Wednesday, October 19, 2011

Wasp Wednesday: Western Yellowjacket

Notorious. That describes the Western Yellowjacket, Vespula pensylvanica (and yes, the species name is spelled correctly). I recall this social wasp from my childhood in Portland, Oregon where it was a constant presence at picnics, barbecues, and garbage cans at the zoo and every other urban park. Here in Colorado Springs they are equally pestiferous, and persist deep into the fall.

Each year, the Western Yellowjacket colony cycle begins with queens searching for nesting sites. These females are larger, and more yellow in color than the worker caste. They typically emerge from their winter hibernacula sometime between March and April, though it may take awhile for a queen to find a suitable subterranean niche where she can start building her nest. I imaged the queen below at the Cheyenne Mountain Zoo on July 9, 2011, so she still had no help from workers in constructing her nest.

Once the queen’s worker offspring can contribute, the colony and nest quickly expand in size. Mature nests, based on records from Pullman, Washington and La Grande, Oregon, can exceed a population of 2,000 workers (average about 1,800) and contain 4,000 cells or more in the paper combs.

Nests are usually located in abandoned rodent burrows ten to fifteen centimeters below the surface of the soil, with entrance tunnels ten to thirty centimeters long. There is often a mud turret surrounding the entrance hole. Look for yellowjackets coming and going from the same spot to locate a nest entrance. You can approach closely to watch their activity without arousing the occupants, but run a lawnmower over the nest and watch out! One of my elementary school science teachers told me he once drove a stake right through a nest. That could not have been a pleasant experience.

Western Yellowjackets are best known for their aggressive scavenging behavior. They have to secure large quantities of protein to feed the larvae in the nest, and the workers take that job seriously. It is much easier to haul away a chunk of your tuna sandwich, chicken leg, or burger than it is to go kill a series of small insects. Still, this species does its fair share of scavenging road kill and preying on true bugs, spiders, flies, grasshoppers, even slugs.

The adult wasps need carbohydrates to fuel their active lifestyle, so that is why they crawl into your soda can. Normally, in nature, they prefer the sweet waste products secreted by aphids and scale insects. This “honeydew” is like the Nectar of the Gods to social wasps. Last week I imaged several workers lapping up the honeydew of Cinara conifer aphids on a pine tree at the Cheyenne Mountain Zoo.

The last chapter in the annual colony life cycle is the production of new queens and males. These reproductives are liberated at the end of the season and they fly out to find mates from other colonies. The males, with their long abdomens and antennae, seem to congregate around the tops of small trees where they perch to await passing females. The males may dislodge each other from prime lookout posts, but they don’t have stingers and such squabbles are therefore not life-threatening.

Western Yellowjackets can be “bad” some years, with much higher than normal population densities. This usually occurs when a warm, dry spring season allows queens to get a head start on establishing colonies. The result can be worker wasps wreaking havoc on fruit tree orchards, logging camps, and outdoor recreation destinations later in the summer.

The Western Yellowjacket is exactly that: a western North American species ranging from southern British Columbia, Alberta and Saskatchewan south through the Pacific coast and Rocky Mountain States, all the way to Arizona and New Mexico at higher elevations. There are scattered records across the northern Midwest as far east as Michigan; and this species has been introduced to Hawaii as well.

Source:Akre, Roger D., Albert Greene, et al. 1981. The Yellowjackets of America North of Mexico. Washington, DC: USDA Agriculture Handbook Number 552. 102 pp.

1 comment:

  1. The non-Pennsylvanian Vespula pensylvanica is also a pest in Edmonton, AB, with other vulgaris group species (alascensis, germanica) also scavenging and threatening to fly into the soda cans. I think it is interesting that the workers in the vulgaris-group have such sweet-tooths, but the males do not nectar at flowers as do the males of Dolichovespula (and the worker Dolichovespula don't seem to care for sweets).

    I'm not sure why, but in early autumn Vespula pensylvanica and other vulgaris-group (alascensis, germanica) males congregate in my backyard. They buzz in, land on grass and other low vegetation, and then crawl to the ground. I can collect a couple dozen in ten minutes. I have no idea what they are doing and crawling around on my belly watching them has left me no wiser.


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