Pop! Goes the Weevil (Larva)

Acorns fill a gallon zip-bag

I shipped seven pounds of acorns to Kentucky.

Not to feed any under-nourished squirrels. My acorns are for the University of Kentucky’s multi-year, genetic study of white oaks. Foresters believe the white oak is in decline, and the project’s goal is to identify trees with traits suggesting a higher likelihood of success in the forest. The research team hopes to acquire acorns from every county in every state in the white oak’s range.

What will they do with my (and everyone else’s) acorns?

They’ll plant the acorns and perform genetic testing on the seedlings, looking for the desired traits. The team will choose the most ‘successful’ seedlings and obtain shoots or branches from the parent trees. Then they’ll create grafted seed orchards, jump-starting the normally decades-long time needed to produce a new generation of acorns with the desired genetic traits. Those acorns will be planted for forest renewal. It’s the same technique that saved French vineyards after devastating losses to a plant louse in the late 1800’s. French vines were grafted onto imported American rootstock that was resistant to grape phylloxera.

I was instructed to collect acorns from just one tree. That was easy. Our huge old white oak next to the driveway has been raining acorns. Per instructions, I put the acorns to the ‘float test’ as I collected.

Acorns in a bowl of water, some floating
The three acorns near the top of the photo are floating. The rest sank.

A floating acorn suggests there is or was an acorn weevil larva inside, or perhaps the caterpillar of an acorn moth. Floaters are unlikely to sprout, so they got tossed. The sinkers went in the fridge until I’d filled a gallon zip bag.

Along the way, I encountered lots of weevil larvae. Some I found crawling on the driveway. Most chewed their way out of floaters in my discard pile. And to my surprise, some little stinkers crawled out of the sinkers! No wonder the Kentucky team needs a gallon of acorns from each sampled tree—the float test can’t catch all the miscreants inside the nuts.

Weevil larvae chew tiny round holes that look too small for such a chubby grub to exit the acorn. It’s funny to see them wriggle through that hole, and quite a feat considering they have no legs to push or pull with.

Acorn weevil larva halfway out of the exit hole it made in an acorn
On its way out.

I discovered lots of larvae when they were already halfway out of the acorn. But I wanted to film one just as it poked its head out of the nut. So, after dispatching my haul to Kentucky, I kept gathering acorns. I kept a keen eye on ones with exit holes, remembering that a mama weevil lays more than one egg in each acorn she selects for a nursery.

Acorn with two weevil-larva exit holes
Two larval exit holes in one acorn.

I eventually noticed some dark, dirt-like debris falling out of an existing exit hole… something was inside, pushing the dirt out as it moved. I turned the acorn over to find a larva peeking out of another hole. So, I plopped that acorn onto the bed of soil I’d placed beneath the camera lens. And waited. After a few minutes, Pop! Goes the weevil (larva)! (I can’t resist, sorry). Here’s what I saw, in real time. That critter worked so hard to wiggle out of that tiny hole! I sense a familiar rhythm in its writhing. Watch the video again. Don’t the larva’s movements match the cadence of ‘I think I can, I think I can?’

Acorn opened to show a weevil larva burruowed into the nutmeat
I opened this acorn to show the larva burrowed inside its bed-and-breakfast.

No weevil larvae were harmed in the making of this film. I returned twenty or so to the soil beneath our oak, where they would naturally burrow some eight inches deep. They’ll stay there for one to two years before emerging as adult weevils.

Adult acorn weevil with its very long proboscis.
The adult weevil uses its long snout, called a rostrum, to drill a hole into the acorn.

Those adults will chew tiny holes in young acorns developing on the tree. Then they’ll position their posteriors on the holes and deposit eggs inside. The larvae will exit soon after the mature acorns fall.

Will my few saved larvae ultimately prevent some future acorns from germinating? Probably, but judging by the flood of nuts covering our woods and driveway, I don’t think they’ll make a dent in the seed stock. And to my surprise, a few acorns with exit holes sprouted.

Sprouted acorn that has a weevil-larva exit hole
I planted this acorn in a pot to see if it will continue developing despite the weevil damage.

I suppose that means those weevil larvae tunneled near the top of the acorn, leaving the bottom, where the germ is, intact. I planted one of the sprouting, damaged acorns in a pot. I planted another sprouter, one with no sign of infestation, in a second pot.  I’ll be watching them for a while… stay tuned!

Learn more…
About the University of Kentucky acorn research.

And about the White Oak Initiative it supports—with some wonderful, informative videos about the need to protect white-oak forests for wildlife support and for oak-wood industries, such as furniture and wine and spirit barrels.

Find info about acorn weevils here.

If you’re an adventurous eater, you can use acorns to make muffins and pancakes.

And… some folks report that acorn weevils are edible, and they’re soft and buttery when cooked.

Related kids’ book: If you or a child in your life ever put an acorn in your pocket, you’ll love reading What’s in Your Pocket?: Collecting Nature’s Treasures. It’s a spankin’ new picture book by Heather L. Montgomery, illustrated by Maribel Lechuga. The book profiles nine scientists who, as children, collected nature’s ‘treasures.’ Their collection habits sparked a lifetime interest in nature and science. This is a wonderful book to encourage curiosity, learn about scientific inquiry, and read about childhood experiences of noted scientists, including Jane Goodall. As a girl, she put worms under her pillow! (My acorn-weevil curiosity didn’t take me that far.)

 

Image Credits: Carol Doeringer.

8 thoughts on “Pop! Goes the Weevil (Larva)”

  1. So interesting, Carol. Our woods and yard on 39th street were full of white oak trees. A few years when there was a heavy mast (acorn production) we had to literally shovel them off the grass. Deer love acorns and would come in the yard to eat them. Now I know even more about those acorns and their boarders. Thanks for sharing this and helping to ensure oak trees are around for a long time.

    1. Carol, we’re almost shoveling now, so it feels like another heavy mast year. Not for the red oaks, though. They showered us with acorns last year. I’m so glad I happened upon information about the U. Kentucky study, and it is such a privilege to be able to help.

    1. Just a soft heart. They do destroy the acorns they live in, although that said, I noticed some of the acorns with exit holes were sprouting anyway. Those larvae had likely lived in the top of the acorn, away from the germ, which is near the bottom. But even if every weevil-infested acorn is prevented from germinating, there are still so, so many good acorns! I think my tree is winning the battle of oak v. weevil.

  2. Hah! You are probably right. Ed says maybe the weevils serve as tree control. Would you really want all those acorns to make new trees?

    Thanks for the invitation to dinner! We are both looking forward to it!

    1. I agree with Ed! We don’t need to clean up any more ‘gifts of the trees’ –especially those long, stringy oak catkins that cover the ground in spring. Survival of the fittest, and if the weevils score some wins, that’s ok by me!

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