There are two key ways in which insects can be considered pests: if they cause health issues, or if their presence has economic implications.
Insects can affect our health in all sorts of ways. Some, like fleas, can bite and cause a huge amount of irritation. Others, like wasps and bees, can leave a painful sting, which is particular dangerous for those with bee and wasp allergies.
Pest insects like weevils can get into food supplies and contaminate them and others, like flies that live on decaying matter, can carry serious diseases like salmonella, E. Coli or campylobacter.