For two reasons: (a) self-defence mainly from humans, badgers and honey buzzards and (b) defence of the colony. They do not use their sting to kill prey.
* Are wasps more aggressive than bees?
No. I was stung just three times in as many years during my research. A colleague working on honeybees was stung at least once a week. Bees are herbivorous and visit flowers. Wasps are omnivorous, so are more attracted to human foods, which means we are more likely to be pestered by wasps than by bees, perhaps giving rise to the misconception that wasps are more aggressive.
* Do all wasps sting?
Only the females (queens and workers) sting. The sting is a modified egg-laying device, so males don't have one. However, as males are the same colour and even pretend to sting, it's best not to take chances.
* How can I avoid being stung?
It really is true that wasps will not sting unless provoked: my research was on this very aspect. Most stings are in self-defence, when a wasp gets into a person's clothing or is flapped at or hit in a panic. The secret is to remain calm and allow the wasp to finish what it's doing without making it feel threatened - try not to move if a wasp is in your clothes: allow it to escape without trapping it.
If the wasp believes its colony is under attack, the situation is more serious. If you unwittingly disturb a wasps' nest, the workers immediately go into "colony defence mode". You will almost certainly get stung at least once - and then the crisis escalates, because wasp venom contains an alarm pheromone. This pheromone only works within about five metres of the nest, but within this radius it attracts other wasps to join in the attack. Rapid movement and dark colours exacerbate the situation, so try to react slowly.
Wearing white while gardening reduces the risk.
The recently introduced "Euro" or "Killer"wasp is notorious because its colonies are fairly large and are invariably hung out of sight in a hedge or garden shrub. German and Common Wasps usually have underground colonies, so can be disturbed when overgrown grass or neglected lawns is cut.
* Can wasp stings be dangerous?
Most people suffer no more than severe discomfort, even from multiple stings, but a few individuals (less than half of 1 per cent of the population) are hypersensitive to wasp venom and exhibit an extreme reaction. However this is very unlikely on the first occasion of being stung - the reaction becomes more severe with successive stings. Local swelling is normal, but if you experience more general symptoms (dizziness, a skin rash away from the site of the sting, or difficulty in breathing), you should contact your GP.
* What use are wasps?
In a "good" wasp year there may be up to 200 nests per square kilometre. If each wasp grub eats 10 bluebottles (or other insect pests) and each colony produces an average of 10,000 wasps, this works out at 20 million bluebottles.
* Which are acid - wasp stings or bee stings?
Almost every science textbook perpetuates this old wives' tale. A colleague of mine measured the pH of both and discovered they were neutral.
* How good is a wasp's eyesight?
Very good. Its two compound eyes can see every colour we can, apart from red, together with one invisible to us - ultraviolet. It also has three small eyes on the top of its head. These can't form a sharp image, but are sensitive to the plane of polarisation of light, so they can detect the position of the sun. They help the wasp navigate back to the nest.
Brian Aldiss, a former teacher, is now an ecological consultant. He has conduced extensive research into what makes wasps sting and their alarm behaviour at the Chemical Entomology unit at Southampton University.