The insect most commonly called a “strawberry fly” is the spotted wing drosophila (SWD), a small fruit fly that lays its eggs directly inside ripening strawberries. Unlike ordinary fruit flies that only go after damaged or rotting fruit, this species attacks healthy berries still on the plant. A second pest sometimes grouped under the name is the strawberry whitefly, which feeds on leaves rather than fruit. Both can cause serious problems, but the spotted wing drosophila is the one most growers and gardeners are worried about.
Identifying the Spotted Wing Drosophila
Adults are tiny, only 1/16 to 1/8 inch long, with red eyes and a pale brown body marked by black stripes on the abdomen. Males are easy to identify: each wing has a distinctive dark spot near the tip. Females lack the wing spots but have a feature that makes this fly uniquely destructive. Their egg-laying organ is large and serrated, essentially a tiny saw blade that lets them cut through the skin of intact, ripening fruit. Most other fruit flies can only deposit eggs in fruit that’s already broken or decaying.
First detected damaging fruit in California in 2008, SWD has since spread across much of North America. It infests strawberries, raspberries, blackberries, blueberries, and cherries, with strawberries in coastal and humid areas being especially vulnerable.
How SWD Damages Strawberries
A female uses her serrated ovipositor to puncture the berry’s skin and deposit one to three eggs just beneath the surface. This leaves a tiny depression, sometimes called a “sting,” on the fruit. She doesn’t stop at one berry. A single female lays between 100 and 600 eggs over her lifetime, spreading them across many fruits. Multiple females may also visit the same berry, so it’s common to find several larvae feeding inside a single strawberry.
The larvae are small white maggots, reaching about 3.5 mm at their largest. As they feed, the fruit’s flesh turns brown and soft. Sunken areas that ooze fluid often develop on the surface. By the time damage is visible from the outside, the larvae have usually been feeding for days. This is what makes SWD so frustrating: a berry can look perfectly fine at harvest but collapse into mush within a day or two.
Life Cycle and Peak Activity
SWD thrives in moderate temperatures and high humidity. Adults are most active around 70°F, and their activity drops sharply when temperatures swing just 15 degrees in either direction. Temperature also controls how fast the next generation develops. At 59°F, the full life cycle from egg to adult takes about 23 days. At 77°F, that shrinks to just 10 days, meaning populations can explode during warm, humid stretches.
Eggs hatch in as little as two hours under ideal conditions, though it can take up to three days in cooler weather. Larvae feed inside the fruit for 3 to 13 days before pupating, a stage that lasts another 3 to 15 days. Pupation can happen either inside the fruit or on the ground beneath the plant.
Strawberry Whiteflies
The other insect sometimes called a strawberry fly is the whitefly, a tiny sap-sucking pest that feeds on the undersides of leaves. Whiteflies don’t damage fruit directly the way SWD does. Instead, they pierce leaf tissue and extract plant juices, causing leaves to yellow and weaken over time. At high infestation levels, they excrete a sticky substance called honeydew that coats leaves and fruit. This honeydew promotes the growth of black sooty mold, which blocks light and further stresses the plant. If you’re finding soft, browning fruit with larvae inside, you’re dealing with SWD, not whiteflies. If you’re seeing tiny white insects flying off the undersides of leaves when disturbed, that’s whiteflies.
Checking Harvested Strawberries
If you suspect your strawberries harbor larvae, a simple saltwater soak will draw them out. Mix a quarter cup of salt into one quart of warm or room-temperature water and submerge the berries. Any maggots inside will float to the surface. Place a light directly over the container to make them easier to spot. After about 15 minutes, nothing more will emerge. For larger batches, scale up to roughly five cups of salt per five-gallon bucket. A little extra salt won’t hurt.
Accidentally eating a larva is not dangerous. The maggots are not toxic and don’t carry diseases that affect humans. The concern is purely about fruit quality: infested berries deteriorate rapidly and taste off.
Exclusion Netting
Physical barriers are one of the most reliable ways to protect strawberry plants. Cornell University field trials found that 80-gram netting with a mesh size of 1.0 by 0.6 mm consistently excluded SWD over multiple growing seasons. An 85-gram option offers additional durability for high-wind or long-season situations. Lighter 60-gram netting, with a mesh of 1.95 by 0.95 mm, failed to keep the flies out.
For netting to work, it needs to go up before SWD populations arrive in your area, and it must be free of holes or gaps throughout the season. A well-designed entryway for harvesting is essential, since leaving the netting open even briefly gives flies access. Removing netting after fruiting ends helps protect it from UV degradation so it lasts multiple years.
Organic and Chemical Treatment Options
Spinosad, a naturally derived insecticide sold under brand names like Entrust, is one of the most effective organic options for SWD. It carries a pre-harvest interval of just one day for strawberries, meaning you can spray and harvest the next day. The re-entry interval after application is four hours. Spinosad has earned a “good” efficacy rating against SWD adults in university trials.
Conventional growers also have access to pyrethroids, organophosphates, and carbamates, all of which show good activity against adult flies. Rotating between different chemical classes helps prevent resistance buildup.
On the biological control front, a parasitoid wasp species has been establishing itself naturally in some regions and has been found to parasitize up to 40% of SWD larvae on certain farms and in wild host plants. Work is also underway to introduce a second wasp species that specifically targets SWD, though that effort is still in early stages.
Cultural Practices That Reduce Infestation
Frequent harvesting is one of the simplest defenses. Picking every one to two days removes ripe fruit before flies have a chance to lay eggs in it, and before any deposited eggs have time to develop into larvae. Overripe or damaged berries left on the plant or on the ground are magnets for SWD, so clearing them promptly matters. Any culled fruit should be bagged and disposed of away from the growing area, or buried, or composted in a sealed system where flies can’t access it.
Good airflow around plants also helps. SWD prefers humid microclimates, so open plant spacing and proper pruning reduce the conditions they favor. Combining these cultural steps with netting or targeted sprays gives the best results, since no single approach eliminates the fly entirely on its own.