Wednesday, September 14, 2016

Bald-faced Hornets

     Bald-faced Hornets (Dolichovespula maculata) have a mostly undeserved reputation for being vicious, pests and safety hazards. They are actually fairly beneficial insects with a role in the environment. Sometimes called White-faced Hornets, Bull Wasps, or Blackjackets, these wasps are actually not true hornets, but large aerial yellowjackets. 
     They mostly go unnoticed and cause no issues until suddenly someone becomes aware of their large paper nests and get scared. Now these insects, which have been living in that same place throughout the earlier part of the spring and summer causing no trouble, are seen as a danger and something to be feared and destroyed.

The typical paper nest we often do not notice until late in the season or when the leaves have fallen from the trees.

     Most of the time, Bald-faced Hornets are actually beneficial. They hunt other insect prey (including some pests) and bring them to feed themselves and their developing young. They predate on quite a few insects, considering that a nest can hold 400-700 workers by the end of the season. Interestingly, one of their favorite food items are their smaller cousins, Yellowjacket wasps. They also serve as minor pollinators (particularly late in the season when they visit more flowers to obtain nectar and hunt less since they have less young to feed). 

A Bald-faced Hornet worker visits a plum flower.

     A typical nest starts when an overwintering queen wakes up in the spring and finds a likely location to start her nest. Though most of the time they choose a location in a tree from 2'-60' high, they sometimes will use the eaves of a building or other structure that might give some cover from the elements. The queen chews some wood fiber, mixes it with saliva, and then shapes it into the first hexagonal paper cells that will house her eggs. Each egg takes about 6 days to produce young. She feeds and cares for these, who then become the first workers and will raise, provide, and protect the young until the end of the season. A new paper nest is quite small and has the opening at the very bottom.

A small paper hornets' nest at the beginning of the spring.

     The queen then has the sole duty of laying eggs for the rest of her life. The workers (all female) hunt for prey, raise the young, build, repair, and protect the nest. They add new layers and hexagonal cells until the nest attains the size of a basketball, though some are up to 2' long. The opening often ends up near the bottom but slightly off to the side. It is at this stage that many people notice and start to be fearful of the nest. Bald-faced Hornets, though not vicious, will certainly defend their nest. They will each sting repeatedly any perceived threat to their young. Unlike other wasps, they are also capable of squirting venom often aimed at the face of an intruder, and which can even temporarily blind. Here's a short video of a nest and its guards:


     By late summer or early fall, the queen starts laying male (drone ) and future queen eggs. Unfertilized eggs become male. The new queens tend to be larger and are hairless (unlike the female workers who have very short "hair" (setae)). Egg production starts to taper off, and eventually there are no more workers or reproductive eggs laid. The old queen dies (some say that in some cases, the workers kill the no longer useful queen). All but the new queens after mating will eventually die off, usually by the time they get a couple of hard frosts. The old nests are durable, easily visible in the winter when the leaves are not around, but start to rot away, with no workers to repair or lap them dry after storms. Nests (except maybe a very few in the deep South) are not used again.

An overwintering Bald-faced Hornet queen.

     The new queen finds a sheltered location to overwinter, often under bark or logs. She will remain in this state (called diapause by some) until mid spring of the following year when she finds a new nest location and starts the cycle all over again. Here's a short video of one I discovered under a log that was overwintering:


     Sometimes nests do become a safety hazard where they might cause issues with people. This is normally when the nest is too close to habitation or where people can bump into them. When this happens, the nests do need to be destroyed, as they cannot be safely moved. This is best done by a professional. If someone does decide to try and eliminate a nest on one's own, please do so with caution and not use such ridiculous tactics as burning them. This should be done at night (when they're all home, sluggish due to lower temperature, and trapped inside) using one of the commercial hornet sprays. A flashlight may help to find the nest initially, but should be placed away from the person spraying or turned off so as to not give the workers a target if they come out in defense. The spray should be aimed directly into the opening, not only to get the pesticide inside and not hit unintended targets, but to also to help prevent them coming out against the stream of the nozzle. 
     Most of the time, these insects should be left alone however. They are beneficial and will leave people alone as long as they don't perceive them as a threat. This year, perhaps because of environmental conditions, we seem to have more nests around than other years. The nests I seem to be finding are much lower down than I typically find also. Old country lore claims that you can foretell the severity of snow by observing how low hornet nests are. The lower the nests are, so it is told, the more snow we will get. There doesn't seem to be any scientific reason that this would work, as the hornets are dead long before the snows arrive and so it doesn't matter where they choose to build, but if this is somehow true, we're in for a really deep snow from what I've noticed this year...


  1. Thanks for the useful information. I was researching them because they built a nest on my bedroom window, so I can see in their bedroom and they can see in mine. We have left them there, since they have caused no harm and I was sure they had a role in our ecosystem. It has been great watching their activity. We appreciate your write up. Thanks.

  2. I've had them on my lilacs in summer several times. This is long past bloom time and not a nest, just lots of individuals. I read they tap the bark for sap and favor lilacs. Have you seen that?

  3. Thank you so much for this post. I have a nest under an eave of my house and have enjoyed watching these fascinating insects. I am going to remove the nest after we get a hard frost and wanted to find out if the overwintering queen would still be inside (therefore needing a safe place for the removed nest). I appreciate your clarification re: the new queen overwintering in logs or other protected locations.