Discovering tiny larvae, often called “worms,” inside raspberries is a common and frustrating occurrence, especially in soft-skinned fruit. Understanding the source of these larvae and implementing immediate and long-term strategies can significantly reduce or eliminate this problem in current harvests and future seasons.
Identifying the Culprit
The larvae found inside raspberries are primarily the offspring of the Spotted Wing Drosophila (SWD), Drosophila suzukii, an invasive vinegar fly. Unlike common fruit flies that target only damaged or overripe fruit, the female SWD uses a unique, serrated ovipositor to cut into the skin of firm, ripening fruit. This allows infestation to begin before any visible damage appears.
The larvae are small, white to cream-colored, legless maggots, measuring up to 3 millimeters long. They feed within the raspberry drupelets, causing the fruit to soften and collapse. Adult SWD are tiny, 2 to 3 millimeters long, with red eyes and a yellowish-brown body. Males are identified by a single dark spot near the tip of each wing.
Immediate Steps for Infested Berries
If raspberries have already been harvested and contain larvae, a simple water bath, or “float test,” can compel the maggots to exit the fruit. This involves creating a solution that irritates the larvae, causing them to float to the surface.
The most common solution uses salt, typically 1 tablespoon of salt mixed into a cup of cold water. Soaking the raspberries in this solution for 5 to 15 minutes is usually enough time for the larvae to emerge. While some use vinegar, salt water is known to be more effective at drawing out the larvae.
After the soak, remove the fruit with a slotted spoon, leaving the larvae behind. Rinse the raspberries thoroughly under running water immediately to remove any residual salt taste. This process is safe but should only be done right before consumption, as washing increases the berries’ perishability.
Infested or damaged fruit not intended for consumption must be disposed of properly to prevent the larvae from completing their life cycle and re-infesting the patch. Do not discard the fruit in a compost pile, as SWD can survive and emerge. The most effective disposal method is to seal the infested berries in a clear plastic bag and leave the bag in direct sunlight for several days to solarize and heat-kill the larvae and eggs. Alternatively, the fruit can be frozen for several days or buried at least two feet deep in the soil.
Long-Term Management and Prevention Strategies
Preventing future infestations requires a multi-faceted approach focused on breaking the pest’s rapid life cycle using cultural controls and physical barriers. Since SWD populations increase rapidly through the summer, peaking in August and September, consistent management is necessary once fruit begins to ripen.
Cultural Controls
Sanitation is a foundational step in long-term control, as adult flies lay eggs in any ripe or overripe fruit. Harvest all ripe raspberries very frequently, ideally every one to four days, to remove the fruit before larvae can develop and emerge. Any unmarketable, damaged, or fallen fruit must be collected and immediately destroyed using solarization or deep-burying methods.
Pruning the raspberry canes to improve air circulation and light penetration also helps manage the pest. SWD adults prefer cool, shady, and humid environments, so opening up the canopy makes conditions less favorable. A comprehensive control strategy is especially important for primocane-fruiting raspberries, which produce fruit later in the season when SWD pressure is highest.
Physical Barriers
Exclusion netting is a highly effective non-chemical method that physically blocks adult flies from reaching the fruit. The netting must be very fine, with a mesh aperture size of less than 1.0 millimeter, as the SWD adult’s thorax is approximately 1.0 millimeter wide. Netting with an opening of 0.6 mm by 1.0 mm has been shown to successfully exclude the pest.
Apply the netting before the fruit begins to ripen and color, which is when female flies start laying eggs. Ensure the barrier is completely sealed around the bottom and has no holes or gaps, as SWD can exploit small openings. The netting should not touch the fruit, as the female can lay eggs through the mesh if the fruit is directly accessible.
Chemical and Organic Control
Chemical control strategies focus on targeting the adult flies, since the larvae are protected inside the fruit. The insecticide spinosad is considered the most effective organic choice against SWD. Other organic materials, such as pyrethrins, are also used but are generally rated as less effective than spinosad.
Timing is critical when using insecticides. Applications should begin as soon as the first adult flies are detected and the fruit begins to turn color. Sprays are often needed every five to seven days to maintain protection against the rapidly reproducing flies. Apply the insecticide with high volume and pressure to ensure thorough coverage of the fruit and foliage, especially in the shady interior of the plant canopy where the flies prefer to rest.