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Holey Moly: Tiny Beetles Make Holes in Acorns

Take a walk in the woods in the fall and you’ll likely find acorns. Walk through a particularly oak-filled forest and you may find your feet rolling around on all the acorns like you’re wearing roller skates. Get up close, or rather down close, to all those fallen acorns. What do you notice? Often, they have a tiny, perfectly round hole in the side. How do you think that happened?

An acorn on an oak leaf.
(Photo by Heather Van Zyl)

An acorn weevil probably made that hole. But how? And why?


Acorn weevils are small, brown beetles. They are only about a half-inch long, the perfect size to fit on your pinky fingernail! Their long snouts help them stand out from other beetles. They look like the anteaters of the insect world.

They bring their own tools

An acorn weevil on a green leaf.
An acorn weevil. (Photo via Shutterstock)

Saw-like teeth at the end of their snout are perfect for — you guessed it — sawing into acorns. While the acorns grow on the trees, weevils drill through the hard outer shell to get to the meaty nut hidden inside. After snacking on some of the nutmeat, a space is left behind.


They build a nursery


Female weevils lay two to four eggs in the space left behind. Then they cover the opening to keep the eggs safely hidden inside. What do you think they use? Believe it or not, their own poop! The matching color makes it the perfect camouflage. Aren’t you glad we build doors on our rooms instead?

 

Words to know

Burrow: To make a hole or tunnel for use as a dwelling.

Gall: An abnormal growth on a plant of tree caused by the presence of insect larvae, mites or fungi.

Snout: The projecting mouth and nose of an animal.

 

Larvae hatch from eggs after a week or two. They stay inside the acorn, eating their fill of nutmeat to grow bigger and stronger. The space is a bedroom and kitchen all in one! Other insects create bedroom-kitchen combinations in the form of galls.


They know when it’s time to move out

Acorn weevil larvae and acorns on a red leaf.
Can you see the acorn weevil larvae on the red leaf? (Photo by Heather Van Zyl)

Weevil larvae stay inside the acorns eating and growing until the acorns drop in the fall. Like a knock at the door, the thud when the acorn hits the ground may be the signal that tells them it’s time to go. They chew their way out (hopefully not through the poop-covered entrance!), leaving the perfect circles you see behind.


They wait to transform


Though finding holey acorns is pretty easy, finding the larvae is not. The small, whiteish, maggoty-looking weevil larvae aren’t fully grown yet. After leaving the acorn, they burrow underground to pupate, or transform, into an adult weevil, snout and all. This can take up to five years!


Leftovers aren’t wasted


Weevils may have eaten most of the nutmeat already, but other small animals, like springtails and snails, will creep in to eat the leftovers.


Fungi also starts to creep in to break down and consume any remaining bits. Then caterpillars, mites and other fungus eaters gobble that up too!


New neighbors move in


Those empty acorns should have “For sale” signs on them after the weevil larvae move out. Acorn moths lay their eggs in empty acorns. When their caterpillars hatch, they cover the hole with webbing to keep others out. Would you rather have a door made of poop or a web?


Can you think of any other tiny critters that could make cozy homes inside an empty acorn?


You can watch the process


Collect acorns without holes to take home. Make sure you have permission to take them. (Everything in the forest preserves has to stay there.) Put them in a container and wait to see what happens. Make sure the container has sides, like an empty jar or a box. Leave the lid off, though. They need fresh air to breath.


Acorn weevil larvae don’t have legs, but they can wriggle away. Not only can the wriggling larvae cause quite a surprise for your grownups, but they won’t be able to burrow through your floor to get underground to pupate. Take the larvae back where you found the acorns so they can safely finish their life cycle.


Visit Squirrels’ Treehouse at Plum Creek Nature Center for some acorn fun. Fill the bucket with acorns then use the pulley to bring them to the top. Pour them down to hear them clatter and roll, then start again!

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