 Biological insecticides, such as "Bacillus thuringiensis", as well as chemical insecticides, are used to protect trees against larvae.Methionine, an essential amino acid in humans, has also been found to be an effective killer of caterpillars, with possible use as a nontoxic pesticide against giant swallowtail larvae.There has been a northern expansion of the range of the giant swallowtail in recent years which has been linked to increasingly warm temperatures, and particularly to a lack of September frosts in regions of expansion starting in 2001.