Interactions between plants and their herbivores have always attracted the attention of ecologists. In the diversity of plant-insect interactions, the relationships of invasive plants and their specialised enemies is a particular field of research. In the present study we observed the blue butterfly species Celastrina argiolus (Lycaenidae) in relation to its host plant Lythrum salicaria (Lythraceae). The first chapter has its focus on the colour polymorphism of the caterpillars of C. argiolus. We ascertained that there is a significant dependence of the larvae colouration to the colour of plant part where it is currently sitting on. It is most probable that the colour pigments in L. salicaria effectuate the colour change of C. argiolus larvae. However we couldn’t prove the causality of this interrelation in an experiment because of unfavourable environmental conditions. The second chapter treats the correlation of L. salicaria habitat patch characteristics and the occurrence of C. argilous. We compared the parameters patch size, vegetation height, plant density, light quantity, distance to forest edges, presence of other host plants, geographical altitude and anthropogenic disturbance to the number of butterfly eggs in twelve patches.