Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada
Clickable Guide
Moths Butterflies Flies Caterpillars Flies Dragonflies Flies Mantids Cockroaches Bees and Wasps Walkingsticks Earwigs Ants Termites Hoppers and Kin Hoppers and Kin Beetles True Bugs Fleas Grasshoppers and Kin Ticks Spiders Scorpions Centipedes Millipedes


Insects inside acorns

In the fall members of my family in more rural areas collect acorns to give to me to feed squirrels here in the city. Some of these acorns end up eaten by insects. Sometimes a squirrel opens a nut, looks at the eaten-up inside in disgust, and asks me for another.

If an acorn actually has insects inside I could cut it open and take pictures. I don't want to cut all of them open. I'd like to know which ones to cut and which to use as squirrel food. Squirrels can tell -- they don't bother to bury insect-infested nuts -- but if you've ever tried to take a nut away from a hungry squirrel you'll realize I can't count on their assistance.

How do I know if an acorn or other nut houses an insect larva? I see ~3mm wide holes in some, so round they could have been drilled. I assume these are made by the adult insect leaving and it's too late.

I recently had started collec
I recently had started collecting copious amounts of acorns for our DNR reforestry program, and I could say at least 3/4 of them were infested by Acorn and Nut weevils. Nothing like dumping out a bucket full of acorns and finding a couple good handfuls of live and dead/liquefying grubs in the bottom of it. The birds migrating through were happy for the snack though.

This year has been a boom year for the weevils. Last year, maybe only 1/3 of the acorns I collected were infested. Our chipmunks and squirrels here are rejecting quite a few of the ones they find. At least the chipmunks can eat the grubs.

Population explosion
I am reviving this issue because of my experience with nut weevils today.
I gathered 12 healthy looking nuts on September 21 (no holes of any sort that I could see) and yesterday (October 1st) a horde of grubs started emerging from them; 92 grubs from 8 nuts, plus another 20 today. Only 4 nuts seem, so far, free from holes.
The whole story is here and in related images.
I may add that the same day (September 21) I found a nut weevil in the same spot.
Several things intrigue me.
1. Early emergence; Jim found some emerging weevil larvae around Thanksgiving; but this is only the beginning of October.
2. Mass emergence, 92 in one day plus 20 the next! What is the cue? Yesterday was the first noticeably cold day this year, but the grubs couldn't have noticed it, being indoors. Or maybe they noticed, the house got a bit chilly and it was even cooler on the window sill where the container was.
3. Attraction to light. Each time that I turn around the plastic container in which they are, they start their slow wiggly gallop toward the light. Don't they want to hide from the light and burrow where they can pupate? Well, granted that they are not finding any soft soil; but, why move toward the light?
I will have to build a terrarium and place it outdoors, so they experience the right climate. I doubt it that I will have any success; I would have to wait until next spring. These grubs will join my mason bees until next year.

 
Update
The nuts are Chinese chestnuts, they are edible. The director of the nature center where this tree grows says that the nuts are delicious roasted, weevils and all. He and his family eat them.
As of October 5, eleven of the twelve nuts are infested. After this smaller second population explosion I have counted a grand total of 163 weevils and I am ending the count here.
On September 21st the trees were raining nuts to the point that it almost wasn't safe to be under them and there were many nice healthy nuts along with the husks on the ground. Now the full crop of husks is on the ground, but there are hardly any whole nuts anywhere, only a few broken empty shells. It seems that the squirrels have been very busy eating and hoarding.
I think that this explains the grubs population explosion; the weevils have to get out as fast as they can to avoid being eaten or buried by squirrels. Maybe it also explains their search for the light. If they are inside a squirrel's cache, they probably want to get out and move away some distance before burying themselves again and pupating.
I got a video of one larva struggling to get out.

 
Chinese chestnuts,
I discovered this fall, are sweet and delicious even when they're eaten raw.

 
very intriguing
Interesting ideas about the reasons for their rapid exodus and light-seeking behavior! Don't think I've heard anyone come up with a better explanation.

