Insects spread disease. Malaria, which is a disease carried by Anopheles mosquitoes, kills over 1.5 million people in Africa every year, most of them children. Simulium flies in West Africa spread a disease to people called onchocerciasis which makes them go blind. If you swallow a flea in your house you run the risk of being infected with tapeworm. There are many other diseases transmitted between humans and between humans and animals by insects and in order to control the spread of these diseases we need to study the vector insects that spread them.
Insects eat our crops.
Millions of rand are spent annually in South Africa controlling insects that eat our crops. Insecticides are the one way of controlling pests but they have undesirable side-effects on our health and the environment so the preferred method is biological control where entomologists find beneficial parasitic and predatory insects such as parasitic wasps and ladybird beetles that can be released to control the pests. Insects eat our stored food. There are many other stored product pests that can cause serious food losses and some of these you will have encountered in your kitchen if dry foods are left unprotected in your cupboard for too long. Insects maim and kill our livestock.