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Diet is one of the major factors directly and indirectly influencing insect’s life history traits and risk of getting infected. Additionally the insect’s fitness is severely affected by the broad diversity of parasites they are exposed to. As a consequence insects have developed well-evolved defences. Behavioural responses include self-medication, the ability of insects to change dietary intake in response to an infection. When studying this ability it is of major importance to consider the insects natural diet range. In this thesis I investigated the effect of different host plants on fitness and immunocompetence of polyphagous Arctia plantaginis larvae and whether the larvae can therapeutically and prophylactically self-medicate by optimising their diet intake. Additionally I examined the long-term effect of prophylactic treatment on lab-reared larvae from the same species. Feeding experiments reveal that the host plants plantain (high in biologically active compounds) and dandelion (high nutritional value) have different effect on the larval development and survival. Results show that a monotonous plantain diet provides lowest protection against an infection, whereas a diet switch from plantain to dandelion increases survival. Immunocompetence seems to be not differently affected by the two host plants. When given the choice, all larvae choose to ingest a mixture of both plants. The ratio of both plants differed depending on the infection status or infection risk in their environment compared to control larvae. This highlights the importance of a mixed diet for some polyphagous species and that dietary variety should be considered when studying insects’ ability to therapeutically and prophylactically medicate. A prophylactic medication applied to laboratory- reared insects showed negative effects on life-history traits and reproductive success, and thus should be carefully considered. In conclusion these findings provide insights in the crucial importance of diet mixing for polyphagous larvae and show evidence for the larvae ability to therapeutically and prophylactically medicate by mixing their diet.