Cabbage, mustard and their relatives store glucosinolates separately from the enzyme myrosinase. When a chewing insect ruptures cells the two mix, and myrosinase cleaves the glucosinolate into isothiocyanate, a reactive compound toxic to most herbivores. Botanists call the arrangement the mustard oil bomb.
Pieris rapae larvae express a gut enzyme, nitrile-specifier protein, that intercepts the reaction intermediate. Instead of forming isothiocyanate, the pathway is diverted to a much less toxic nitrile, which the caterpillar excretes. The defence is not merely tolerated but redirected, and the same compounds that repel generalist insects act as feeding and egg-laying cues for this specialist.
The result is an insect closely bound to a crop family and, through it, to agriculture. Pieris rapae spread from Eurasia across North America after arriving in Quebec in the 1860s, and reached Australia and New Zealand later. Control now relies largely on parasitoid wasps and on Bacillus thuringiensis rather than on the plant's own chemistry.