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

Kristina Karlsson Green

doi:10.1002/ece3.7802

Abstract

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, for example 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 littoralis have 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 how host plant species affects immune investment in generalist insects, and whether 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.

Highlights

The immune system is a costly trait, and there are often trade- offs between investment in an increased immune response and other traits (Lochmiller & Deerenberg, 2000; Schwenke et al, 2016), such as between immune function and reproduction (Ilmonen et al, 2000), or growth and intraspecific competition (Kraaijeveld & Godfray, 1997)
Given that immune function studies with manipulated artificial diet have shown that differences in nutrient content could have large effects on various immune parameters (e.g., Ponton et al, 2020), the host plant diversity of both nutrients and toxic defenses that generalist insects are exposed to present excellent opportunities to study the effect of natural diet variation for immune responses (Singer et al, 2014)
I studied the effects of host plant species, larval density, and their interaction on different immune responses in S. littoralis, asking whether plant species has the potential to mediate the prophylactic investment in immune function and whether certain plant species are more advantageous for the immune system than others

Summary

| INTRODUCTION

The immune system is a costly trait, and there are often trade- offs between investment in an increased immune response and other traits (Lochmiller & Deerenberg, 2000; Schwenke et al, 2016), such as between immune function and reproduction (Ilmonen et al, 2000), or growth and intraspecific competition (Kraaijeveld & Godfray, 1997). Given that immune function studies with manipulated artificial diet have shown that differences in nutrient content could have large effects on various immune parameters (e.g., Ponton et al, 2020), the host plant diversity of both nutrients and toxic defenses that generalist insects are exposed to present excellent opportunities to study the effect of natural diet variation for immune responses (Singer et al, 2014). I studied the effects of host plant species, larval density, and their interaction on different immune responses in S. littoralis, asking whether plant species has the potential to mediate the prophylactic investment in immune function and whether certain plant species are more advantageous for the immune system than others

| MATERIALS AND METHODS

Findings

| DISCUSSION

| CONCLUSION