10.5 Wednesday, Jan. 4 Adaptive melanism and immunity to fungal infection in the migratory grasshopper SRYGLEY, R.B.**; JARONSKI, S.T.; USDA-Agricultural Research Service; USDA-Agricultural Research Service firstname.lastname@example.org
Many ectotherms respond to cooler temperatures by increasing their ability to absorb sunlight via adaptive melanization of exposed surfaces. In insects, phenoloxidase (PO) is a key enzyme for both cuticular melanization and the generalized immune response of insects to invasion. Hence, higher temperatures might impact susceptibility to disease making populations more vulnerable to climate change. We asked: is decreased melanism in response to higher temperatures associated with less enzymatic immunity and greater vulnerability to fungal attack? Migratory grasshoppers Melanoplus sanguinipes were reared from the 3rd instar in either a hot (39°C) or cool (27°C) environment. To control for physiological age, adults were moved to a common 33°C. We assayed PO and proPO activity of the blood and measured cuticular darkness. In a second group of adults, we assayed survival to attack by the entomopathogenic fungus Beauveria bassiana. We topically applied a dose of fungal spores that typically causes high mortality within 14 days and followed survivorship. Application of sunflower oil alone served as a control group. Grasshoppers reared in the hot environment were paler and blood PO and proPO titers were significantly less than those reared in the cool one. Consistent with PO and proPO activities, those reared in the hot environment had greater mortality from fungal infection and shorter median survival time. We have shown experimentally that melanism in response to ambient temperature is also associated with changes in immunity. Although the change in coloration is generally considered adaptive for thermoregulation, its association with enzymatic immunity makes it detrimental from the standpoint of combating invasion. More importantly, adaptive melanism may be detrimental to a population’s resilience to climate change.