A general model of the thermal constraints on the world’s most destructive locust,Schistocerca gregaria

Abstract

All terrestrial ectotherms are constrained to some degree by their thermal environment and the extent to which they can behaviorally buffer variable thermal conditions. New biophysical modeling methods (NicheMapR) allow the calculation of the body temperature of thermoregulating animals anywhere in the world from first principles, but require detailed observational data for parameterization and testing. Here we describe the thermoregulatory biology of marching bands of the desert locust, Schistocerca gregaria, in the Sahara Desert of Mauritania where extreme heat and strong diurnal fluctuations are a major constraint on activity and physiological processes. Using a thermal infrared camera in the field, we showed that gregarious nymphs altered the microhabitats they used, as well as postural thermoregulatory behaviors, to maintain relatively high body temperature (nearly 40°C). Field and laboratory experiments demonstrated that the preferred body temperature accelerated digestive rates. Migratory bands frequently left foraging sites with full guts before consuming all vegetation and moved to another habitat before emptying their foregut. Thus, the repertoire for behavioral thermoregulation in the desert locust strongly facilitates foraging and digestion rates, which may accelerate developmental rates and increase survival. We used our data to successfully parameterize a general biophysical model of thermoregulatory behavior that could capture hourly body temperature and activity at our remote site using globally available environmental forcing data. This modeling approach provides a stronger basis for forecasting thermal constraints on locust outbreaks under current and future climates.

Additive genetic variance for traits least related to fitness increases with environmental stress in the desert locust,Schistocerca gregaria

“…However, this trend for a stress-dependent increase in V A that mainly stands for morphological traits has been recused by Rowinski and Rogell's (2017) meta-analysis, which showed a general increase in V A with environmental stress, irrespective of the category of the traits considered. genetic parameters have been obtained, our results should help to predict more accurately the future range and severity of attacks by this agricultural pest in response to global change over long timescales (Ayali, 2019;Maeno et al, 2021;Meynard et al, 2017Meynard et al, , 2020.…”

“…In this ectotherm at the top end of the thermal tolerance spectrum (Maeno et al, 2021), our study explored as a thermal stress a low temperature that reflects winter local conditions. While our laboratory environments did not capture the full microclimatic variability from deserts and did not allow either for behavioral opportunities (e.g., warm-up by basking in the sun; Maeno et al, 2021), they are likely to capture most of the cold stress related to seasonal variation. In any case, the imposed low temperature lengthened the development time up to three times (i.e., range of 20-59 days), costs of which are likely to reduce the fitness of individuals in nature (e.g., F I G U R E 3 Simulation analysis of the statistical power of our quantitative genetic analysis.…”

“…In order to further address this fundamental issue, we investigated in the laboratory how the genetic parameters of 10 traits with different levels of proximity to fitness responded to a lifetime sublethal thermal stress in a population of the desert locust. In this ectotherm at the top end of the thermal tolerance spectrum (Maeno et al., 2021 ), our study explored as a thermal stress a low temperature that reflects winter local conditions. While our laboratory environments did not capture the full microclimatic variability from deserts and did not allow either for behavioral opportunities (e.g., warm‐up by basking in the sun; Maeno et al., 2021 ), they are likely to capture most of the cold stress related to seasonal variation.…”

“…By bringing insights in the potential evolutionary trajectory of traits involved in demographic processes (i.e., growth, survival, and dispersal) under environmental stress, our results should help predict more accurately how global change might affect populations' dynamics and invasions (Maeno et al., 2021 ; Meynard et al., 2017 , 2020 ). At a short timescale, the heritable variation that is released under the more intense selection regime of stressful conditions provides a reservoir of standing genetic variation that can contribute to the adaptation of populations (Endler, 1986 ).…”

“…However, expected change for those latter plastic traits is not easy to quantify, though it might be rather low on average, with responses likely to be more specific, depending both on the intensity at which phenotypic compensation is compromised with stress and on the genetic constraints in which they evolve. Beyond evaluating V A predictions in relation to environmental stress in a desert locust population, thereby expanding the range of taxa for which values of genetic parameters have been obtained, our results should help to predict more accurately the future range and severity of attacks by this agricultural pest in response to global change over long timescales (Ayali, 2019 ; Maeno et al., 2021 ; Meynard et al., 2017 , 2020 ).…”