Attack of the Acorns

Crack! Rumble, rumble, rumble. Crack! The sound of hard objects pelting my metal roof shot through my open bedroom window, rousing me from the last wisps of sleep. Then silence. I braced myself as a soft hush of wind drew closer. Crack! The wind triggered a new spatter of noises. The house was under attack—by acorns.

Two large red oak trees reach the edges of their canopies out over the roof of my house. Each fall, they create a racket as acorns drop on the metal roof, tumble down the steep slope, and launch out over the driveway. Some years are worse than others, since oaks are mast trees who will produce a bumper crop in one year, then spend subsequent years rebuilding their stores of nutrients and not producing as many acorns. This is clearly a mast year.

The acorn attack had preceded my alarm clock, but I decided to get up anyway. Taking my coffee with me into the crisp fall morning, acorns nestled in the grass rolled under my feet. I bent down for a closer look at the offending projectiles. The acorns with caps intact captured my attention first. They are the most adorable, after all. And the caps can be helpful in telling apart different kinds of oaks. Red oaks have a low-profile cap with artfully arranged concentric scales. Burr oaks, in contrast, have fringed brims on their acorn caps. Many of the acorns were cap-less though, a pale ring marking the newly exposed shell.

On the driveway, many acorns had been cracked open by my car tires. Some showed pure, cream-colored nutmeat. On others the insides were blackened and bedraggled. I’d read that trees will discard immature acorns that have been attacked by insects or fungi, and they fall to the ground with the cap intact. On the other hand, trees release healthy, mature acorns from their cap, which stays attached to the tree. Was this true?

Gathering up a handful of acorns with and without caps, I got out a cutting board and a Mason jar to use as a nutcracker. I chose a cap-less acorn first. Sure enough, a couple of bangs with the jar split the nut open to reveal intact tissue, ready to fuel the growth of a seedling next spring.

Next, I picked a capped acorn. It looked normal, but from the first tap I could tell it was mostly hollow. Sure enough, when the shell split, I discovered a fat white larva with a brown face had eaten more than half of the two fatty seed leaves called cotyledons that make up the nutmeat. Another capped acorn produced at least four smaller larvae, all eating around the edges. One nutmeat was just shriveled and brown, with some white webby stuff at the bottom—likely a victim of fungi. Wasps, sap beetles, and acorn moths also attack acorns and consume the nutritious tissue inside. In my sample size of 6, all the capped acorns were being attacked, and the bare-headed ones were intact.

I chuckled as I remembered learning this lesson a different way. For our MuseumMobile visits to kindergarten classrooms in the fall, we fill a little cup with acorns. Kids shake the cup, listen to the rattle, and try to guess what’s in there. One fall, our educator filled the cup with fresh acorns, and when we went to show the kids, the cup contained several white larvae, and the acorns each had a small, round hole where they’d chewed their way out.

Likely, they are the young of acorn weevils. These little insects using their long, saw-like snout, called a rostrum, make a tiny hole just under the edge of the cap and lay one or more eggs inside the young acorn. The larvae are fine with the tree’s habit of discarding infected acorns, since they need a ride to the ground. Once there, they tunnel out of the acorn, burrow into the soil, and eventually pupate into an adult weevil.

While some squirrels seem to avoid weevil-infected acorns, others have been observed feasting on the tender protein-filled morsels. Perhaps it’s a question of whether the squirrel is going to cache a nut and needs it to survive the winter, or wants a juicy snack right now. Squirrels might shake or weigh an acorn to determine what it contains, but a quick way for a human to separate viable acorns from predated ones is to do a float test—viable acorns sink and the rest can be skimmed off the top and discarded.

The float test using two of my sample acorns. 

A scratchy rustle on the roof made me look up from the pile of cracked acorns just in time to see a full oak twig launch off the roof. Two fluffy gray squirrels looked guiltily down from the branches. A cluster of empty acorn caps on the lower part of the twig marked the tree’s success—or maybe a squirrel’s full tummy? Out toward the tip, though, in the axis of each leaf, were what looked like miniature acorns forming from the remnants of female flowers.

The squirrel who had tossed this twig onto my roof did me a favor—female oak flowers only occur high in the canopy and are hard to see. The male flowers—dangly yellow catkins that release pollen—are much easier to observe in the springtime. Once pollinated, the female flowers bide their time, and don’t fully mature until their second summer.

Unfortunately, what this squirrel also showed me is that next fall might be noisy, too!

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too. Natural Connections 3 will be published in November 2025!

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our Fall Calendar is open for registration! Visit our new exhibit, “Becoming the Northwoods: Akiing (A Special Place). Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.