Have We Found Spotted Wing Drosophila Yet in Fruit Crops this Season?

Posted: May 29, 2012

I’ve been getting a lot of calls from growers asking whether spotted wing drosophila (SWD) has been found in fruit crops in Pennsylvania yet. The good news is that it hasn’t been showing up in traps, not even in locations with SWD problems last year. So far, so good in Maryland as well. However, this doesn’t mean that growers should let their guard down.
Kathy Demchak, Penn State Department of Horticulture


Below is an article that will also be printed as a fact sheet in the next few weeks.  This outlines how to monitor for SWD so you know whether it is moving into any fruit crops on your farm.

Monitoring for Spotted Wing Drosophila

K. Demchak, Dept. of Horticulture, Penn State
D. Biddinger, Dept. of Entomology, Penn State
B. Butler, Univ. of Maryland Extension in Carroll County

Spotted wing drosophila (SWD) is an invasive vinegar (fruit) fly that was first detected in Pennsylvania in 2011. This pest lays eggs in ripening fruit, so its larvae may be present in fruit at harvest.  Some PA growers lost large portions of fall raspberry and day-neutral strawberry crops to this pest.  By monitoring for SWD, growers can know whether and when action is needed.

Monitoring for Adult SWD with Traps 

Using bait traps allows positive identification of the adult male flies. Trap use as described here is for monitoring, and not for providing control.  See the Penn State fact sheet “Managing Spotted Wing Drosophila” for information on control measures.

Types of Traps and Lure

Homemade traps can be made inexpensively from 16 to 32 ounce clear drink cups with lids, deli containers, or rigid screw-top wide-mouth plastic jars. Six to twelve holes 3/16” to 3/8” in diameter should be drilled or burned with a soldering iron in the upper half of the container extending about 2/3 of the way around.  Do not drill holes around the entire container or flies will be lost when the vinegar bait is poured out.  To hang the trap, thread a wire through two holes drilled opposite each other near the top (Fig. 1), or a paper clip or screw can be inserted through a small hole in the lid.  Applying red tape or paint to the trap may make it more effective. Commercially produced traps are available.  One common brand is Contech.  This trap is convenient to use and catches only small insects, excluding bees, flies, and sap beetles. This trap catches fewer SWD than homemade traps because it has only two entrance holes (Fig. 2).

One or two inches of apple cider vinegar plus one drop of unscented dish detergent is the recommended lure.  Yeast mixtures and various juices may attract more flies, but are messy, attractive to animals, and/or opaque making observation difficult.  Vinegar both traps and preserves SWD. The detergent breaks the vinegar’s surface tension so the flies sink rather than escape (Figs. 1 and 2). 

Sticky Cards

Sticky cards do not improve trap attractiveness, allow adults to degrade over time, and make identification of female SWD more difficult for researchers or regulatory personnel who may check the cards later. Sticky cards should be used only when individuals intend to check samples themselves.

When and Where to Place Traps

Traps should be in the field when fruit begin to color. Female SWD fly earlier in the season than males and may be caught first, but identifying them is difficult without a microscope. Any thin-skinned fruit (strawberries, cherries, raspberries, blackberries, grapes, etc.) should be monitored.  SWD has been found on cherry tomatoes and wild berries. Late season fruit crops are especially vulnerable as SWD populations are highest then. Even slightly attractive crops in tunnels should be monitored. 

Traps Should be at Fruit Level

For stability in low-growing crops, dig a depression to hold the trap, tie it to a stake, or use a short container.  SWD is more likely to be found on the shady side of the row and where humidity is highest. Traps should be placed on the north side of rows in mid-field (Fig. 3).

The optimum number of traps per area has not been determined.  A good starting point is one trap per acre or in smaller fields, one trap per susceptible crop.  Traps can be moved to later crops as they ripen.  As with all fruit flies, SWD will continue to breed in dropped fruit residues after harvest.

Checking the Traps

Traps should be checked about once per week.  Used vinegar should be replaced with fresh vinegar as attractiveness may decrease.  Discard old vinegar away from the planting.

Draining off the vinegar and replacing it with water makes it easier to see the wing spots and black bands on the front legs (see the fact sheet “Spotted Wing Drosophila: Overview and Identification”).  Pouring the solution into a shallow white container or a clear container on a white background increases contrast.  More water can be added to disperse the flies.  A magnifying or hand lens will be needed.  A close-up digital photo can be taken if the water is shallow - this keeps all the flies in focus (Fig. 4).  The photo can be viewed on a computer screen, zooming in as necessary.

Thresholds for Treatment

Thresholds have not been established due to SWD’s recent arrival. One adult male per trap per week is cause for concern, so fruit should be checked for larvae (maggots) as outlined below.  Fifteen adult males per trap per week indicate a population that is threatening to the crop. These numbers are likely to change as we learn more about SWD.

Storing and Shipping Samples

If samples must be transferred to a container, the solution can be strained through one half of a mesh tea ball (Fig. 5), or a funnel lined with fine netting or fabric (tent netting, organza, or tulle fabric).  The holes in typical kitchen strainers and screen door netting are too large.  Flies can then be washed into a container. Alternatively, you can blot the strainer on a paper towel to wick out moisture which frees up the flies, then gently gather the flies with a craft brush and roll them into another container (Fig 6). 

Apple cider vinegar can be used to store and ship samples for about a month. If shipping samples, the containers should be sealed with electrical tape and placed in a zip-lock bag.  Samples should be labeled with the collection date, crop, location, and other pertinent information, preferably in pencil.

70% ethyl alcohol (ethanol) is typically used to store samples in laboratories. Rubbing (isopropyl) alcohol makes SWD brittle.  Both types of alcohol are flammable, dissolve the writing of alcohol-based marking pens, and may be subject to shipping restrictions.

Checking Fruit for Larvae

Larvae (maggots) may be present in fruit even before adults are caught.  Monitoring fruit for larvae also indicates whether sprays are effective. To check, mash fruit in a zip-lock bag and add a salt solution (1/4 cup salt to 4 cups water).  The larvae will float at the solution surface, while fruit will sink.  SWD larvae are white, about 1/8” long when full-sized, and have no obvious head.  Differentiating SWD from maggots of other species is nearly impossible.  However, if maggots are found in recently ripened fruit, they are likely to be SWD.  If larvae are larger and have a head, they may be that of sap or picnic beetles.  Pupae of SWD can be differentiated from other species with the help of a specialist. 

Fruit can also be pulled apart and checked for larvae.  If raspberry receptacles are stained with juice, SWD larvae may be present, though staining may also simply mean that the fruit is overripe.  A small oviposition hole surrounded by decomposing fruit tissue is a clue regarding where to look for larvae or pupae.

This article is part of a project funded by the Northeastern IPM Center through the Urgent IPM Grant Program.

Photos:  Figs. 1, 2, 4, 5, and 6, K. Demchak.  Photo 3, B. Butler.
Reviewers:  S. Spichiger and E. Rajotte.