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Wednesday, June 15, 2011

Do it yourself - Spotted wing drosophila monitoring

As we begin to find spotted wing drosophila (SWD) in more and more locations, growers and homeowners are expressing interest in monitoring for adult flies.  Our volunteer monitoring network uses simple, homemade traps that can be easily adapted for use by others. (Enlarge photos by clicking on them.)  

Trapping supplies include:
Apple cider vinegar (not "flavored")**
1 to 3 quart sized plastic containers with 10 to 12 1/4 inch holes one inch from the top
Unscented dishsoap
Forceps (not shown)
Sieve (not shown)
Solid containers (not shown)
Sorting tray (not shown)
Sample vials (not shown)

Yellow sticky cards (optional)
Cellophane (optional)
Microscope, magnifier, or hand lens (preferred)
**Update, 18 April 2013 - For 2013, we are suggesting that yeast & sugar baits be used instead of apple cider vinegar. Read about how to make and use yeast & sugar lures here.

Many of the trapping supplies can be either be purchased at the grocery store (the deli section will often let you have a few quart sized (32 oz) containers) or through online retailers.  I either use a soldering iron or a 1/4 inch drill bit to punch 12 holes in each trap.  I then string a hanger (nylon cord) through 2 opposite holes.  Yellow sticky cards, forceps, and magnifiers can be purchased from several sources*; I order ours from Great Lakes IPM or BioQuip.

Traps are baited with 1 to 2 inches of apple cider vinegar.

Yellow sticky cards increase the attractiveness of the traps, but also make it more difficult to distinguish female flies from native Drosophila spp.
Because yellow sticky cards improve trap attraction, we are using them at SWD volunteer monitoring network sites where we have not previously caught SWD.  However, because the yellow sticky cards complicate SWD identification & trap processing, we stop using them once SWD is confirmed at a location.

Traps should be hung directly from plants (blueberries, caneberries, grapes, fruit trees, etc.) or placed in the fruit zone of low growing plants (strawberries) near fruit.  SWD prefer shady areas with available moisture, so traps hung in these areas are most likely to catch flies.

This trap was hung from the trellis wire near a heavily fruiting blackberry plant.
Traps should be checked at least weekly.  If using yellow sticky cards, they should be removed, wrapped in cellophane, and marked with the date and trap number.

Sticky cards are only changed if suspected SWD are present or if they are no longer sticky.
A regular kitchen sieve (8 or 10 gage mesh) is small enough to prevent adult SWD from passing through.
Using a 8 to 10 gage mesh sieve (a standard kitchen sieve or sink strainer works well), filter apple cider vinegar into a solid container.

Be sure to line up the holes in the trap with the sieve to avoid spilling vinegar. 
After you have filtered the vinegar, return it to the trap and refill to 1 to 2 inches.  You can reuse the apple cider vinegar but should completely change it at least every 4 weeks.  Used apple cider vinegar should be discarded away from the field so as not to attract flies away from traps.  We are currently studying how long apple cider vinegar and other lures are attractive to SWD, so recommendations on reusing lures may change.

Replace traps after they have been refilled.  
Rinse collected insects off the sieve in to the solid sample container and return them to the office  or lab to process.
The collected insects should be compared to voucher specimens (right) or images of known SWD.
While male SWD are readily distinguishable with little or no magnification, female SWD are more difficult to identify.  Ideally, suspected female SWD (flies that are similar is size, shape, and color to voucher specimens) should be observed under 20x magnification.  The key distinguishing characteristic for female SWD is their large, serrated ovipositor.

The ovipositor of female SWD is large, serrated, and readily distinguished from native Drosophila spp.
Can you spot the male SWD?
Compare suspected SWD to known voucher specimens (available upon request from Hannah).
If you suspect that you have captured SWD adults, check your identification with your cooperative extension agent, extension specialist, the NC Plant Disease & Insect Clinic, or other experts.

Because we are still learning about SWD biology in North America (and the southeast), our current management recommendations are conservative.  I suggest the growers who capture SWD at their farm utilize pesticide treatments (either conventional or organic).  See here for a list of management tools available for North Carolina small fruit growers and their likely efficacy against SWD.  This list is only applicable to North Carolina growers.  If you are outside NC, consult your local cooperative extension agent for registered management tools.  You can find your local cooperative extension office here.

*Does not imply endorsement of named vendors over others.

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