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The three instars of SWD will emerge from fruit immersed in a salt solution. The smallest instar is about 0.5 mm long, the largest about 2 mm long.

Effective use of salt flotation will help you determine if your fruit are infested with SWD and if your spray program is working. It also will give you a perspective on what your customers may find when they take the fruit home to eat fresh or to make pies, jellies, jams and preserves.

I learned that variations on the salt flotation method helped a couple NY blueberry growers decide when to close this season. After a bad 2017 SWD season, when many NY blueberry growers suffered significant crop loss and shut down early, it was time to take action to monitor their fruit. Here are their methods.

Grower 1.

We analyze a batch of berries picked off bushes and a batch gathered that have fallen to the ground. Blueberries are collected randomly across our 5-acre patch. We test batches of 20-30 berries from these two sources separately and then compare.

Mix a solution of one gallon of water to one cup of salt. Place collected blueberries in two separate, labeled bags. Slightly squeeze the berries to help release larvae. Some say to give it about an hour, but in most cases, if larvae are present, they will show up in the solution as early as 15 minutes. Of course, you will want to use a magnifying device such as a jewelers loop or magnifying glass. You will see small white larvae if infestation is present.

Their results this year:

  • Aug 08:  negative, both from bushes and on the ground.
  • Aug 10:  negative, both from bushes and on the ground.
  • Aug 11:  positive, both from bushes and ground, but more pronounced with the latter.

In 2017, at least once, fruit tested positive for berries that were on the ground, but negative when picked from the bush.

Grower 2.

The salt flotation method we use is basically the same as the method demonstrated at the SWD workshops in 2014-2015. But instead of pouring the salt solution into a low tray and visualizing larvae over a black paper with a hand lens, we pour the solution through a very fine stainless steel mesh permanent coffee filter and check for larvae under a dissecting microscope. It's faster overall, and much easier to find the hard-to-see 1st instar larvae (probably to my detriment, since in the past I could ignore what I couldn't see).

We collect 100 berries randomly from throughout the planting. These are covered with salt solution (1 cup salt in 1 gal water) in a plastic bag. I don't bother pressing on the berries to crack the skins as they suggest, but gave them plenty of time to exit on their own (at least an hour, usually 2 or more).

Results this year, percent fruit with larvae, in our unsprayed blueberry planting:

  • Jul 26: 1.5%
  • Aug 3: 4%
  • Aug 9: 16.5%
  • Aug 11: 30%
  • Aug 13: 78% - in two days, the SWD numbers rose dramatically!

A male spotted wing drosophila (SWD) on blueberry, photographed in early September.

In all cases, collect what appears to be sound, perfect fruit to test for SWD infestation using salt flotation. SWD entrance and exit holes in fruit are less than half a mm in diameter and practically invisible.

I hope these two growers’ experiences using salt flotation will motivate you to monitor your fruit in this way to check for SWD infestation.

There are still a lot of delicious berries out there; lets check them and protect them from this nasty insect!

A blueberry grower in Tioga County monitoring traps daily for male SWD in their planting caught 6 males in one day, July 21, 2018. No SWD were found in traps up to and including Friday, July 20th.

Homemade SWD trap. The red cup contains an apple cider vinegar drowning liquid and a specimen cup with a wheat dough bait. Traps are checked daily for males.

The two traps are set in the hedgerow adjacent to the blueberry field. To quote the grower,

"Spraying is going to be hard with wet weather for the rest of the week."

The blueberry grower is making their own traps, using whole wheat dough as the bait and apple cider vinegar as the drowning liquid. Instructions for these whole wheat dough traps are on the SWD Monitoring page.

Only two Counties reporting zero SWD trap catch (gray) - Herkimer and Orange - as of July 23, 2018.

 

Elsewhere, across the CCE SWD monitoring network, all locations have reported in. Only the sites in Orange County and in Herkimer County have yet to catch SWD. Dutchess and Erie Counties are now at sustained catch. Details of these findings will be reported in a separate blog.

In the last week of July 2017, my program found up to 100 SWD in some of the SWD traps in our raspberry research plot in Geneva, NY. Without a doubt, pressure from SWD and fruit infestation levels will be increasing as the SWD population explodes.

Sampling cherries in 2016 in western NY to check for SWD. None was found, but in 2017 the situation was different.

