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...