How-to guide: Larval sampling in cherries to determine spotted wing drosophila infestation
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EmailNew video provides instructions on a quick and easy method for detecting drosophila larvae in your cherry fruits.
The following video provides instructions on how to detect spotted wing drosophila (SWD) larval infestation in cherry fruits. Research has shown that trap captures of adult SWD are not always accurate representations of actual larval infestation in fruits. Effective SWD management relies on understanding true infestation levels using the following larval detection methods.
Materials:
- Deli cups (16 oz)
- Markers
- Table sale
- Measuring cup
- Water jug or carboy
- Plastic funnel (2.5 x 10 inches)
- Wire mesh (0.08-inch holes)
- Reusable coffee filter with mesh bottom (4 cup)
- Optional: Stereo microscope with 10x eye piece
Methods:
- Start by collecting fruit into 16 oz plastic deli cups.
- Fruit samples should be taken from areas with a high chance of infestation, such as near a tree line with wild fruits that can be alternative hosts for SWD.
- Collect fruit from different areas of the tree canopy to get a representative sample.
- Collected fruit should be starting to ripen/soften so that they are vulnerable to SWD egglaying.
- Remove all stems from the fruits to ensure there is an opening for salt water to enter fruits and larvae to swim out.
- You don’t want to squish the fruits, this will cause excess fruit pulp in the sample which can look similar to larva.
- Next, we make our saltwater solution using 1 cup of table salt to 1 gallon of water.
- It can be helpful to use a small water carboy with a nozzle for easy filling.
- The salt serves to force the larvae out of the fruit, kills the larvae and causes them to float to the top of the sample.
- Some researchers prefer a sugar solution to salt because the larvae stay alive longer and continue to wiggle, making it potentially easier to identify them.
- Make sure the salt is well dissolved within the water by thoroughly mixing.
- Fill each deli cup of collected fruit with saltwater solution so that all fruit are fully submerged in water.
- Fruit should soak in the salt water for a minimum of 30 minutes.
- An hour of soaking can improve the likelihood of detecting larvae but is not always necessary.
- Once the sample has sat in salt water for at least 30 minutes, it can be filtered to extract the larvae.
- We use a modified funnel with a wide opening that fits well into a reusable plastic coffee filter. Inside the funnel we add wire mesh which stops fruit from falling through but allows water and larvae to wash into the coffee filter below.
- Fruits are dumped into the top of the funnel and thoroughly rinsed.
- Agitate the fruit gently to make sure each cherry is washed. Also rinse the cup, lid and your hands over the funnel to ensure any larvae are rinsed through to the filter.
- Now you can discard the rinsed fruits and inspect the filter under a microscope to look for SWD eggs or larvae.
- Eggs and the smallest larvae will only be visible through a microscope.
- The medium and large larvae will be visible without a microscope.
You can use this information to guide your SWD management programs prior to harvest, decide how intensively you need to spray, if you need a curative spray, or which blocks you might prioritize harvesting.
For more information on SWD management in cherries, view the Michigan State University Extension resource, “Managing Spotted Wing Drosophila in Michigan Cherry,” which provides more details on scouting strategies and management options.