekly.
5. Count the number of male flies (the ones with spots on the wings).

Counting male flies should be adequate for determining whether or not spotted-wing drosophila is present, and for observing if numbers are going up, down, or remaining the same.

If trapping programs indicate the presence of adult spotted-wing drosophila, apply an insecticide when the first berries begin to turn pink, with a second application just prior to harvest. Green fruit are very poor hosts and there is no indication that management after harvest has any benefit for the following season. These treatments target adults before oviposition occurs because effective larvicides are not available.

After treatments, if traps indicate that adult flies are still present, especially if flies are migrating into the field from other crops such as cherries or caneberries, additional treatments may be warranted at 7- to 14-day intervals until harvest is over.