What's That Hole in my Acorn?

January 22, 2019

Written by Mary Lou Flint

Round exit hole of a filbert weevil.


We are all familiar with the important role acorns play in the diets of the squirrels, scrub jays, woodpeckers, deer and other vertebrate wildlife in our reserve, but what do you know about the insects that make their niche within an acorn? 

In autumn, the children on my nature hikes often pick up acorns and find one with a small round hole in it. What made that hole?

The likely culprit is the filbert weevil, Cucurlio occidentis, (sometimes called the California acorn weevil); although a moth, Cydia latiferreana, known as the filbertworm, could also be involved.  

 Filbert weevil showing its long snout next to al old larval exit hole.

Adult filbert weevils are golden brown beetles with a long slender snout. Adults emerge from pupae in summer just as oak trees begin to develop acorns. After mating, the female uses her snout to make a deep puncture in a green acorn and then turns around to use the ovipositor at the rear end of her body to insert one or more eggs inside.  

Puncture made by filbert weevil to lay eggs. These egg punctures close and heal over.


Weevil larvae are legless, C-shaped grubs. They feed within the nut during the summer and early fall, completing development after the acorn drops to the ground. After reaching maturity, each grub chews a round exit hole in the wall of the acorn, leaves the nut and buries itself in leaf litter. The buried grubs pupate in late spring or early summer and later emerge as adults. There is only one generation a year. 

 Legless filbert weevil grub (and blck excrement) in an acorn.

The filbertworm has a similar life cycle, although the adult moths lay their eggs singly on the surface of acorns and the newly hatched larvae must bore a hole to enter the nut. The larva of the filbertworm is a caterpillar, so it has three pairs of true legs and is longer and much more active that the sluggish filbert weevil larva. Like the filbert weevil, it continues to develop in acorns after they fall from trees, bores an exit hole in the acorn shell when itís mature, and pupates in the soil in autumn or early winter to emerge as a moth in summer.

 Filbert worm caterpillar and exit hole.

Both these insects can damage acorns from any of our native oak species and most infested acorns will fail to sprout. Insect holes also provide an entryway for bacterial or fungal pathogens and can be associated with drippy nut disease.   Infestation levels vary from year to year, but in bad years, these acorn pests can hinder oak reproduction. 

Do other acorn feeders benefit from consuming insects in acorns? Researchers in Pennsylvania found that gray squirrels test acorns by shaking before collecting them and are more likely to eat weevil-infested acorns right away and store clean ones for later consumption. Sometimes they just pick out the weevils and eat them. The University of California Oak Woodland Management group notes that scrub jays will selectively choose clean acorns, if presented with a tray of infested and noninfested acorns, although other UC research indicates that acorn woodpeckers do not discriminate between clean and insect-infested acorns for storage.

 Sqirrels test nuts for quality and insects before consuming them.

Acorns were an important food source for the Native Americans who lived in the Sacramento Valley hundreds of years ago, and they recognized these insects as serious pests. Maidu and other California tribes regularly burned leaf litter under oaks to kill overwintering larvae and keep weevils and filbertworms under control.  

When collecting acorns for eating or planting, check for holes and signs of insect infestation and toss out damaged nuts. Weevil feeding will create air pockets making them lighter and easily deformed when squeezed. If placed in a bucket of water, infested acorns will float.


Text:  Mary Louise Flint, Docent, Effie Yeaw Nature Center and Extension Entomologist Emerita, Department of Entomology, UC Davis

Photos: Jack Kelly Clark and Bruce Hagen, used with permission of the University of California UC IPM Program and UC ANR.