In addition, fruit crops normally escaping infestation are reporting problems with SWD. Early blueberry varieties are at high risk and the later varieties will be even more prone to infestation. A tart cherry grower in the Lake Ontario region reports a load of fruit rejected by the processor due to SWD infestation.

When harvest dates are close to insecticide application dates, the available insecticides that can be used against SWD on berries or on tree fruit are few because of the needed days-to-harvest intervals. The heavy rains washing off applied materials creates a greater challenge to keeping fruit clean of infestation.

A mild winter; early arrival of SWD; warm, cloudy, rainy weather; abundant fruit; prolonged ripening windows — these likely have created a perfect storm for SWD in 2017. We continue to learn about this pest, what drives it, and what we can do about it. We thought summer raspberries, early blueberry varieties, tart cherries and sweet cherries weren't at high risk, perhaps we need to rethink this in light of this year's situation and be more vigilant in 2018 for conditions that favor SWD infestation in our early fruit crops.

Reports from blueberry growers have come in. SWD — caught in traps, found in fruit, and plantings shut down. One of these growers in the Southern Tier of NY, caught a single male in a trap on Sunday, July 23, 2017, and then,

"In a matter of 3 days my 4 traps exploded with SWD. A minimum of 3 females, the same for males, (in each trap). I have been spraying, but the weather is a problem."

For organic growers, managing SWD in blueberries this year will be nigh on impossible. It is essential to rotate insecticide active ingredients (ai), that is: not using the same active ingredient back-to-back, repeatedly. Entrust is the most efficacious organically-approved insecticide against SWD (ai spinosad), but it is essential to rotate with other ai's such as pyrethrin (Pyganic) or azadirachtin (AzaSol) or the biological Grandevo, which aren't as efficacious. The weather, with heavy and frequent rainfall, washes off the insecticides applied, making it necessary to reapply sooner.

Two male SWD on a blueberry, photographed in early September 2013. SWD populations typically build to very high levels in late summer and early autumn.

For no spray growers, the best approach will be to invest in exclusion netting for a long term solution to protecting the crop. Even early varieties of blueberries have been hit hard in locations where SWD was found early, whereas in prior years these varieties escaped infestation. This underlines the importance of monitoring in your local area, whether with the use of SWD traps or by sampling fruit using salt flotation to detect infestation. Sampling 100 fruit allows you to quickly get a rough estimate of the percent infestation level from the number of larvae detected in those (12 larvae found/100 fruit = 12% infestation).

There is no magic number or percent infested fruit at which a field should be shut down to pickers, for a u-pick or roadside market. Be proactive with customers and make sure they refrigerate or freeze fruit soon after purchase. However, for the processing market, there may be zero tolerance for SWD-infested fruit. And for some direct market growers when infested fruit is found that signals the time to shut down.

Eight female and one male SWD were caught on July 23, 2016 in traps set on the edge of a red raspberry planting in St. Lawrence County by Paul Hetzler, who works with Cornell Cooperative Extension of St. Lawrence County.

One female SWD and two males were caught on July 16, 2015 in traps set in blueberries in Columbia County and being monitored by the grower. This report came in courtesy of Laura McDermott, E NY Commercial Horticulture Program, who adds the site is in the southern part of Columbia County. The grower will begin a spray program for SWD on their blueberries.

Three males and one female SWD were caught in a trap today, set in a hedgerow near a blueberry planting in Tioga County. This trap is being monitored by the blueberry grower.

The suggestion I give growers is to check traps frequently, if they plan to set traps to monitor.  Frequently would be about every day. This way, not a lot of fruit flies and other insects get caught in the traps and it is easier to sort through the insect to check for SWD.

The treatment threshold is first trap catch and ripe fruit present in the planting. Keep this in mind and plan accordingly for an effective spray program that will protect your fruit for your markets.

Three SWD males were confirmed from traps set in raspberries and checked the week ending July 18, 2014 in Westchester County. A science project is underway at Stone Barns Center for Food and Agriculture, Pocantico Hills, NY, where children are learning to monitor and identify SWD males in the raspberry, blueberry, and currant plantings at the Center. Faruque Zaman, Suffolk County Cornell Cooperative Extension, and Juliet Carroll, NYS IPM Program, are assisting these young scientists and their mentors. (GDD 1441; day length 14:42)

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