Studies on chemical ecology of host plant selection by herbivores were largely carried out toexplain the diversity of secondary plant chemicals and host specialisation of herbivores. Theglucosinolate – isothiocyanate system in Brassicaceae plants was considered as a plantdefence and was known to influence the feeding behaviour of phytophagous insects such asthe cabbage aphid, Brevicoryne brassicae. Purified myrosinases from the latter and fromwhite mustard, Sinapis alba, were used to hydrolyse two glucosinolate compounds.Identification of the degradation products, mainly isothiocyanates, was performed by gaschromatography – mass spectrometry. While sinigrin hydrolysis gave identicalisothiocyanates for insect and plant enzymes, B. brassicae myrosinase showed a particularactivity toward sinalbin. A lost of an hydroxy group was observed for the two latter substratesrelated products when compared to the S. alba enzymatic specificity. Aphid and plantmyrosinases have different properties and specificities which can be discussed in relation to aco-evolution approach.
