Each year insects heavily attack northern red oak acorns and destroy a large percentage of them, greatly reducing the number of acorns available to produce seedlings and feed wildlife.
~Lester P. Gibson
No, this isn’t what you think…it really is a post about acorns (I need a break from the other nuttiness). It is a quick follow-up to my recent post on acorn weevils. I became fascinated with the goings-on inside acorns after seeing the weevil larva pull itself out of the nut and wanted to learn more.
I collected 100 acorns and did the float test I mentioned in that first post (I used only white oak acorns for this test although the photo above shows both white and northern red oak acorns). An astonishing 45 out of 100 acorns were floaters, indicating they were “unsound”, which means they possibly had acorn weevil larvae or some other insect inside. I separated those out and placed them in a plastic tub and have been collecting the larvae that emerge. In the last week, 13 insect larvae were found crawling around in the tub. Most were the large chubby weevil grubs I found before, but a few were different.
The photo above shows samples of the three types of larvae that have emerged…the large weevil grub on the right; a much smaller weevil grub in the middle; and a moth larva on the left.
A few of the grubs were small versions of the chubby acorn weevil larvae. These may be from a different species of weevil rather than simply smaller versions of the dominant larvae I have seen. Online resources state there is another acorn weevil with a short (less than half its body length) rostrum that lays its eggs in acorns that are on the ground (the species I showed in the last post, with the long rostrum, lays eggs in developing acorns on the tree). And it appears there may be more than one species of weevil that lays eggs in developing acorns, so the small larva shown in the middle above could certainly be that of a different species.
There was also one caterpillar that crawled out of an acorn this week. From what I can decipher from scattered references, there may be a couple of species of small moths that lay eggs in acorns. The information I found suggests they lay eggs into existing cracks or openings in acorns (including the exit holes of acorn weevil larvae), although one reference also stated at least one species of moth caterpillar can chew through the shell of an acorn.
The moth caterpillars are easily distinguished from the weevil larvae by their more elongate shape, and the presence of three pair of legs just behind the head capsule (the weevil larvae lack legs). I placed several of the insect larvae in small containers with potting soil and hope to rear them to see what emerges next spring (or whenever since some may take more than one year). The literature indicates a wide range in the percent of any years’ acorn crop that is infected with insect larvae, depending on location and oak species. The study cited in the opening quote found an average of 52% of the northern red oak acorns at a site in Ohio were damaged by insects of various sorts. My very limited “study” indicates 45% are unsound.
And it’s not just in acorns. It is also a good year for the hickory nuts in our woods, and, much to my surprise, I am finding a small percentage that have very neat exit holes in them. These nuts have much harder and thicker shells than acorns, so it will be interesting to see what is making them (I am assuming a weevil larva of some sort). Seems like there are some pretty amazing things going on out there in the forest.