Since my last visit to our Colorado farm, a gray, basketball-sized, turban-shaped globe has appeared in one of our apple trees; not to be mistaken for a pinata, this is, of course, a hornet nest. No doubt present during my visit in July, the nest has "grown" to a more conspicuous size over the past two months.
Pregnant female hornets emerge from their winter retreat in spring and lay eggs in a cluster of paper cells that they attach to a tree limb; unlike other wasps, they enclose these cells in a protective envelope (the "paper" is formed by salivary digestion of wood scraps). Once the eggs hatch, the attentive queen feeds them until the larvae pupate; emerging as sterile female workers, this initial brood returns the favor by expanding the hive and tending to the queen, who will continue to lay fertilized eggs throughout the spring and summer. Space for the new egg chambers is made by scraping away the inner layer of the envelope and adding a new layer to the outside; over the months, the enlarging envelope houses a growing colony of worker hornets, larvae, pupae and eggs. Access to the nesting cells is via an opening on the underside of the globe; watching from a safe distance, one can see worker hornets arriving and departing throughout the day as they gather nectar and paper substrate.
As the autumn chill arrives, the queen lays eggs that will hatch into fertile males and females; these non-workers will mate and the pregnant females will become next year's queens. Once the winter freeze sets in, the hive is abandoned, the future queens retreat to warm dens (either underground or in structures) and all others perish.
