Research was carried out on the effects of insecticides used in agriculture on butterflies. Field experiments showed that insecticides have a harmful effect on young caterpillars. As a result of drift, pesticides settle in field margins and grass verges, killing off many of the caterpillars. The effects of insecticides are greatest for butterfly species with a larval stage in spring or summer when the use of pesticides is high. This study and risk analysis show that insecticides are a cause of the decline of a number of common butterflies in the Netherlands.