Summary Associational refuges – reduced herbivory on plants in presence of other plant species – may be caused by general and species-specific plant characteristics. However, the species specificity of associational refuges has rarely been evaluated. This study examined the species specificity of one known example of associational refuges, the forb Lythrum salicaria and the monophagous insect herbivores Galerucella calmariensis, G. pusilla and Nanophyes marmoratus. The underlying mechanism was examined in order to evaluate connections between mechanisms and species specificity. Laboratory studies showed that N. marmoratus but not Galerucella individuals were attracted by odour from undamaged host plants, and that neither species was distracted by odour from Myrica gale. Field experiments showed that three non-host plant neighbours with an appearance similar to M. gale, and artificial shrubs, reduced the abundance and egg-laying of Galerucella species by 70–90%. The abundance of N. marmoratus was increased 18-fold on plants in thickets compared with outside. . The different responses by N. marmoratus and the Galerucella species to plant neighbours appear to be because N. marmoratus, but not Galerucella, uses olfactory information in the initial host-finding process.