Speciation of herbivorous insects may be driven by specialization on host plants. Plasticity in host plant preferences might promote sympatric speciation of herbivorous insects if plants affect mating signals and thus, lead to assortative mating. The general aim of this thesis is to understand the causes and consequences of phenotypic plasticity of mating recognition systems of herbivorous insects. The investigated species are the syntopic leaf beetles Phaedon cochleariae and P. armoraciae which have a common host plant range, but use divergent host species when occurring at the same site. Their sexual behavior is mediated by their cuticular hydrocarbon (CHC) profiles which function as contact pheromones for mate and species recognition. Behavioral bioassays and chemical analyses are used to study the question whether the host plant species affects the CHC pattern of the beetles, and thus, their mate recognition. Within a species, males prefer mating with females feeding on the same host plant species to mating with females feeding on an alternative host plant. Sexual isolation between species ceases when beetles feed upon the same host plant species. A discriminant analysis reveals that the beetles' quantitative composition of CHC profiles clearly differ in dependence of sex, host plant and insect species. However, the profiles of the two beetle species are more similar when feeding upon the same host plant species. These findings give rise to the idea that plant-induced phenotypic divergence in mate recognition cues of herbivorous insects may act as an early barrier to gene flow between insect populations on different host species, thus preceding genetic divergence and thus, promoting ecological speciation