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Diet Drives the Collective Migrations and Affects the Immunity of Mormon Crickets and Locusts: A Comparison of These Potential Superspreaders of Disease

R. B. Srygley
Integrative and Comparative Biology 2016 v.56 no.2 pp. 268-277
Anabrus simplex, Beauveria bassiana, Metarhizium, adipokinetic hormone, adults, antibacterial properties, cannibalism, carbohydrates, disease transmission, entomopathogenic bacteria, entomopathogenic fungi, high protein diet, hormonal regulation, immunity, lipids, locomotion, locusts, migratory behavior, monophenol monooxygenase, mycoses, risk, risk reduction, starvation
The need for resources is a major driver of animal migration and yet migration itself is energetically demanding. Mormon crickets and nymphal locusts readily engage in cannibalistic attacks that result in aligned, coordinated movement of individuals in massive bands that march daily for weeks at a time. Coordinated movement reduces contact frequency, which not only reduces cannibalism but the risk of disease transmission. In addition, Mormon crickets and locusts elevate their constitutive immunity when in groups, which further reduces the risk of disease transmission. Dietary deficiencies of the migrants also determine whether Mormon crickets will be susceptible to bacteria or entomopathogenic fungi. Mobilization of lipid stores when Mormon crickets seek carbohydrates, as observed in some migratory bands, engages a lipid-transporting protein compromising its role in combatting bacterial invaders. Starvation in locusts results in release of adipokinetic hormone and mobilization of lipids, suggesting that the trade-off between locomotion and anti-bacterial activity is under hormonal control. Mormon crickets in other migratory bands prefer protein over carbohydrates, indicating protein-deficiency. In these bands, the generalized immunity of Mormon crickets, measured as phenoloxidase, is compromised, and the insects are more susceptible to Beauveria bassiana fungal infection. In locusts, a high protein diet resulted in greater susceptibility to another entomopathogenic fungus, Metarhizium acridum, whereas in Mormon crickets, both phenoloxidase titers and immunity to M. acridum increased with adult age. Infection with either of these fungi diminishes cannibalism, further reducing disease transmission. As long as the insects are not succumbing to infection, and show no signs of infection that ward off their conspecifics, then we would expect that these bands would be superspreaders of disease. However the diseases that they are most likely to harbor are likely to depend on the dietary deficiencies exhibited by members of the band.