Bald-faced Hornet

Winter-degraded bald-faced hornet nest, showing cells,
some empty, some capped, with dead pupae inside the 
latter, that had failed to mature before freezing temperatures
brought the nest to an abrupt end. Click to enlarge. 

Those greyish, roughly football-shaped nests one spies in trees after leaf-fall has left them bare are the products of bald-faced hornets. (The nests of various ‘paper wasp’ species are smaller and do not have an outside wall.) Bald-faced hornets are easily distinguished from their yellowjacket relatives by being colored in black and white rather than shades of yellow. Each now remnant nest is the end-result of one fertilized queen emerging from hibernation in spring. Over the course of spring to fall she churned out eggs, first for workers, then as fall approached, for a mix of workers, new queens and males.   

Fertilized queens over-winter in a weather-sheltered place to avoid being wet, but are still subjected to temperatures below freezing . Extracellular fluids freeze, whereas glycerol (anti-freeze) is produced to protect against intracellular ice crystal formation. The queens emerge in the spring and immediately creating the beginnings of a plum-sized nest in order to raise the first cadre of workers. Once those reach maturity – in about 25 days after eggs are laid – the queen will remain inside the nest while the workers forage for food, enlarge the nest and tend to the next generation of workers.

Early nest, built by a 
queen for the first workers

At maximum size, each nest will contain 300 to 600 workers. The nest wall consists of multiple layers of ‘paper’ made from wood pulp and saliva. Close observation (NOT of an active nest) shows strips of brown to grey depending on the type of dead wood the workers scraped with their jaws, chewed, swallowed, then regurgitated back at the nest. Inside, there are horizontal disks of six-sided cells, each cell open-end downward, in which one egg is laid. These eggs hatch to become wingless larvae, which when full-sized, pupate into winged adults.

Bald-faced hornets are omnivorous and are considered to be beneficial due to their predation of flies and caterpillars; they also consume nectar from flowers, sip tree sap and nibble over-ripe fruit. Toward the end of summer the nest is enlarged downward and a new layer of cells created for eggs that will become new queens and males. All this, mind you, from the sperm the queen had accepted and stored from the previous fall’s mating. Meanwhile, she also continues to produce eggs that will mature into workers. The mix is crucial to next generation success, as too many new queens and males, and/or too early, will mean not enough workers to feed the nest, whereas too few queens and males, and/or too late, will increase risk than fewer new queens will survive winter.

An odd fact: unlike honeybees, female bald-faced hornet workers are able to lay eggs that will become males. This is competition with the queen, who is laying worker, queen and male eggs. A sampling of seven nests showed that one-fifth of the males were from worker eggs. Another study reported that in some nests in the reproductive stage there were no queens. The researchers theorized that egg-laying workers had killed the queen – committed matricide! – so that a larger percentage of the males would be an exact match to their worker DNA rather than the partial match for queen-laid male eggs.

Bald-faced hornet
(internet download)

The aggressive defensive nature of bald-faced hornets makes them a threat to humans who wander too close to a nest or when a nest is constructed too close to human habitation. Their stingers deliver a nest-defensive, pain-inflicting venom rather than the prey-subduing venoms of wasps that sting insects or spiders into living immobility to lay their eggs upon. The act of stinging releases volatile pheromones that serve to alarm and recruit other hornets in the colony and identify the target to be stung. Unlike honeybees, the stinger is smooth rather than barbed, allowing each hornet to sting repeatedly. Venom can also be sprayed at eyes, causing temporarily blindness. Pain lasts five to ten minutes, swelling longer.

Venom components include histamine, dopamine and noradrenaline, and the proteins phospholipase A, phospholipase B, antigen 5 and hyaluronidase. For people who have had an anaphylactic reaction to being stung, immunotherapy greatly reduces the risk of a subsequent severe reaction, although people with this medical history are still advised to have access to an EpiPen (a prescription epinephrine injector).

On the Schmidt Sting Pain Index, bald-face hornets are rated a 2.0, putting them in same class as yellowjackets and honeybees. Luckily for us, no insects in New England cause pain scores of 3.0 or 4.0.