Bug juice

“Better eat that peach on the kitchen counter before it draws fruit flies.”

Nearly everyone has either heard or said that. Although common fruit flies are easy to manage in the kitchen, genetically similar spotted wing drosophila (SWD) adults are close to the same size and also seek fresh, ripe fruit. The difference is in their reproductive habits and habitat.

Dr. Kelly Hamby, associate professor and Extension specialist, University of Maryland, has done extensive work on SWD. Since its arrival in the U.S. from Southeast Asia, SWD has become the key insect pest of soft-skinned fruits in America.

It’s easy to see SWD damage in the field: tell-tale pitting on fruit after females pierce the fruit skin to lay eggs inside. Firmer fruits such as cherries and blueberries have sunken areas, and soft fruits such as cane berries and strawberries show bruising.

Hamby emphasized the importance of understanding the SWD lifecycle to better manage it. The female SWD uses a sharp ovipositor to cut into fruit so she can lay eggs. Depending on temperature, eggs hatch into larvae, go through three larval instar stages and become adults in about 10 to 15 days.

“They reproduce very quickly,” Hamby said. “During the crop growing season, they are typically active around dawn and dusk when it’s cooler and more humid. Most of the population is in the juvenile stages: eggs, larvae and pupae.”

Throughout the day, SWD adults move between woods and crop areas. They may start in wild vegetation, move into crops, then return to wild areas. In winter, SWD is typically more active in the middle of the day when it’s warmer. SWD overwinter as adults in wild vegetation.

“Adult SWD that successfully overwinter comprise the early season population, which is the lowest throughout its lifecycle,” Hamby said. “As mature SWD lay eggs throughout the season, pressure increases. The earliest crops get the least damage, and the latest crops have the most.”

SWD prefers blackberries and black and red raspberries, so those who grow them are likely to see the heaviest SWD pressure. One solution is choosing earlier varieties to reduce SWD challenges. However, solving the problem isn’t that simple. In some years, there is significant damage in early crops.

While earlier cultivars experience lower SWD pressure, insect populations build as the season progresses. Summer-bearing cane berries typically have less pressure than autumn-bearing berries, and growers can stay on top of the problem more easily than with autumn-bearing berries.

One option for preventing SWD damage is growing firmer fruit with thicker skin. Hamby noted research into using biofilms and foliar calcium fertilizers to make fruit skin firmer to deter egg-laying. However, this strategy only works when there are other food options for the SWD.

Another solution is to block flies from fruit using exclusion netting tunnels. For most effective exclusion, mesh should be less than 1 mm. Hamby recommends installing mesh tunnels before SWD is in the fruit.

“Continue to keep flies out of the tunnels,” Hamby said. A double door will help prevent SWD adults from entering.

Exclusion netting can inhibit pollination of fruit such as cane berries because these plants continue to fruit and flower after netting is up. Blueberries have one-time pollination, and the netting can be put up after pollination. Bumblebees in the tunnel can increase pollination.

“One of the advantages of this strategy is that tunnel-grown fruit is often higher quality,” Hamby said. “It’s possible to get 100% control of spotted drosophila without additional insecticide sprays, especially in blueberries. Netting helps control SWD and reduces damage due to weather events and birds.”

Monitoring and risk assessment are critical in SWD control. When scouting, be aware that only the males have spots. Food lure-based baits and synthetic lures help attract adult SWD, and a drowning solution or sticky cards capture them.

Adult traps should be placed in shady areas, including field and interior areas of field borders. Border traps can help show where SWD are active on field edges. Interior traps indicate how far SWD has come into the crop area. Place lures at fruit level on cooler and shadier sides of the plants where flies are more active. While traps can indicate SWD presence, they don’t indicate populations.

“We recommend [traps] in the field before fruit starts to color,” Hamby said, adding that careful timing helps with spray scheduling. “You can also monitor for SWD larvae. There are other larvae in soft-skinned fruits. SWD are flies, so there isn’t a head capsule like on raspberry fruit worm or a sap beetle. They have a darker area in their head region – that’s the mouth part used to scrape fruit.”

Hamby noted the importance of diversified farms to check for overripe fruit, including pre- and post-harvest crops and not just the crops under active management because SWD are often present in higher numbers and easier to detect. Primocane and floricane raspberries are good indicators of SWD presence, especially in the interior and lower canopy areas, which tend to be hot spots because they’re darker and cooler.

Chemical management is the primary method for controlling SWD. Some organic products such as Entrust® rank well for control. Rotate products according to mode of action to delay resistance. Hamby said there are fewer insecticide products that are effective on cane berries compared to blueberries and cherries. Check label for complete usage instructions and product rain fastness.

“Raspberries can be hard to spray,” Hamby said. “You need to get thorough coverage throughout the canopy into the center of the canopy to control them.”

Options for cultural control are based on creating a less favorable crop habitat. Because SWD may be pupating in the soil, mulches and irrigation can impact survival in the soil. Remove cull fruit and consider pruning and trellising to open the crop canopy to increase temperature.

Shorter harvest intervals can help eliminate cull fruit. Harvesting frequently and removing cull fruit can reduce infestation up to 60%. Cull fruit should not be discarded in the field; instead, bag and solarize culls or bury them at 10 inches or more.

“If it’s gotten away from you and you really have a lot of infested fruit, you can reset by stripping off all of the fruit then implement management strategies again,” Hamby said. “On diversified farms, it can be hard to find all the places where cull fruit is producing SWD.”

by Sally Colby