Generalism should be favoured evolutionarily when there is no genetic constraint or loss of fitness across alternative environments. However, evolution of generalism can require substantial evolutionary change, which can confer a general stress response to other aspects of the environment. We created generalist lineages from an ancestral, resource-specialized laboratory population of seed beetles (Callosobruchus maculatus) by rearing lines over 60 generations on a mixture of both ancestral and novel host species to test for costs associated with the evolution of generalism involving evolutionary changes in gene expression and correlated phenotypic responses during a shift to generalism. Evolved lines had higher fitness on the novel resource, with no loss of fitness on the ancestral resource, indicating that they overcame initial fitness trade-offs. This involved upregulation of major stress response (heat shock protein) genes and genes coding for metabolic enzymes, suggesting an underpinning metabolic and physiological cost. Resource generalist populations also evolved greater thermal tolerance breadth, highlighting that the evolution of resource generalism might pre-adapt species to respond favourably to other environmental stressors, following selection for generalized stress response gene upregulation. The rapid gain of novel hosts during a pest invasion might also confer greater thermal resilience to ongoing climate change.