First guess: Painted Hickory Borer
Second guess: Powder Post Beetle
According to Univerisity of Missouri Extension Link
Insect problems and damage
"Insects can cause damage or be bothersome when you store wood. Pests include wood borers (round and flat head wood beetles), wood roaches, and termites.
People who often use wood to heat their homes usually have had experience with wood borers. In the summertime you can frequently hear the borers chewing wood in most older wood piles. Once borers become established in a wood pile, they can reduce its weight by 20 to 30 percent per year
. This means that from the standpoint of borer damage, it is probably not a good idea to hold your wood outside for more than a year or two at most.
If you can delay felling trees cut for cordwood until after the first frost, your wood will be relatively free of wood borers for at least the first year.
The adult borers do not fly after the first frost and thus do not infest the logs. If adequate drying takes place before spring, the adult borer will be less attracted to the wood. We are not advocating that everyone should always wait for a frost before cutting trees for cordwood. However, it is a good idea to consider this as an option in your time schedule.
People frequently ask about the consequences of bringing borers inside the home with the cordwood. Borers do not reproduce inside nor do they contaminate the wood of your home. Their normal habitat is outside. Inside, they emerge from the wood as adults and die within a short time. The worst scenario is that you will sweep or pick up a few adult borers where the wood is piled."
Interesting stuff, but I think the estimate of 20-30% weight loss per year is a little overblown. Maybe a worst case scenario.