How was nicotine used as an insecticide?

Because nicotine is highly toxic to many insects, it was widely utilized as a natural insecticide beginning in the 19th century.1

The compound serves as a natural chemical defense for the tobacco plant, which synthesizes it in its roots and transports it to its leaves to deter insects and other herbivores. While its direct agricultural application has declined due to concerns regarding its toxicity and the availability of safer alternatives, nicotine remains significant as the chemical model for the development of neonicotinoids, a class of synthetic insecticides that are now widely used in global agriculture.123