While there are only about 20 species of true hornets in the world, there is only one specie of true hornet in the United States: the European hornet. The European hornet measures about 1 1/2 inches long and is brown and yellow in color. People may refer to other insects as a hornet without them actually being a true species of hornet, such as the baldface hornet. The baldface hornet is what people typically think of when picturing a hornet. The behavior of both species is very comparable to that of yellow jackets, but hornets are significantly larger in size.
Hornets nests are first constructed by the queen and reach about the size of a walnut before sterile female workers take over construction. Baldfaced hornets make aerial nests, circular in shape, about the size of a football or basketball and usually built in trees. European hornets tend to make their nests in secluded, aboveground locations like hollow trees, attics, porches and inside wall voids. The queen initially starts the nest by making a single layer or canopy and working outwards until she reaches the edges of the cavity. Beneath the canopy she constructs a stalk to which she can attach several cells; these cells are where the first eggs will be laid. The queen then continues to work outwards to the edges of the cavity after which she adds another tier. This process is repeated, each time adding a new tier until eventually enough female workers have been born and matured to take over construction of the nest leaving the queen to focus on reproduction. For this reason, the size of a nest is generally a good indicator of approximately how many female workers there are in the colony and some hornets’ nests eventually grow to the size of beach balls.
Social hornet colonies often have populations of between three and ten thousand female workers at maturity, although a small proportion of nests is seen on a regular basis that are over three feet across and potentially contain upwards of twenty thousand workers and at least one queen. What has also been seen are nests close to one another at the beginning of the year growing quickly and merging with one another to create nests with tens of thousands of workers.