by Sally Colby
Dr. Amy Dreves, Oregon State University, says Drosophila suzukii, more commonly known as the Spotted wing Drosophila (SWD) is destroying the IPM programs developed in the 1940s and 50s.
“The SWD invasion has forced a lot of fruit growers to drastically increase chemical applications,” said Dreves. “IPM pulls together a number of practices — it doesn’t rule out pesticides, however, there is a heightened awareness of the downfalls and concerns of using only pesticides as a control method.”
Dreves believes understanding the life cycle of the SWD is an integral part of a good management program. She explains the SWD has four growth stages, starting with the adult. As for most insects, growth for any of the stages is dependent on environmental conditions such as temperature, moisture and host availability. “The adults appear, at this point in the Pacific Northwest, to be the only life stage in that overwinters,” said Dreves. “They can live up to five months. But they typically only live a month or so during summer, and many don’t make it through winter.”
Adults are opportunistic and will lay eggs in spring if a host is available. The ovipositor of the female SWD is a unique, saw-like appendage that burrows deep into soft fruit. The larvae pupate, sometimes inside the fruit and sometimes outside.
Dreves says that it’s critical that growers know how to identify SWD accurately, and can learn to do so with the help of websites such as www.uspest.org/swd . The unique ‘spot’ is clearly visible on the male, but not on the female. The female’s prominent, saw-like-ovipositor may be difficult to see without a good hand lens and keen observation skills.
Monitoring for SWD requires a good trap and bait. “It’s important that traps and bait are sensitive to early catch,” said Dreves, “particularly in areas that are very cold like Michigan and Colorado, and specific to SWD females.”
Through trapping, growers can determine when SWD is in the area and in a particular crop, and gauge population levels. It’s important to identify hot spots in each production area in order to plan treatments and measure the effectiveness of a treatment. Good records throughout the season will help growers track shifts in populations and concentrations of the insect throughout the season.
Traps vary in size and style, but visual clues such as red stripes are important, and stripes are more appealing. Most traps have a side or bottom entry, although some are larger and can hold more insects. Homemade traps can work well if they’re designed to meet the specific requirements that attract the SWD. Yeast and sugar water are good basic attractants, and recipes for homemade baits are available on the website above.
The SWD has a relatively short range attraction, so the bait should hold up in the field. Traps must be serviced regularly, depending on which trap is used. Some commercial traps with commercial bait last as long as four to six weeks.
Dreves says that growers can take advantage of monitoring systems such as the Integrated Plant Protection Center at Oregon State University, which uses a temperature model to predict and alert egg-laying in spring.
Even with monitoring, growers should be aware of the signs of SWD presence in the field, including bruising, tissue collapse and softening, juice at egg deposit sites, presence of larvae and larvae feeding inside fruit. It’s especially important to be aware of ripening times so crops can be monitored for damage at the right time. Growers should train workers to be aware of the signs of SWD, and encourage them to report any signs that SWD is present.
Dreves wants growers to become proactive and use a four-season approach to effectively manage SWD. “In winter, temperatures drop and light is reduced,” she said. “In the Pacific Northwest, we’re getting precipitation, and Colorado may be getting snow. This is a prime time to find out where they’re concentrated.” Dreves explains that in the post-harvest stage, the SWD is in diapause, but when temperatures are above 48 degrees, the insect begins to feed.
When the SWD wake up in spring, they will begin to search for a food source, but at first they remain mostly in their winter refuge. Dreves encourages growers to begin thinking about what kind of monitoring techniques and traps will work best before the insect starts to move out of their winter refuge. “What works for you?” she said. “Do you want to build traps or buy them? What fits in your budget, time and objectives?” Dreves suggests monitoring both the crop and perimeter areas; watching for shifts in numbers that suggest insects are moving into production areas. Be aware of the season changes, environmental conditions change and as fruit starts to ripen.
In one study that measured temperature and insect activity, researchers found that the majority of flies remained in the protective tree habitat when temperatures were lower, and moved into the crop as temperatures rose. “We’re trying to identify those points of when they’re in the trees and when they’re in the crop, and then use that information,” said Dreves. “We identified hot spots (of insects), and they seemed to be in clumps. Where are SWD hanging out in the off-season? Perhaps in riparian areas, since they love moisture, and in protected areas.” Non-crop hosts, including citrus, could be the off-season habitat for SWD in California. “Not when the citrus fruit is on the tree, but when it falls on the ground,” said Dreves. “That is their main choice for feeding. They could be there for the pollen, sap, honeydew, protection or refuge. Know where they are, and flag those areas.”
Keeping records of counts in each monitored area will help determine a pattern. Be aware of whether fly count numbers are higher than the year before, because numbers will eventually build up. “In the PNW and in Canada, we had higher numbers in traps in 2014 and 2015 because we had mild winters,” said Dreves. “That’s good information.”
Trap catches are always higher in locations where there is high moisture, so it’s important to know where excess moisture is accumulating. Discourage ‘stray’ moisture by repairing leaky lines and burying hoses whenever possible. Proper pruning will open up and expose berries to higher light and lower humidity, thus discouraging SWD from establishing habitat. Trellis plants to provide support that encourages upward growth, and eliminate excess canes and sprouts that can become habitat for SWD.
Increase air movement, decrease shade and maintain mowing and trimming to eliminate tall grass. “Incorporate black weed mats, particularly in a blueberry system,” said Dreves. “They heat up and kill eggs and larvae in fallen fruit.” Cleaning up waste by raking, crushing or drying fallen berries also helps. Dreves noted that research showed greater than 50 percent reduction in SWD simply with the use of weed mats.
Always strive for good plant health and nutrition. “With the proper nutrition, you have fruit firmness,” said Dreves. “It’s fitness advantage and pest resistance.”
Get a head start on managing Spotted wing Drosophila
by Sally Colby