I believe you are right, Helena, the Vespula spp. (especially the females) are much more aggressive, and they make their nests in the ground or in hollows. I read that 'Though avoidance is not always possible, do not swing at the Yellow Jacket as they can be easily provoked to attack.' My sister was nearly killed by these when she was three years old. She was staying with an aunt while my parents went to Sweden for a couple of days. She happened to sit on the ground near a nest, and could not get away, because she was paralysed from polio when she was 1 1/2 years old and could not walk, she was stung by more than 40 wasps, which is a lot for a small child.