How seasons affect population structure, behavior and risk on spotted-wing drosophila

Key points

Spotted-wing drosophila are less active during cold winter months, and mostly survive as adult females during this period.
SWD exist as adult In two morphs or body types. In cold-winter climates, SWD adults can assume a larger, darker-colored winter morph or body type..
When temperatures rise above 50°F (10°C) for more than five consecutive days, populations can increase.
When mean daily temperatures are below 50°F (10°C) or above 82°F (28°C), SWD become less active and reproduce more slowly.
As the growing season progresses, generations overlap, resulting in a relatively stable distribution among different life stages.
There is a stable population structure during the growing season. During this time, populations consist largely of immature life stages that are protected inside fruit.
During the growing season, the population structure consists of 90% eggs, larvae or pupae and 10% adults.
During hot summer periods, most activity, including egg laying, occurs during dusk and dawn.

Seasonal population structure and reproduction

Most spotted-wing drosophila that survive the winter do so as adult females. Winter morph females mate during fall, store the sperm over winter, and infest the first ripening fruits in early spring. As temperatures fall below 50°F (10°C), few SWD females carry eggs and the population decreases. When temperatures rise above 50°F (10°C) for periods longer than five days, the fertilized eggs within SWD females start to mature and the population begins to increase above the overwintering level (see Figure 1).

When temperatures reach 64°F (18°C), adult females lay more eggs (Figures 1 and 2). Eggs hatch into larvae, which develop into pupae and then adults at increased rates as temperatures climb during the summer. As the season progresses, it is possible for the generations to overlap, resulting in a relatively stable distribution among different life stages (egg, larva, pupa, adult). In Oregon, approximately 90% of all life stages are eggs, larvae and pupae during this period, with only 10% in the adult life stage (Figure 3).

This illustration is explaning the process of the spotted-wing drosophila's population reproductive status in a graph. The right has the percentages from 0-100 in increments, on the left the temperature in farenheit, the top is the year and the bottom of the graph is the months during that year. of 104 shades of green from dark to light representing no eggs, unripe eggs, ripening eggs and mature eggs. The graph ultimately shows that at lower temperatures there are less mature eggs in comparison to warmer.

Figure 1. SWD reproductive biology during three consecutive years (2013–15). Different green shades represent the proportion of the different reproductive stages of SWD females at any time; specifics could vary in another region or growing season. The red line represents temperature. The population distribution illustrates a general trend; specifics could vary in another region.

Credit: Grassi et al.

This graphic displays the optimal tempertures for the spotted-wing drosophila in the style of a graph. There is a thermomiter on the left with the temperature in celsius starting at 0 and going up to 40 degrees in increments of 10, at the bottom it shows development rate ranging in colors from red to green with red meaning low and green meaning high. The graph ndicates that the most optimal temperature is around the 25°C, and sub-optimal temperatures being around 15°C and 33°C.

Figure 2. Lower and upper temperature thresholds of SWD.

Credit: Torchen et al.

In Oregon, approximately 90% of all life stages are eggs, larvae and pupae during this period, with only 10% in the adult life stage (Figure 3).

This graphic is explaining the percentage of spotted-wing drosophila in each of their stages throughout the year. The percentages are on the left in increments of 10 from 0 to 100, on the bottom its the months of the year from January to December. The adults are at 100% throughout the year, while the eggs stay in 50% range from February until November, Larvae are in the 80% range between February and November and the Pupae are above the 90% range from February until late November.

Figure 3. Generalized population structure of SWD through the growing season. Notice that during the winter months, 100% of the population is overwintering as adults. During the growing season, much of that population is in the immature egg, larval and pupal stages and are protected within the fruit or in the soil. Adults are most susceptible to insecticide sprays.

Credit: Wiman et al.

One reason SWD is so difficult to manage is that eggs, larvae and pupae are well protected within the fruit or in the soil. Also, a steady stream of adults reaches reproductive maturity and attacks the crop. When average daily temperatures drop below 50°F (10°C) or increase above 82°F (28°C), SWD become less active and reproduce at lower rates. However, the insect can optimize its behavior to daily temperature cycles. During hot summer periods, the majority of activity, including egg-laying, occurs during the cooler dusk and dawn periods (Figure 4).

This graphic explains in 3 graphs the day and night activity of the spotted-wing drosophila. On the left is a bar representing the level at which the activity is being done in 2 colors yellow as low and red as high. on the bottom of the graph is the time of day. Graph a is the flight which shows dawn and dusk being at its highest. Graph b shows that mating is at its highest at dawn. Graph c shows that egg laying only happens at dusk which is also its highest point.

Figure 4. Daily flight (a), mating (b), and egg laying (c) activity of SWD during the growing season. These behaviors are concentrated during cooler parts of the day, and diminish during the hotter parts of the day. All of these activities drop off during the dark because SWD uses vision to navigate and because its circadian rhythm causes it to rest during the night.

Credit: Hamby et al.

About this series

This publication is one of a series of nine publications focused on strategies for controlling spotted-wing drosophila in Oregon. Find them at https://catalog.extension.oregonstate.edu/. The publications in this series include:

About the authors

Vaughn Walton

Entomologist

Linda Brewer

Senior Faculty Research Assistant II, Department of Horticulture

Daniel Dalton

Graduate student, horticultural entomology