Various herbivorous insects prefer plants of the Brassicaceae family as their hosts, although they are toxic. The two-component chemical defence system of the Brassicaceae against herbivores consists of glucosinolates (GLS) and the activating enzyme myrosinase. GLS hydrolysis by myrosinase leads to isothiocyanate (ITC) products, which are toxic and deterrent to many insect herbivores. Some insects that feed on Brassicaceae, however, have evolved specific adaptations (called counter-defences) against GLS. Two different types of counter-defences can be distinguished: a preemptive counter-defence that prevents the GLS from being hydrolysed to ITC due to metabolic redirection and direct counter-defence, where the ITC is formed, but then metabolized to a non-toxic conjugate. Preemptive counter-defence is believed to be more efficient due to the lower exposure to ITC, but this has not been well demonstrated experimentally. Here, we prove on theoretical grounds that preemptive counter-defence reduces exposure to ITC compared to direct counter-defence by studying the dynamics of GLS defence and counter-defence with two separate ordinary differential equation models. By quantifying the specific ITC concentrations that herbivores are exposed to during feeding with the two types of counter-defences, we show that herbivores with a preemptory detoxification system are less exposed to ITC. In addition, our models explain how the decline in the level of ITC is achieved by both counter-defences, which helps to understand the overall mechanisms and benefits of these techniques.