RESUMO
The high diversity of phytophagous insects has been explained by the tendency of the group towards specialization; however, generalism may be advantageous in some environments. The cerambycid Apagomerella versicolor exhibits intraspecific geographical variation in host use. In northern Argentina it is highly specialized on the herb Pluchea sagittalis (Asteraceae), while in central and southern areas it uses seven Asteraceae species. To study host species geographical variation from ecological and evolutionary perspectives, we investigated field host availability and use across a wide latitudinal range, and performed laboratory studies on insect oviposition preference and larval performance and mitochondrial DNA (mtDNA) variation in a phylogeographical framework. Geographic variation in host use was unrelated to host availability but was highly associated with laboratory oviposition preference, larval performance, and mtDNA variation. Genetic studies revealed three geographic races of A. versicolor with gene flow restriction and recent geographic expansion. Trophic generalism and oligophagy within A. versicolor seem to have evolved as adaptations to seasonal and spatial unavailability of the preferred host P. sagittalis in cooler areas of the species' geographic range. No single genotype is successful in all environments; specialization may be advantageous in environments with uniform temporal and spatial host availability, while being a trophic generalist may provide an adaptive advantage in host-constrained environments.