Western Yellowjacket

The western yellowjacket shares the basic yellow and black pattern with other species of social wasps in the genus Vespula. As a result, it is often misidentified. Specifically, it closely resembles V. germanica. Though both species have a similar diamond-shaped black mark on the first tergum, V. pensylvanica can usually be differentiated by the continuous yellow ring (often referred to as an eye-loop) present around each eye. The eye-loop is not always present, however. Though few females lack this distinguishing characteristic, males often do not have the eye-loop. In such cases, V. pensylvanica can be identified by a spotted mark on the frons below the antenna, a slender aedeagus, and a denser apical region of tergum 7. The fore wing length ranges from 8.5–10.5 mm for workers, 12.5–14.5 mm for females, and 12.5–14.0 mm for males. Western yellowjackets build large nests made of wood fibers, enclosed in a hexagonal paper envelope with a small 10– to 30-cm entrance tunnel at the bottom. Nests are usually found 10–15 cm below ground or in other dark cavities, such as rodent burrows or old logs.

The western yellowjacket is native across the western half of North America, in temperate zone climates. Its range overlaps with that of V. atropilosa. Individuals have been identified in the Canadian and Transition Zones in western North America. It has been found in all states west of the Rocky Mountains, from Mexico through western Canada. It is also found in Hawaii on the islands of Kauai, Oahu, Maui, and Hawaii. V. pensylvanica can be found in open forest, prairies, urban zones, gardens, parkland, meadows, and houses.

Mating behavior of V. pensylvanica has been observed to be very similar to mating behavior of V. germanica. Pairing occurs midday and receptive queens remain stationary throughout. Mating occurs between one male and one female, with the female generally upright on her legs and the male attached solely by genitalia. Other males in the nest do not interfere with the mating, but often antennate the pairs during the mating process. Sometimes, queens may be unreceptive and will exhibit this by twisting away from the males and biting. Unreceptive behavior from females, however, does not always prevent mating from occurring. Vespula queens often mate more than once with the same or different males. Mating involves only one male.

Periodic population outbreaks every three to five years have created problems within fruit orchards. The tendency of V. pensylvanica to cluster in areas of human impact has created problems for pickers in fruit orchards who are frequently stung. The costs to the grower to control the outbreaks can be extremely high. V. pensylvanica is often immune to efforts to control other pests, which often results in high costs to growers who have to use additional methods to control V. pensylvanica.