Increasing temperature reduces cuticular melanism and immunity to fungal infection in a migratory insect

1. Few studies have considered how global warming affects disease resistance. Many ectotherms depress the melanisation of exposed surfaces to reflect more sunlight in warmer habitats. Because phenoloxidase (PO) enhances both cuticular melanisation and the innate immune response to invasion, we asked: is decreased melanism in response to higher temperatures associated with less PO activity and greater vulnerability to entomopathogenic fungal attack? 2. We measured spontaneous PO activity from wounding and circulating total PO activity before topical application of Beauveria bassiana fungus and 2 days following inoculation. 3. Melanoplus sanguinipes grasshoppers reared in higher temperatures were paler, and had lower spontaneous PO activity and total PO both before and after the fungal attack. They were also more susceptible to Beauveria bassiana fungal attack than those reared in cooler temperatures. Hence, thermoregulatory benefits of decreased melanism for growth, reproduction, and survival can be compromised by reduced PO activity and the immune response to fungal infection. 4. As the Earth warms, these two functions of PO, thermoregulation and immunity, are likely to remain associated with one another, which may cause thermal melanism to be detrimental to an insect population's resilience to climate change.