Yellowjackets build nests with fibers scraped from wood that is mixed with their saliva. Nests consist of multiple tiers of cells that are enclosed in a paper-like envelope. Late in the season, there may be as many as 5,000 workers in the nest. The Dolichovespula spp. of yellowjackets construct aerial nests on structures, or among trees and shrubs. The Vespula spp. primarily build nests in the ground or in other protected areas. Common sites for these nests are in abandoned rodent burrows, wall voids, or in attics. Yellowjackets are considered to be among the most dangerous stinging insects in the United States. Their behavior is often unpredictable. When their nests are disturbed they aggressively attack intruders. Most complaints about yellowjackets come in late summer or fall when the numbers of foraging workers are greatest. These foragers are attracted to human foods such as chicken, beef, or fish and sugary drinks, thus are more likely to come into contact with humans.