If you came to visit us, you might wonder why we have old hollyhock stalks still standing in our garden. I know my husband does!
If you look very closely, you might find a clue.
The hole is a sign that something is living inside. When we opened one of the stalks recently we found:
The pithy center of the stalk had been hollowed out and a small carpenter bee had made cells for her offspring.
The bee would have started by creating a cavity. She would prepare a small ball of bee bread, a mixture of pollen and nectar. When the bee bread was ready, she would lay an egg on it and close the end with a plug of plant material. (It looks like sawdust here). Then she would make some more bee bread and repeat the process until the cavity was full. Within each cell, a larva would hatch from the egg, eat the bee bread, pupate and then become an adult bee. Eventually the new bees emerge from their chambers.
You might wonder if the tunneling by the bee harms the plant. It turns out that the living part of the plant is in the outside tissue that is not disturbed. Because a hollow tube is very nearly as strong as a solid one, the stalks are no more likely to break. Bees, like small carpenter bees, are important pollinators and should be encouraged whenever possible.