[en] Studies on chemical ecology of host plant selection by herbivores were largely carried out to orplain
the diversity of secondary plant chemicals and host specialisation of herbivores. The glucosinolate -
isothiocyanate system in Brassicaceae plants was considered as a plant defence and was known to
influence the feeding behaviour of phytophagous insects such as the cabbage aphid, Brevicoryre
brassicae. Purified m5nosinases from the latter and from white mustard, Sinqis alba, were used to
hydrolyse two glucosinolate compounds. Identification of the degradation products, mainly
isothiocyanates, was perficrmed by gas chromatography - mass spectromeûy. While sinigdn hydrolysis
gave identical isothiocyanates for insect and plant enzymes, B. brassicae myrosinase showed a
particular activity toward sinalbin A lost of an hydrory group was obsen'red for the two latter
substrates related products when compared to the S. alba enzymatic specificity. Aphid and plant
myrosinases have different properties and specificities which can be discussed in relation to a
co-evolution approach.