Grubs in your acorns? Meet Curcuio, or the Acorn Weevil

I’m pretty fascinated with acorn weevils these days, since I’m seeing a lot of them while processing my acorns. I finally looked them up, and it turns out they have a fascinating life cycle.

There are two types of acorn weevils (beetles), long snouted and short snouted, Curculio and Conotrachelus, respectively. They both plant their eggs in acorns, but the short snouted one seems to do this in cracked acorns once they are on the ground. The long snouted variety is the one that you’re going to run into acorn processing, because unless you’re drinking your bathwater for breakfast, you’re not going to be picking up cracked acorns.

The female weevil, whose snout is as long as her body (about 3/8″), digs a hole in a green, developing acorn with tiny appendenges on the end of her snout. She sucks the oily nutritious juice out of the acorn, and thus fortified, lays her eggs in the hole, and plugs the hole with her own poo. The grubs hatch in the continuous buffet which is the acorn, and snuggled up in there, snacking, until the acorn falls from the tree. By this time (as Nature is smart) they are ready to leave the acorn, and they take the fall to the ground (which must be quite a shock) as a signal to start chewing their way out of the acorn. How fast this happens depends on how thick the acorn’s shell is — anywhere from a few hours to three days.

The grubs always chew a perfectly round, 1/8″ hole. It’s just big enough for their head, and they have to squeeze and wiggle their fat, shiny acorn-stuffed body through the hole to escape. Once they fall to the forest floor, they hurry to bury themselves in the soil before something comes along and eats them. If they make it, they take a multiyear nap underground (I’ve read anywhere from 1-5 years). They don’t eat, but they somehow metamorphose into their adult beetle form. When they wake one fine summer day, they crawl out of the soil, mate soon after, and start the process all over again.

There’s some points to be taken here for the forager. The first is that just because there’s no hole in the acorn doesn’t mean that there’s not a grub in it. A hole means a grub has already emerged. It may have siblings which will also be emerging soon. Or not. Or if the acorn has been on the ground for a while, another insect may have moved in. No hole means nothing.

However, if you’re collecting fresh acorns, you’re going to know that you’ve got about a 3 day window in which you may see larvae emerge from your stash, leaving their distinctive holes behind. (You may even see individual acorns in your stash wiggling, like giant jumping beans!) This is not a problem, just something to know, for the sake of storage, or squeamish loved ones.

The acorn they emerge from may or may not be useable. You can open it and check it out–or opt not to. It will be likely to be at least 50% spoiled, in any case. It may also have more fresh grubs in it, trying to make their way out. You may well chop them in half with your knife, and feel oddly bad about the whole thing.

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  1. I hope these grubs in the ground are what my hens are getting as they destroy the ground under my oaks with their furious and incessant scratching and pecking.

    When I eat roasted pistachios, I see the same size hole you describe and wonder what ate its way into or out of the nut. Now, I have the heebie jeebies.

  2. I love weevils – they look like tiny elephants. I didn’t know much at all about bugs, but learned a lot in my first job after leaving grad school. I was a county extension advisor, and frequently scouted farm fields showing bug damage. It was always interesting, and I had to learn to mask my enthusiasm for the bugs who were destroying the farmers’ plants.

  3. The grubs will go through paper and plastic bags in their search for the ground. I learned this after leaving a bag of acorns at work overnight, officemates were not amused.
    I made shallow trays of hardware cloth which just fit over a storage tub, then stacked the acorn filled trays, with wood spacers between for airflow, over the tub and left them there till no new grubs appeared. This was usually at least a week. Could stack them over a chicken run and let them have the grubs for dinner.
    I found that using a knife to slice them open led to the occasional slicing open of my hand, but using my pruning shears, well scrubbed, was much easier and safer. After having dried out a bit in the trays the nut came out of the shell right quick.
    A neighbor uses the running water leaching method by suspending a mesh bag of acorns in the toilet tank, every flush changes the water for you. Tank, not bowl, mind you!

    • LOL– the poor office mates!

      The stacking system sounds excellent. And yes, it does seem that drying the acorns out a bit before processing really helps with the cutting. I was processing with a friend this week and we had stashes of all different ages. Older acorns definitely smash and slice better than fresh ones.

      I’ve heard of the toilet method. It makes sense, yet…I just can’t quite embrace it.

  4. i ran out and gathered a whole mess of acorns after your acorn burger post and left them on the counter and then – yep – had a bunch of chubby grubs all over my counter so i carefully gathered them and the acorns up and put them all outside and the squirrels came.
    so now i am confused and appreciate this post but still need to research more – i just dont want to be killing all the grubs and not understanding the whole cycle i suppose….

    • I just had grub friendly idea. If after harvest the acorns were put in a wire cage of some sort, and allowed to sit for a week, say, the hatching grubs could return to the soil. (The cage would have to be placed near an oak for the sake of the future waking beetles.) Meanwhile, the cage would keep the squirrels from taking all the good acorns (since they avoid the grubby ones).

    • That is an interesting idea….thank you much mrs h. for the reply….I definitely will be putting a lot of thought into this because those acorn burgers looked/sounded incredible!

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