Genetic variation contributes to intraspecific differences in the chemical defence in many insect species, yet the underlying genetic mechanisms remain poorly understood. The horseradish flea beetle, Phyllotreta armoraciae , sequesters glucosinolates from its horseradish host plant and activates them using endogenous myrosinase enzymes. Of the three known myrosinases in P. armoraciae , PaMyr1 functions primarily in adults, whe...