The small, white, C-shaped “worms” you find inside acorns are almost always the larvae of acorn weevils, a group of snout beetles in the genus Curculio. Less commonly, the culprit is a filbertworm moth or an acorn moth. These larvae are harmless to humans and animals, but they can destroy a large portion of an oak’s acorn crop, with infestation rates reaching as high as 80 percent in some stands.
What’s Actually Inside the Acorn
Three insects are responsible for most acorn infestations. The most common by far is the acorn weevil (also called the filbert weevil). The larvae are legless, creamy white with a brown head, and grow up to about a third of an inch long. When you crack open an acorn and find one, it will typically curl into a C-shape. These grubs move slowly and look like tiny, pale caterpillars without legs.
The filbertworm is a moth larva that also feeds inside acorns. Unlike weevil larvae, filbertworm larvae have three pairs of short legs and are noticeably more active when exposed. The adults are brown or copper-winged moths with black and gray markings. A third and less common species, the acorn moth, produces similar-looking larvae that also have visible legs and move quickly when disturbed. If the larva you found is sluggish and legless, it’s a weevil. If it wriggles actively and has tiny legs near its head, it’s one of the moth species.
How the Larvae Get Inside
The life cycle of an acorn weevil is remarkably well-timed to the oak’s fruiting season. Adult weevils are small brown beetles, about a quarter inch long, with a distinctively long, thin, curved snout. In mid-summer, the female uses that snout to bore a tiny hole into a still-developing acorn on the tree and deposits her eggs inside. The hole is so small it’s nearly invisible.
Once the egg hatches, the larva spends weeks feeding on the acorn’s interior, hollowing it out from the inside. This all happens while the acorn is still hanging on the branch, looking perfectly normal from the outside. When the acorn falls to the ground in autumn, the larva chews a perfectly round exit hole about an eighth of an inch across. That neat little hole is the telltale sign of a weevil infestation. After emerging, the larva burrows into the soil, where it stays for one to two years before pupating and emerging as an adult beetle to start the cycle again.
This extended underground phase is part of why weevil populations are so persistent. Even a year without a heavy acorn crop doesn’t starve them out, because larvae from the previous season are still developing below ground.
Impact on Acorn Crops and Oak Forests
In heavy infestation years, these insects can damage up to 80 percent of the acorn crop in a given area. That matters for more than just the trees. Acorns are a critical food source for squirrels, deer, turkeys, jays, and other wildlife. A bad weevil year means a lean winter for animals that depend on the fall mast.
The damage also affects oak regeneration. Infested acorns are far less likely to germinate, and even those that do sprout tend to produce weaker seedlings with lower survival rates. For forests that depend on acorn germination to replace aging trees, sustained heavy infestation can slow regeneration over decades.
How to Sort Infested Acorns From Good Ones
If you’re collecting acorns to grow oak seedlings, eat, or use in crafts, you’ll want to separate the infested ones. Start with a visual inspection. Look for small round exit holes, soft spots, or caps that are still attached (healthy, ripe acorns drop their caps cleanly). Press gently on the cap scar at the top of the acorn. It should feel firm. Discard any acorns that show holes, mold, rot, or feel spongy.
The classic next step is the float test: drop your acorns into a bucket of water and throw away the ones that float. Floating usually indicates insect damage, drying, or rot. This works reasonably well as a quick screen, but it’s not perfect. Research has shown that insect-damaged acorns sometimes float even when they can still germinate, and acorns that dried out in hot conditions may float despite being perfectly healthy inside. The float test is useful for removing the worst acorns from a large batch, but it will reject some viable ones along the way. If you’re planting just a few trees and want to maximize your chances, cutting a small sample open to check the interior is more reliable than floating alone.
Are the Larvae Harmful?
Acorn weevil larvae are not toxic or dangerous. If your dog chews up an infested acorn, or your chickens scratch one open and eat the grub inside, there’s no cause for concern. The larvae are essentially just protein. In fact, wildlife actively seeks out infested acorns because the grubs are a rich food source. (The acorn itself poses more risk to dogs than the larvae do, since acorns contain tannins that can cause stomach upset in large quantities.)
For people processing acorns for food, finding a larva is unpleasant but not a safety issue. You can simply discard the damaged acorns during sorting.
Can You Prevent Infestation?
For individual homeowners with a few oak trees, there’s no practical way to prevent weevils from reaching your acorns. The adults fly to the canopy in summer and lay eggs in developing nuts that are far out of reach. No sprays are recommended for home landscapes, and the weevils’ underground pupation phase makes soil treatment impractical.
Collecting acorns as soon as they fall gives you the best chance of beating the larvae out. Weevil larvae typically exit the acorn within days or weeks of it hitting the ground, so early collection means fewer exit holes and less interior damage. If you’re storing acorns, freezing them for 48 hours after collection kills any larvae inside and prevents further damage during storage.