Easing the transition: Nutrition’s role in the host shift of a tumbling flower beetle from goldenrod stems to galls

Host-race formation in herbivorous insects usually involves a shift from a natal to novel host plant. The several potential challenges associated with such a change in niche, including the presence of new natural enemies and defensive chemistries, must be offset by benefits to ensure the success of the shifted population. Enhanced nutrition in the novel environment is a possible, previously unexplored, advantage facilitating the transition from one host to another. The role of nutrition in host-race formation was examined in the genetically distinct host races of Mordellistena convicta beetles. These beetles inhabit nutrient-poor stems or nutrient-rich galls (induced by Eurosta solidaginis) of Solidago gigantea. According to phylogenetic mapping, Mordellistena convicta has shifted from stems to galls. Therefore, stem beetle larvae were transplanted as early instars into the stems and galls of S. gigantea plants. Final mass determination at the end of larval growth stages showed that the average mass (± standard error) of beetle larvae growing in gall tissues was significantly greater than the average mass of those growing in stems (n_gall=138, m_avg= 4.185 ± 0.084mg; n_stem= 80, m_avg= 3.211 ± 0.088mg; n_total= 218, F= 61.2, d.f.= 1,  P< 0.0001). Chemical analyses revealed elevated levels of key elemental nutrients within gall tissues (n= 110; N: t= 8.687, P< 0.001; K: t=14.687, P< 0.001; P: t= 19.405, P< 0.001).Greater mass among herbivorous insects usually confers enhanced fecundity and survival; thus feeding on nutrient-rich gall tissue could have mediated M. convicta’s host shift and, therefore, the formation of host-races within this species.