The effects of host plant species and larval density on immune function in the polyphagous moth Spodoptera littoralis

Immune functions are costly and immune investment is usually dependent on the individual’s condition and resource availability. For phytophagous insects, host plant quality has large effects on performance, e.g. growth and survival, and may also affect their immune function. Polyphagous insects often experience a large variation in quality among different host plant species, and their immune investment may thus vary depending on which host plant species they develop on. Larvae of the polyphagous moth Spodoptera littoralishave previously been found to exhibit density-dependent prophylaxis as they invest more in certain immune responses in high population densities. In addition, the immune response of S. littoralis has been shown to depend on nutrient quality in experiments with artificial diet. Here, I studied the effects of natural host plant diet and larval density on a number of immune responses to understand if host plant species affects immune investment in generalist insects, and i...