Salt water assessments for SWD larvae in fruit
When SWD are active, all aspects of a management program, including regular insecticide applications, frequent and timely harvest, and post-harvest cooling, are needed to keep damage to a minimum.
Growers can keep tabs on SWD by monitoring fruit closely for damage. One way to do this is by salt water assessments of marketable fruit. When marketable fruit are covered in salt water, larvae in the fruit will float to the top. Routine assessments can be used to determine how well your management program is working, when it's time to shorten intervals between sprays, or the best way to market your crop.
To do a salt water test you will need; water, salt, a measuring cup, a large jar or jug, a dark tray or pan about 3-4 inches deep, and a hand lens or magnifying glass. A potato masher or heavy spoon is also handy (Figure 1) .
Figure 1: Supplies for salt water assessments.
Mix up a salt water solution in advance so the salt has time to dissolve. Use 1 part salt to 16 parts water. For example, mix 1/4 cup of salt in 4 cups of water.
Next, head to the field and collect 50-100 sound, ripe fruit of good quality. Do not use damaged or over-ripe fruit. Choose the earliest ripening fruit and continue until harvest is completed. Select only marketable fruit, collecting some from sheltered areas of the bush or field, where SWD is likely to hang out. Do the test immediately or set the fruit at room temperature for up to 1 day. Do not refrigerate and do not place in an air tight container. Protect the fruit from other fruit flies during this holding period.
When you are ready to do the test, place the fruit evenly in the pan, no more than 2 fruit deep. Record the number of fruit in the sample.
Shake up the salt water solution and pour into the pan until the fruit is completely covered (Figure 2) . Gently mash the fruit and set it aside for 2-3 minutes (Figure 3) .
Figure 2: Pour salt water over a layer of fruit.
Figure 3: Gently break up the fruit just to break the skin. (Don't smash the fruit into a pulp)
If SWD larvae are present they will wiggle out of the fruit and float to the surface. Look for small white larvae, pointed at both ends (Figure 4). When magnified, black mouth hooks are visible. The larvae will be moving at first, in a serpentine pattern. Some will be very small, only 1 mm in length. Larger larvae may be 3-4 mm in length. (Figure 5). Watch for small larvae moving towards the sides of the pan. Record the number of larvae observed, the number of fruit and the date of the test.
Figure 4: SWD larvae floating in salt water.
Figure 5: Larvae of all sizes are visible here. These fruit were infested in the lab for demonstration purposes, so they are plentiful.
SWD has many hosts and presents a season-long challenge to production. Once detected, numbers and risk to fruit will continue to increase through the season. Using this salt water test will help growers detect early problems and adjust their SWD management programs if necessary.
For more information on spotted wing drosophila in Ontario visit the SWD website or give us a call.
Thanks to Geraldine Ryan and Lisa Emiljanowicz, University of Guelph, for photos in Figures 2-5.
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