Climate change expected to improve digestive rate and trigger range expansion in outbreaking locusts

Abstract

Global climate change will probably exacerbate crop losses from insect pests, reducing agricultural production, and threatening food security. To predict where crop losses will occur, scientists have mainly used correlative models of species' distributions, but such models are unreliable when extrapolated to future environments. To minimize extrapolation, we developed mechanistic and hybrid models that explicitly capture range‐limiting processes, and we explored how incorporating mechanisms altered the projected impacts of climate change for an agricultural pest, the South American locust (Schistocerca cancellata). Because locusts are generalist herbivores surrounded by food, their population growth may be limited by thermal effects on digestion more than food availability. To incorporate this mechanism into a distribution model, we measured the thermal effects on the consumption and defecation of field‐captured locusts and used these data to model energy gain in current and future climates. We then created hybrid models by using outputs of the mechanistic model as predictor variables in correlative models, estimating the potential distribution of gregarious outbreaking locusts based on multiple predictor sets, modeling algorithms, and climate scenarios. Based on the mechanistic model, locusts can assimilate relatively high amounts of energy throughout temperate and tropical South America; however, correlative and hybrid modeling revealed that most tropical areas are unsuitable for locusts. When estimating current distributions, the top‐ranked model was always the one fit with mechanistic predictors (i.e., the hybrid model). When projected to future climates, top‐ranked hybrid models projected range expansions that were 23%–30% points smaller than those projected by correlative models. Therefore, a combination of the correlative and mechanistic approaches bracketed the potential outcomes of climate change and enhanced confidence where model projections agreed. Because all models projected a poleward range expansion under climate change, agriculturists should consider enhanced monitoring and the management of locusts near the southern margin of the range.

An increase in management actions has compensated for past climate change effects on desert locust gregarization in western Africa

“…However, it is important to note that the relationship between climate change and locust dynamics can be complex and context-specific. For similar species, rising temperatures can accelerate population development by improving their digestive rate for example, leading to more swarms and range expansions during outbreaks [ 61 ]. For desert locusts, temperature increases can have positive effects on gregarization in the areas that were at the lower temperature range to start with, but in extremely hot and arid deserts these further temperature increases could also impede population development [ 62 ].…”

How Nutrients Mediate the Impacts of Global Change on Locust Outbreaks

“…Because metabolic rates increase exponentially with temperature (44), carbohydrate intake may be expected to increase to fuel higher energetic demands. However, higher temperatures also increase growth rates for developing insects, at least in part due to improved digestion rates (49,147), and faster growth rates increase protein consumption (130). Grasshopper studies have shown mixed results.…”

“…Temperature has many interactive effects on digestion and nutrient assimilation (33). Higher temperatures (40°C) improve digestion rates and are predicted to trigger range expansion in outbreaking South American locusts (S. cancellata) (147). Fittingly, migratory locusts (L. migratoria) select higher temperatures (38°C) to maximize growth rates, even at the expense of efficient nutrient use (93).…”

“…Rising temperatures are generally predicted to expand locust ranges to higher elevations and latitudes [e.g., S. gregaria (147), Calliptamus italicus (103), and Dociostaurus maroccanus (74)]. Grasshoppers, including locusts, are excellent behavioral thermoregulators.…”

“…Grasshoppers, including locusts, are excellent behavioral thermoregulators. Warmer climates will likely allow grasshoppers to achieve body temperatures closer to optimal over a greater part of the day, allowing them to eat, digest, and grow faster (147). At lower altitudes and latitudes, however, grasshoppers may be unable to avoid damaging temperatures, which begin at roughly 45-49°C, resulting in a predicted range contraction in some areas (91,147).…”