When it comes to monitoring for Spotted Wing Drosophila, (SWD), early detection is important. Traps for SWD, baited with apple cider vinegar or yeast bait are useful, especially when trap catches from several locations in a region are combined. However, in 2013 we found SWD larvae in fruit at approximately the same time we found the first SWD in traps. This means that on-farm trapping is not sensitive enough to detect the earliest SWD infestations on the farm. Until better baits, traps and thresholds for SWD are developed, growers need a second tool for monitoring this invasive pest. Fruit sampling at harvest is recommended to detect early infestations, and to determine the effectiveness of your SWD control program.

Larvae from infested fruit will float out if fruit is placed in salt water. Dissolve 1 part salt in 16 parts water (1/4 cup salt in 4 cups water). Place a sample of sound, marketable fruit (approx. 100 ripe, healthy-looking fruit) in a shallow dish or tray. Pour salt water over the fruit until fruit is completely covered. Mash fruit lightly with a potato masher. In 10- 15 minutes, larvae will float to the surface of the solution. To detect very small larvae you will need a hand lens and good lighting. Look for small, white larvae, tapered at both ends, and 1-4 mm in length. It is impossible to distinguish SWD from other drosophila larvae at this stage; however only SWD attack sound marketable fruit. If you are using ripe, not overripe, fruit in the tests, emerging larvae are probably SWD.

Salt water fruit immersion provides a quick test to determine if SWD is present in fruit, although numbers observed may underestimate the level of infestation. Success with this technique could be improved if fruit is collected and held at room temperature for 1-2 days before immersion in salt water. This will provide time for eggs to hatch and larvae to grow to a size that is easier to see in the salt water solution.

Figure 1: Materials needed for salt water test

Figure 2: Small larvae floating out of fruit submerged in salt water