Spotted Wing Drosophila fruit flies are an important economic pest on a variety of fruits and berries. In non-tropical areas, Spotted Wing Drosophila overwinter as adults and first emerge in spring, after which 8-10 generations may occur in a year. Researchers from Oregon State have developed a growing degree day model for Spotted Wing Drosophila that can help predict when overwintered females will first begin to lay eggs and when other life-cycle stages, such as egg-laying, will occur. It can be used to determine the best time for setting up monitoring traps, or when to apply insecticide treatments.

*Validation of this model has not been documented outside of the Pacific Northwest, so it should be used with caution and tested for accuracy all locations.

Calculating Growing Degree Days

The Spotted Wing Drosophila model uses a lower threshold of 50F. It can be calculated with daily high and low temperatures, using the sine model to estimate growing degree day units, or the Pest Prophet app can be used to calculate GDD accumulation from hourly temperature data for a specific field.

The first egg laying by overwintered females begins at 260 GDD(F), using Jan. 1st as a starting point, (biofix) for beginning the model accumulation. After this the first new generation of adults will begin to emerge at 510 GDD (F) and peak at 755 GDD (F). This range is a good opportunity for setting up monitoring traps. First egg-laying by this generation will begin at 565 GDD (F) and peak at 995 GDD (F).

Adjusting the Biofix Date

When adults are first consistently caught in traps, the biofix can be adjusted to reflect this first date that adults were consistently caught. Egg laying will begin 55 GDD (F) after this date. This can be easily done in the Pest Prophet app by setting the biofix stage to “First Adult Emergence – First Generation”. Adjusting the biofix date will help improve the accuracy of the growing degree day model over using Jan. 1st as the starting point without readjustment.

Re-adjusting the biofix for each generation, based on adult trap catches, is recommended, especially following insecticide sprays. Later in the season many generations will overlap in the lifecycle stages, so the growing degree day model will be less useful, unless they are used in conjunction with insecticide treatments and trapping / monitoring.

Sources:

Kanzawa, T. 1939 Studies on Drosophila suzukii Mats.,

Sakai, M. and Sato, R. 1996. Bionomics of Drosophila pulchrella (Diptera: Drosphilidae) Fukushima Fruit Tree Exper. Sta