Diet drives the collective migrations and affects the immunity of Mormon crickets and locusts: A comparison of these potential superspreaders of disease

Authors: Srygley, Robert

Submitted to: Integrative & Comparative Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2016
Publication Date: 7/25/2016
Publication URL: http://handle.nal.usda.gov/10113/63254
Citation: Srygley, R.B. 2016. Diet drives the collective migrations and affects the immunity of Mormon crickets and locusts: A comparison of these potential superspreaders of disease. Integrative & Comparative Biology. 56(2):268-277. doi:10.1093/icb/icw035.

Interpretive Summary: In this paper, I address the question: Are Mormon crickets and locusts superspreaders of disease? Superspreaders are infected individuals that tend not to show symptoms of infection but make frequent contact with uninfected individuals. Mormon crickets and nymphal locusts readily attack their own species, which forces them to march to avoid cannibalism. To avoid attack, individuals aligned themselves when densities are sufficiently high, resulting in coordinated movements of individuals in massive bands. Collective motion of individuals within the band reduces contact frequency, reduces cannibalism, and also reduces the risk of disease transmission. In addition, because the risk of disease transmission is higher when in groups compared to solitary individuals, Mormon crickets and locusts in groups have greater immunity to disease. The insects’ diet also influences the immune system. Fat is used as a fuel when Mormon crickets lack sugars, and in these bands, the insects are weaker to bacteria. In other migratory bands, Mormon crickets prefer protein over sugars, and the insects are more susceptible to infection by an insect-killing fungus. In addition, fungal infection repels potential cannibals, further reducing disease transmission. As long as the insects are not succumbing to infection, and show no signs of infection that ward off potential cannibals, then it is likely that these bands harbor superspreaders of disease.

Technical Abstract: 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.