The discovery of small, white larvae emerging from a freshly picked strawberry is common for many home gardeners and consumers. This phenomenon, often called “strawberry worms,” can make people question the safety of their fruit. This article clarifies the identity of these tiny invaders and provides practical solutions for cleaning harvested berries, along with long-term preventative measures for growers.
Identifying the Culprit
The “worms” found in strawberries are the larvae of the Spotted Wing Drosophila (Drosophila suzukii), a specific type of fruit fly. This species is distinct because the female possesses a specialized, serrated egg-laying organ, or ovipositor. This tool allows the fly to cut into the skin of healthy, ripening fruit to deposit its eggs, unlike other species that only target overripe or damaged produce.
The resulting larvae are tiny, legless, and white or cream-colored, typically measuring about one-eighth of an inch (3 to 4 millimeters) at maturity. They feed and develop just beneath the fruit’s surface, causing the berry to soften prematurely. Since the eggs are laid inside the fruit while it is still firm, the infestation can be completely hidden until the larvae grow large enough or the fruit is soaked in water.
Post-Harvest Removal Techniques
For berries that have already been harvested or purchased, a simple, non-toxic method can encourage any hidden larvae to exit the fruit. This involves preparing a mild salt water solution, which uses the principle of osmosis to draw the larvae out of their protected feeding sites. Create this solution by dissolving one tablespoon of salt for every cup of water you use.
Gently submerge the strawberries completely in this salt solution for 15 to 30 minutes. The change in salinity irritates the larvae, causing them to emerge from the fruit and float to the surface of the water, where they can be skimmed away. The larvae are not considered harmful if accidentally consumed.
Once soaking is complete, thoroughly rinse the strawberries under cool, running tap water. This removes any residual salt taste and washes away remaining larvae or debris. Immediate refrigeration after rinsing and drying helps maintain the fruit’s quality, as cooling to 35°F (about 2°C) halts the development of any remaining eggs or larvae.
Strategies for Field Prevention
The most effective approach for growers is to implement strategies that prevent the adult fly from laying eggs, thus eliminating the problem at its source. A foundational strategy is meticulous sanitation throughout the growing season. Adult Spotted Wing Drosophila are strongly attracted to fermenting, damaged, or overripe fruit as breeding grounds.
Sanitation and Cultural Practices
Gardeners must perform frequent and complete harvests, ideally picking all mature fruit every day or every other day. Any unmarketable, dropped, or rotting fruit should be removed from the field immediately and destroyed, perhaps by sealing them in clear plastic bags and leaving them in the sun to kill the larvae and pupae. Pruning the plants to create a more open canopy helps reduce the humidity and increase light penetration, making the environment less appealing to the adult flies, which prefer shaded, moist conditions.
Exclusion Netting
A highly effective physical barrier is the use of exclusion netting, which prevents the tiny adult flies from accessing the fruit. This fine mesh must be installed over the crop before the fruit begins to color and must be secured completely to the ground to prevent fly entry. Netting with a mesh size of 1.0 millimeters by 0.6 millimeters, often referred to as 80-gram netting, is successful in blocking the flies.
Monitoring and Trapping
Trapping is primarily for monitoring the pest population rather than for significant control. Simple traps can be made using a container baited with a fermenting mixture of yeast, sugar, and water, or apple cider vinegar. The presence of adult flies in these traps signals that the fruit is at high risk and that more aggressive control measures should begin immediately as the fruit starts to ripen.