How to Extract Spotted Wing Drosophila (SWD) and Blueberry Maggot (BBM) Larvae from Harvested Blueberries using the Salt Flotation Method
A number of growers have asked about the methods we have been using to examine berries for the presence of spotted wing drosophila larvae. While there are several methods, we have used a salt water flotation method adapted from Washington State (Bruck et al. 2011). This method uses ¼ cup of salt in 1 qt of warm to room temperature water, but is adapted here for larger quantities. We have used a sample size of 2 qt of freshly picked berries, never refrigerated, but held at room temperature if needed. This method will also extract blueberry maggot larvae, but these are rarely found in commercial blueberries, and they are ever present in other types of soft fruit.
- A baking pan, 9″ x 13″ dark or painted black
- ¼” hardware cloth, cut to fit inside the pan (Figure 1)
- A weight such as a metal rod
- Three 5 gallon buckets, one for the fresh solution, and the other two for the used berries and the used solution
- Colander, to separate the berries from the salt water when done
- Hand lens
- A direct light source
This procedure utilizes salt solution made in 5 gal. buckets. For the original recipe of ¼ cup of salt per qt of water, you need 5 cups of salt per bucket. One standard 26 oz can of salt contains 2 ⅓ measured cups by volume. You can estimate the solution by using 2 ¼ cans of salt per 5 gal bucket. If you use a little more salt, it is not critical.
- In a 5 gallon bucket add 2 ¼, 26 oz cans of salt, fill with water and stir. Allow the solution to rest for about 20 minutes until it is room temperature and the salt is dissolved. A 5 gallon mix of solution is enough to test about 10 quarts of berries.
- Place 1 quart of berries in black pan and pour 2 quarts of salt solution over berries. (Figure 2)
- Place hardware cloth over berries in pan and use a weight such as a metal rod to hold it in place. (Figure 3)
- If necessary add more salt water solution until the water level is about a half inch above the berries.
- Any maggots will immediately float to the surface; place light directly over the pan to increase visibility (Figure 4) and use the hand lens to scan the surface for any larvae. SWD larvae are fairly small and may not be visible without magnification.
- The maggots will be alive and may move to the edges of the pan or the edges of your weight, examine those areas carefully and move your weight around to dislodge any maggots that may be hiding there. After about 15 minutes nothing more will emerge from the fruit.
- Count and record the number of each type of maggot found, blueberry maggot larvae can be much larger and are a slightly different shape. BBM larvae are “squared off” on the posterior end, while SWD larvae are tapered at each end. SWD larvae will be up to 3–4 mm in length, but young ones will be smaller. BBM larvae can be up to 5-6 mm in length. The vast majority of what you will find in picked blueberries will be SWD larvae.
- When finished, use the colander to strain the berries away from the salt water solution; the used solution can then be disposed of down a drain.