Building and using traps for monitoring SWD in fruit plantings
By Rufus Isaacs, MSU Entomology, Version 1, October 2010
Monitoring for Spotted Wing Drosophila (SWD) flies can be done with a trap consisting of a large plastic cup container with holes and apple cider vinegar to attract the flies. To create holes that allow entry of small flies while excluding larger insects, drill or melt small (quarter-inch diameter) holes towards the top of the container sides. It is a good idea to leave 1/3 of the circumference around the cup without holes to allow easy pouring out of the liquid. Add a drop of dish soap to the vinegar to ensure flies remain trapped. If using this basic design, check weekly for SWD in the vinegar and replace with fresh vinegar to retain optimal attraction.
Adding a sticky card to the inside of the trap provides a surface on which to catch the flies, and this can be replaced easily each week. Small-sized traps are available for greenhouse monitoring programs, and they work well for SWD traps. The card can be hung on a small, plastic-covered paperclip that is poked through the lid. We have found that rain or irrigation on these traps can enter through the hole, thereby diluting the vinegar. Sealing the hole with a small dab of hot glue keeps the water out. Use a piece of wire to hang the trap in the plant canopy.
Traps of this design are estimated at approximately one dollar per trap to construct, including raw materials and assuming 100 traps per hour can be built by someone earning $10 per hour. Constructing large numbers of traps provides some economy of scale.
In the image above is an SWD trap showing the plastic container with holes around the upper edge to allow fly access. The yellow sticky surface catches flies when they are attracted to the vinegar in the bottom of the trap. Place traps in the fruit zone with the traps in the shade
Traps for SWD are best placed in areas where the flies most like to spend their time, so place them in the shade, in the fruit zone. For strawberries this is on the ground, whereas for bush fruit or tree fruit, the trap needs to be hung up in the plant. Place the trap on the north side of the plant to remain in the shade. Early experience with SWD monitoring in Michigan suggests traps are most likely to catch flies if placed in areas of fields that remain in the shade or have ripe fruit remaining on the plants. Using multiple traps per field will help pinpoint areas of farms where flies are active.
Check traps weekly to determine the first presence of SWD flies. Keeping clear records is important to be able to identify trends in fly populations, and for seeing whether management programs are reducing fly activity. The vinegar should be replaced every 1-2 weeks to retain activity. Dispose of old vinegar away from the trap location to minimize competition with the newly-charged trap.
If resources will allow, having a second set of traps on hand allows rapid changing of traps in the field, and reduced time during field visits. The collected traps can then be sorted through back indoors where it might be easier to see the distinguishing features of flies. If this is not possible, placing the small yellow sticky traps or the vinegar into a ziplock bag can aid in easy transportation of collected insects to a location for identification. Be sure to write the farm, field, date, and trap checker initials on the traps or bag so that fly numbers can be matched with trap location.
Flies stuck to a trap (left). Close-up of same trap showing SWD males (right).
It is important to be able to accurately identify the number of SWD on traps, especially when numbers are low and the population is starting to build. The wing dots of male flies (see images below) are relatively distinctive and can be clearly seen with the use of a basic hand lens. The foreleg is banded.
Male SWD identifications.
Identification of female SWD on traps is more challenging because the ovipositor is not always visible and may be stuck under the fly if it is on the glue. A small pin and higher powered handlens will be very helpful for teasing flies into a position where the ovipositor can be seen clearly. Female SWD have a serrated ovipositor (see image below) with dark teeth along its length. Seeing this with a handlens is possible, but being able to look at flies under a microscope will greatly aid in ability to accurately count female flies.
Female SWD identifications.
For a detailed pictorial key, see web hosted by the Oregon Department of Agriculture.
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