How do I get it open?
What's the best way to open a possibly-infested acorn without hurting myself or mangling the insect inside?

 
Oh come on....
you're not doing anything if you don't bleed a little :-D.

I used the nut cracker off the living room table....don't tell my wife.
Just crack it gently then pull it apart. I'm sure there's a better way but it worked for me.

Try this website
This article should answer your question.

 
My mistake...
I may have made a mistake this year by keeping my acorns warm. The article suggests that the parasites expect to be cold over the winter they spend inside.

 
probably better cold
but the acorns I have at my house this winter are indoors, for various reasons I won't go into. Insects are emerging nevertheless.

We seem to remember something
about healthy acorns sinking in a tub of water while infested ones float, but maybe that's just an old wives' tale.

 
sinking in tub of water
That's exactly what it says on the web site Joseph links to above. Next time I have to try that!

A few months ago I brought home about 25 coast live oak acorns. I was trying to figure out which might contain a cynipid wasp that's known to occur in them.

I brought home acorns that were big and perfect, acorns that were stunted but without holes, and acorns of various sizes that had those round perfect holes. I've had insects emerge from each of the types of acorns. So far I've seen moths, unidentified larvae (possibly weevils?), and a parasitoid wasp. It made me think that ALL acorns have insects in them -- I have no idea how squirrels can tell (or how I can tell) which have insects in them. Next year I'll try the sinking-in-water test.

I still have a long wait to see what else emerges from the acorns. Apparently the cynipid wasps can take 2, 4, or 6 years. :-( I'm not really sure my rearing conditions are good enough for having them survive that long (if I have any cynipid wasps at all).

 
Ok but
if there's no evidence externally then how do they get inside the acorn in the first place ?

 
Simple
The eggs are laid when the acorns are quite immature, and the very small holes are healed over as the acorns grow.

 
Cool!
Do you know of any pics or info anywhere?

 
Nevermind....
I went out and found my own. See here.

 
Good job :-)
And very nice photos! I'll try to post some of my photos shortly... I had an anobiid (beetle) come out of one of my acorns yesterday, which was very exciting.

 
Very cool!
I can't wait to see them!

 
ok
I posted this beetle and some of the acorns. (The acorns in that container are particularly moldy now, though they weren't so obviously moldy when I brought them home.) Also I posted this weevil larva and this moth. I don't have a photo of the parasitic wasp that emerged (and no longer have the wasp).

Some cynipid researchers more knowledgeable than I told me that these acorns are stunted because of a cynipid wasp inside them. The cynipid wasps don't emerge until months or even years after the acorns have fallen off the trees, so the holes visible in these photos were made by parasitoids or inquilines.

Also, while I was looking around in the Bugguide acorn weevil section I found Jim's photos of an acorn weevil larva and also his photos of a fly that emerged from acorns. There are a few other acorn weevil larvae in the acorn weevil section ... it probably didn't need my two additional photos.

 
Those are awesome!
How long did you have to keep them before they emerged ?

I would never have thought a moth would come out of an acorn!

I'm going to update the overwintering article to include these in the " BugGuide DOCUMENTED OVERWINTERERS" section.

 
I brought my acorns home arou
I brought my acorns home around November 10th. Weevil larvae emerged over the span of the next couple of weeks. The moths emerged after 1.5 months. The beetle emerged after 2+ months. The parasitoid wasp emerged as I was collecting the acorns -- it was coming out of one of the holes that went through the cap, and I speeded up its emergence by pulling the cap off by accident.

Since I've had all these acorns indoors, the dates of emergence are not necessarily when they would emerge if the acorns had been left outdoors. A better study would be to keep the acorns outdoors somewhere or in an unheated shed/building.

 
Something else
Also check acorns during the summer. Sometimes small ants will nest inside an old acorn.

Comment viewing options
Select your preferred way to display the comments and click 'Save settings' to activate your changes.