Impact of plant nutrients on the relationship between a herbivorous insect and its symbiotic bacteria

Abstract

The interactions between herbivorous insects and their symbiotic micro-organisms can be influenced by the plant species on which the insects are reared, but the underlying mechanisms are not understood. Here, we identify plant nutrients, specifically amino acids, as a candidate factor affecting the impact of symbiotic bacteria on the performance of the phloem-feeding aphidAphis fabae.Aphis fabaegrew more slowly on the labiate plantLamium purpureumthan on an alternative host plantVicia faba, and the negative effect ofL. purpureumon aphid growth was consistently exacerbated by the bacterial secondary symbiontsRegiella insecticolaandHamiltonella defensa, which attained high densities inL. purpureum-reared aphids. The amino acid content of the phloem sap ofL. purpureumwas very low; andA. fabaeon chemically defined diets of low amino acid content also grew slowly and had elevated secondary symbiont densities. It is suggested that the phloem nutrient profile ofL. purpureumpromotes deleterious traits in the secondary symbionts and disturbs insect controls over bacterial abundance.