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 if the density-dependent prophylaxis could be mediated by host plant species. While host plant species in general did not mediate the density-dependent immune expression, particular host plant species was found to increase larval investment in certain functions of the immune system. Interestingly, these results indicate that different host plants may provide a polyphagous species with protection against different kinds of antagonisms. This insight may contribute to our understanding of the relationship between preference and performance in generalists, as well as having applied consequences for sustainable pest management.

Immune assay data (degree of cuticular melanisation, area of encapsulated nylon piece, darkness of encaupsulation, log lysozyme activity in hemolymph, square root transformed phenoloxidase content of hemolymph as well as square root transformed protein content of hemolymph) for larvae that were raised in either high or low density (4 or 1 larvae) and fed either cotton, cabbage or maize plants. Missing values exist because not all immune assays were possible to perform with every larvae e.g. due to that the nylon piece inserted for encapsulation assay fell out or that the extracted hemolymph was not enough for all assays.