Horticulture News: A tiny menace – Spotted Wing Drosophila
Published 8:21 pm Saturday, July 18, 2015
With the season progressing, our berries are starting to ripen and ready to pick. In the past we had to worry about deer, squirrels, birds and beetles competing with us for those tasty little morsels. Add to that a fairly new pest, the Spotted Wing Drosophila (SWD).
Drosophila suzukii, also known as SWD, was first detected in Kentucky in 2012 and is a serious invasive pest of soft-skinned fruits. By midsummer of 2013, SWD could be found throughout Kentucky at high levels. Last year, many producers noted that SWD infestations did not appear to be as bad as 2013.
It is still unclear the extent of their activity this year.
This fruit fly lays its eggs in soft-skinned fruits as they soften before harvest, which not only causes issues for the commercial grower but the home gardener, as well.
SWD attack many different soft-skin fruiting crops, but the most serious damage in 2013 was to raspberries, blackberries, blueberries and grapes. It also breeds in the fruit of native wild plants like pokeweed, mulberries, wild blackberries and nightshade.
The female SWD uses her serrated egg layer (ovipositor) to cut into the skin of otherwise intact soft-skinned fruits to deposit her eggs. Each female deposits seven to 16 eggs a day and up to 300 eggs during its lifetime.
Eggs hatch in two to 72 hours, resulting in small larvae in harvested fruit. SWD will overwinter in Kentucky as it survives in states to the north and parts of Canada.
Adult SWD have an amber body color, red eyes, dark banding on their abdomen, and clear wings. Male SWD, easier to identify than the female, have a black spot on each wing and have two small dark bands on its front legs.
Female SWD, lack the spots on their wings and banding on the legs, have to be identified by looking at their serrated ovipositor and wing-vein characters. Many gardeners find larvae in the fruit before noticing the fruit flies around the plants themselves. The larvae are white and may reach ¼-inch in length.
When harvesting, take the time to thoroughly look over each berry. Externally, berries may look uninfested but larvae may still be inside. If a berry is very soft, collapsing on itself, or is watery near the cap of the fruit, SWD larvae are most likely present.
These berries should be discarded away from the field, and preferably bagged for disposal. There is not a known risk to human health by ingesting SWD eggs and larvae.
Small fruit growers can use the fruit floatation method to sample small fruit periodically for SWD. Only samples of fruit that appear undamaged should be tested for SWD.
To sample fruit for SWD larvae with the floatation method, place about a dozen ripe, apparently undamaged fruit into a gallon bag. Add a cup of sugar syrup (mixture of ½-cup sugar mixed into 1 quart of water) to the bag and seal the bag. Thoroughly mash the berries, letting the berries settle to the bottom of the bag. Any small, white larvae that are present should float to the top.
Freezing, netting, sanitizing, monitoring and applying chemicals are effective ways to manage this pest.
Freezing: Berries with no visible damage should be placed in the refrigerator immediately. Placing the berries in the refrigerator will stop the development of the hatched and unhatched SWD if present.
Freezing berries will kill SWD and holding berries at 34F for 72 hours will also kill most of the eggs and larger larvae.
Netting: Mechanical control maybe an option for homeowners, as with many of the small fruit crops we often use netting to keep the birds from eating the berries. The same concept applies for SWD except the netting must be a fine mesh, with opening of less than one millimeter.
ProTekNet is a brand of netting that provides netting small enough (less than a one millimeter square opening) that can exclude SWD, but spun bond row covers should work, as well.
Netting should be placed over the planting when the earliest berries begin to turn color prior to harvest. Netting may interfere with pollination of later raspberry flowers, but this may be a sacrifice to save the rest of the crop. The netting must be secured along the ground to prevent any openings for SWD to enter.
On larger plantings a structure could be installed to help support the netting and allow a person to pick underneath the netting. The netting will have to remain over the crop until harvest is finished.
Sanitation: It is also important to remove any damaged fruit from the field, as these fruits may contain SWD eggs and larvae. Overripe, damaged, or rotting fruit should be collected, placed in clear bags, and left in the sun. Burial of infested fruit is ineffective as the larvae can emerge from depths of one foot or more.
Monitoring: Trapping can be used to monitor SWD and possibly reduce their numbers. Traps are used to detect SWD presence and are being evaluated in the Northeast United States for use in home gardens as a method of control when placed no more than 30 feet apart.
Many different types of flies and species of drosophila are captured in these traps. To identify SWD, the flies should be poured into a light-colored pan and examined with a hand lens. The males are recognized by the single black spot on each wing.
These fruit flies are attracted to several kinds of traps using apple cider vinegar or yeast baits and are simple to make at home. For information on recommended baits and trap designs see http://www.fruit.cornell.edu/spottedwing/pdfs/SWDTraps_CornellFruit.pdf.
For commercial plantings, traps have not been able to reliably predict SWD presence prior to their detection in fruit.
Chemical control: Insecticides can also be used as management for SWD. Using insecticides is costly for the homeowner with the need for spraying equipment, cost of insecticides and the amount of time needed to spray each week.
If a homeowner decides to spray, applications will need to start when fruit begins to turn color and soften. Good spray coverage is essential, especially the leaf undersides and the denser parts of the plant canopy where the SWD can be found throughout most of the day.
Insecticides must be applied weekly as the crop starts to ripen and through harvest. Persons using insecticides to control SWD need to be familiar with and follow Pre Harvest Intervals (PHIs) for each specific insecticide spray.
Managing SWD takes a lot of work but it is doable. Important points for SWD management are recognizing the pest, trapping/monitoring, removing wild host plants, using netting prior to the berries ripening, placing picked berries directly in the refrigerator and cleaning up after harvest.
For more information or printed publications, contact the Franklin County Extension Office. Call 502-695-9035 or email Adam.Leonberger@uky.edu.
Source: Jessica Cole and Patty Lucas, Extension Associates, and Ric Bessin Extension Entomologist