Managing spotted wing Drosophila update

Trapping, fruit sampling and fruit protection methods can help manage spotted wing Drosophila.

Many berry growers are aware that we have seen an earlier arrival of spotted wing Drosophila (SWD) during this scorching summer. As of July 2, 2012, these detections have been in Berrien, Van Buren, Allegan, Ottawa and Muskegon counties. Detections in these counties likely reflect the higher density of our monitoring traps in this region of the state, and we advise growers of susceptible fruit in all Michigan counties to initiate active monitoring for this pest. SWD have been trapped in raspberry, strawberry, blackberry and blueberry plantings. This update provides information on monitoring, fruit sampling and fruit protection to help growers meet quality standards for their crops.

The first detections of SWD have been getting earlier in the past three years: Sept 23 in 2010, then July 3 in 2011 and May 29 this year (2012). The earlier activity here in Michigan and elsewhere may reflect better survival through the recent, mild winter. This is the second season that growers have needed to manage for this pest during the harvest season, but with detections coming a month earlier, there is more of the harvest period that overlaps with this pest’s activity.

There are some important insights into this pest that we have learned in the past few years of research, and some new results that can help growers optimize their management for this pest. Much of this information is posted at the MSU IPM Spotted Wing Drosophila website and berry growers can check for more details at the management guides posted there.

SWD trapping update

For the second year in a row, the traps baited with a yeast-sugar mix (1 Tbsp. active dry yeast: 4 Tbsp. sugar: 12 oz water) are outperforming those with apple cider vinegar, by a long way. This pattern is being seen in both raspberry and blueberry fields. For example, from 73 traps of each of these two baits deployed across four blueberry farms, we have trapped 390 SWD in the yeast-baited traps this season so far, but only seven in the apple cider vinegar baited traps. Most of these SWD have been trapped in the liquid, so this is drained across a mesh surface before sorting to look for SWD flies. At least 80 percent of the SWD found in these traps have been females, which do not have the distinctive wing spots that males possess. To identify the females, you must look for the long, dark ovipositor and this will take a microscope or 30x hand lens. For more on identification of SWD, we have developed a pictoral guide posted at our SWD website.

Yeast-baited trap
Yeast-baited traps are catching more SWD
than those baited with apple cider vinegar.

Whichever bait is used in the traps should be checked and changed weekly at a minimum. We have found declining activity of both apple cider vinegar and the yeast mix baits after one week. The attractive radius of a trap for SWD is also expected to be quite small, so different fields that might be ripening at different times should each be monitored if growers want to know when SWD activity starts in each field.

Sampling for SWD in berries

Sampling for SWD in berries can help inform management decisions by providing indication of infestation and an assessment of how well a spray program is working. Now that berries are ripening, weekly samples can be taken to look for the number of SWD larvae. These are small (2 to 4 mm long) and require a boil test or salt solution test to detect them. To do the boil test, simply cover a sample of berries with water just to the top of the fruit then boil them in a microwave for a minute. Pour the sample through a 0.25-inch hardware cloth onto a tray, mash the berries with the back of a spoon and look for the larvae in the liquid in the tray. Alternatively, berries can be collected and placed in a Ziploc bag, mixed with a salt solution (quarter cup salt per quart of water = a cup per gallon) and left to sit for 30 minutes or more. These can then be assessed for larvae. Watch a video of the salt bag method.

SWD control

Summer and fall ripening berries must be protected from first color until harvest if SWD is active in your farm. The most effective SWD materials are the organophosphates Imidan and malathion; the synthetic pyrethroids Asana, Brigade, Bifenture, Danitol and Mustang Max; the carbamate Lannate; and the spinosad insecticides Entrust and Delegate. Neonicotinoid insecticides such as Actara, Assail or Provado are not effective against SWD and should not be used for this pest.

Due to the zero tolerance for fruit infestation in the food industry and the increasing captures of SWD being detected in monitoring traps, if ripe fruit are present and SWD is active in your immediate area, we recommend fruit protection at this time. Maintaining fruit protection will require reapplication based on the product used previously and its expected longevity, the weather conditions and the harvest schedule. Refer to the table below for guidance on residue effectiveness in blueberries under dry, normal conditions.

Insecticides for SWD control in blueberries

Class

Trade name

Active ingredient

PHI
(days)

Days of
activity#

Organophosphate

Malathion*
Imidan

malathion*
phosmet

1*
3

5-7*
7

Pyrethroid**

Mustang Max
Danitol
Asana
Brigade/Bifenture

zeta-cypermethrin
fenpropathrin
esfenvalerate
bifenthrin

1
3
14
1

7
7
7
7


Hero

bifenthrin+zeta   cypermethrin

 1

7

Carbamate

Lannate

methomyl

3

3-5

Spinosyn

Delegate
Entrust (organic)

spinetoram
spinosad

3
3

7
3-5

Pyrethrum

Pyganic (organic)

pyrethrum

0.5

2-3

* The new label for Malathion 8F allows only 1.25 pints per acre and this rate has not been tested in efficacy trials. A maximum of three applications are allowed per season. Also, check the label for your specific Malathion formulation for the correct PHI. Most are one day, but some may allow 0.5 day PHI.
** Residual control will be reduced during hot sunny weather.
# Estimated residual activity from experience with other insect pests in Michigan and from SWD studies in Oregon.

However, this summer is far from normal. Hot, sunny weather and rain can both reduce the longevity of pesticides, so it is important to consider how the current or future environmental conditions will affect spray intervals. With the continued 90 degree Fahrenheit heat we are experiencing, the residual control of pyrethroids is expected to be shortened, whereas the organophosphates are less susceptible to this effect.

Relative rainfastness of Imidan and Mustang Max residues on blueberries

Imidan - Fruit Residues

Imidan - Leaf Residues

Mustang Max - Leaf Residues

Mustang Max - Fruit Residues 

Although we have had too little rain this summer, it is also worth reviewing recent studies on the wash-off potential for pyrethroid and organophosphate (OP) insecticides on blueberries. This varies according to tissue type (fruit or leaf) and the amount of simulated rainfall applied to the fruit. OPs formulated as wettable powders are highly susceptible to wash-off following precipitation, with residue losses on fruit up to 75 percent following 0.5 inch of rainfall, and similar losses on leaf tissue after 1 inch of rain. Synthetic pyrethroids, like Mustang Max, adhere well to the waxy surfaces of blueberries. Although less persistent than OPs, they tend to be more resistant to wash-off with residue losses on fruit up to 60 percent following 2 inches of rainfall, but relatively little negative impact on fruit and leaves when rainfall levels are less. A rainfall decision chart has been developed in the E-154 Michigan Fruit Management Guide published for 2012 by MSU Extension.

Overall, the pattern is for greater rainfastness of pyrethroid insecticides and greater “sun-fastness” of organophosphate insecticides. This information can help guide selection of sprays to protect fruit based on the predicted weather conditions.

Related MSU Extension News article: Spotted wing Drosophila: Training is essential for management and control

Drs. Isaacs’ and Wise’s work is funded in part by MSU’s AgBioResearch.