SWD series #2
Cultural controls can reduce spotted-wing drosophila population pressure by modifying the microclimate within the crop so the environment does not promote SWD reproduction and survival.
The rate of SWD development and reproduction depends on temperature and humidity. Flies can develop from the egg stage to reproductive adults within one to two weeks when temperatures are within optimum range: 71–82°F (22–28°C) and humidity is 80%–90%.
SWD undergo a complete metamorphosis. Eggs develop into three larval stages, pupae and finally adults. During the growing season, the egg, larval and pupal stages last from one to three, five to seven, and four to 15 days, respectively. In the mid-Willamette Valley, SWD have five to ten generations during the growing season (Figure 1, page 2).
Adult life span during the growing season is about three to five weeks. Flies emerging late in the season are adapted to overwintering and may live longer. A single female can lay up to 40 eggs per day and 200–400 eggs total in her lifetime. A female fly lays approximately one to three eggs per oviposition site. Multiple females may lay eggs in the same fruit, giving rise to greater infestation of individual fruits and multiple clutches of larvae.
Immature life stages are sheltered inside the fruit. Eggs are laid directly beneath the skin of the fruit. As the eggs hatch, the larvae consume the fruit pulp. Ultimately, they molt into pupae. Life stages within fruit are protected from adverse conditions, such as dehydration, temperature fluctuations, and full-cover insecticide applications. SWD pupae and adults are more susceptible to adverse environmental conditions than earlier life stages. SWD can pupate inside or outside the fruit. At high infestation levels, a greater proportion of pupae will be found outside the fruit. In such cases, larvae will drop out of fruit before they develop into pupae. Once out of the fruit, pupae tend to burrow into the soil to find a more suitable and protected environment.
All factors that affect temperature and humidity in an orchard will also affect the environment for SWD (Figure 2). Overhead irrigation increases humidity and can lead to higher rates of SWD reproduction and development compared to drip irrigation.
Weed fabric prevents pupating SWD larvae from burrowing into the soil. SWD find more suitable temperature and humidity conditions an inch under the soil surface, as opposed to warmer and drier conditions found on the soil surface or weed fabric.
Pupae are very susceptible to desiccation and do not survive when exposed to low humidity.
Surrounding vegetation provides more optimal habitat and microclimates for SWD during the hottest hours of the day. Proper management of surrounding vegetation may alleviate SWD pressure by reducing “hot spots” outside the crop where SWD can overwinter or escape field sprays.
Detailed work showed that SWD lay eggs at elevated levels in the center of the plant canopy. Fewer eggs are laid in the upper and outer portions of the canopy. Eggs laid in fruit in the upper and outer portions of the canopy are more prone to high temperatures during the summer period (Figure 3).
Heavier pruning of dense canopies where SWD hotspots occur can reduce suitable microclimates and result in lower SWD pressure on the crop. Areas where pressure is reduced may require less pruning.
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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: