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

Authors: Srygley, Robert; JARONSKI, STEFAN - Retired ARS Employee

Submitted to: Ecological Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2021
Publication Date: 1/12/2022
Citation: Srygley, R.B., Jaronski, S.T. 2022. Increasing temperature reduces cuticular melanism and immunity to fungal infection in a migratory insect. Ecological Entomology. 47(1):109-113. https://doi.org/10.1111/een.13088.
DOI: https://doi.org/10.1111/een.13088

Interpretive Summary: Warmer temperatures increase vulnerability of grasshoppers to pathogenic fungi. The color of the exocuticle of some insects is lighter at warmer temperatures to reflect more sunlight. Because a key enzyme of the insect immunity (phenoloxidase) is also involved in darkening of the cuticle, ARS researchers at Sidney, Montana investigated whether the migratory grasshopper Melanoplus sanguinipes has lower phenoloxidase activity and is more susceptible to entomopathogenic fungi when it is reared at higher temperatures. Grasshoppers were reared at 27 C and 39 C, and placed in a common 33 C when they molted to adults. Grasshoppers of the same age but reared at a higher temperature were paler, had lower phenoloxidase activity, and were more susceptible to Beauveria bassiana fungal attack than those reared in the cooler temperature. This study demonstrates that warming compromises the immune system of insects that become lighter at warmer temperatures. Although the response to temperature enhances microbial control of insect pests like M. sanguinipes, it may put pollinators and predators at greater risk with global warming.

Technical Abstract: Few studies have considered how global warming affects disease resistance. Many ectotherms depress melanization of exposed surfaces to reflect more sunlight in warmer habitats. Because phenoloxidase (PO) enhances both cuticular melanization 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? We measured spontaneous PO activity from wounding and circulating total PO activity before topical application of Beauveria bassiana fungus and two days following inoculation. Melanoplus sanguinipes grasshoppers reared in higher temperatures were paler, and had lower spontaneous PO activity and total PO both before and after 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. 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.