Spotted Wing Drosophila (SWD) in Ontario: Frequently Asked Questions
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Temperature affects the rate of insect development. SWD has four life stages: egg, larva, pupa and adult. Development from egg to adult can occur in as little as 10 days (or less) under optimal conditions. At lower temperatures, it may take 2 to 4 weeks. SWD prefer moderate temperatures and humid conditions. Hot, dry weather may have a negative impact on SWD activity and reproduction. There are multiple, over-lapping generations each growing season.
Residual activity describes the persistence of an insecticide as a crop protectant against a given pest; knowing the residual activity is important in terms of knowing when re-application is required. The products that we have available for Emergency Use against SWD in 2012 target the adult fly, not the eggs or the larvae. Most of the products should be effective for 7 days, with the exception of Pyganic (maximum 5 days) or Entrust (maximum 5 days). Thorough coverage is critical. Keep in mind that temperature and precipitation events may affect the residual activity of products. Hot, dry conditions such as those seen in most parts of Ontario this year may cause pesticide droplets to evaporate very quickly and before they reach the crop. Pesticide residues (and sometimes their activity against a pest) tend to decrease at high temperatures. You can reduce this effect by applying insecticides at times of the day when it's cooler. SWD adults tend to avoid hot, dry conditions, and are more likely to be active in the crop canopy when it is cooler.
SWD will lay eggs in residual fruit, potentially contributing to a population build-up later in the season, and potentially to pressure the following year. Adults that emerge from infested fruit can migrate into other nearby crops. Renovate June-bearing strawberries after harvest. For other crops, remove all hanging fruit and culls. In crops where it is difficult to remove all fruit, consider the use of a post-harvest insecticide following harvest. Growers with a decreased crop load (example cherries in 2012) and who implemented a reduced spray program are at risk for higher pressure in other crops.
SWD females do not appear to lay eggs in cull pile fruit. However, if the fruit was infested prior to disposal in the cull pile, immature forms will complete their development and emerge as adults. Therefore, it is important to manage culled fruit regularly (at least weekly and daily if possible) using one of the following methods:
Since high temperatures are required to kill immature SWD, composting is not recommended. Crushing fruit is not a reliable way of destroying eggs and larvae.
SWD is an introduced pest of Asian origin. It was first identified in California in 2008. This pest has been present in Hawaii since the 1980s. SWD has now been found in fruit production areas throughout North America and parts of Europe. The most likely pathway for long-distance and local introduction into an area is through infested fruit. Overwintering adults are susceptible to freezing conditions. In Ontario, they are likely to survive only in protected areas and in low numbers. Populations build up over the growing season. Pest pressure may fluctuate from year to year depending on overwintering mortality and environmental conditions early in the season. Because SWD is now widespread, there is potential for re-introduction though infested fruit into areas where the pest does not overwinter successfully. SWD appears to be established in Ontario. It has been found in isolated natural areas, where wild hosts like uncultivated raspberries, blackberries, mulberry, elderberry and wild blueberries are commonly found. These act as potential reservoirs of SWD in the landscape that growers need to consider. SWD will move into crops from unmanaged or wild areas where they may be established earlier in the season.
There is ongoing research to evaluate post-harvest treatments to kills eggs and larvae that may be present in fruit. Cooling may be an option in produce that can tolerate extended periods (more than 4 days) at low temperatures (< 2°C) without a resulting impact on quality. More information is required before we can make recommendations for post-harvest treatments in fruit.
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