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Nutrient composition varies greatly across landscapes, influencing the spatiotemporal dynamics of populations. However, few studies have explored this pattern across multiple scales. We tested how nutrient limitation affects herbivore populations, from individual behavior to landscape‐level dynamics, using the Australian plague locustChortoicetes terminifera. Our study combined field populations and long‐term survey data across their range. At the individual level, juvenile locusts selected a carbohydrate‐biased intake target of 1 protein (p) to 2 carbohydrate (c) and exhibited the highest growth rates and shortest development times when fed artificial diets matching this 1p:2c ratio during their final juvenile instar. In the field, locusts exposed to protein‐biased plants corrected their nutritional imbalance by initially selecting carbohydrate‐heavy diets (up to a 1p:20c ratio). Over a week after removal from the protein‐rich environment, they returned to the 1p:2c intake target once the deficiency was balanced. At the landscape level, locust outbreaks were negatively correlated with soil nitrogen and exhibited a non‐linear relationship with soil phosphorus, peaking at approximately 4% phosphorus content. By disentangling the interaction between mean annual precipitation and soil nitrogen, using comprehensive locust surveys and remotely sensed soil and weather data spanning decades, we show how environmental factors drive population dynamics. This study integrates lab, field and remote sensing approaches, highlighting the importance of nutrient balancing across scales for herbivores. Specifically, we demonstrate that low‐nitrogen environments promote locust outbreaks, likely by reducing plant protein‐to‐carbohydrate ratios. Incorporating soil quality data into locust plague forecasting models could significantly improve prediction accuracy. 

Organisms require dietary macronutrients in specific ratios to maximize performance, and variation in macronutrient requirements plays a central role in niche determination. Although it is well recognized that development and body size can have strong and predictable effects on many aspects of organismal function, we lack a predictive understanding of ontogenetic or scaling effects on macronutrient intake. We determined protein and carbohydrate intake throughout development on lab populations of locusts and compared to late instars of field populations. Self-selected protein:carbohydrate targets declined dramatically through ontogeny, due primarily to declines in mass-specific protein consumption rates which were highly correlated with declines in specific growth rates. Lab results for protein consumption rates partly matched results from field-collected locusts. However, field locusts consumed nearly double the carbohydrate, likely due to higher activity and metabolic rates. Combining our results with the available data for animals, both across species and during ontogeny, protein consumption scaled predictably and hypometrically, demonstrating a new scaling rule key for understanding nutritional ecology. 

Locusts are grasshoppers that can migrate en masse and devastate food security. Plant nutrient content is a key variable influencing population dynamics, but the relationship is not straightforward. For an herbivore, plant quality depends not only on the balance of nutrients and antinutrients in plant tissues, which is influenced by land use and climate change, but also on the nutritional state and demands of the herbivore, as well as its capacity to extract nutrients from host plants. In contrast to the concept of a positive relationship between nitrogen or protein concentration and herbivore performance, a five-decade review of lab and field studies indicates that equating plant N to plant quality is misleading because grasshoppers respond negatively or neutrally to increasing plant N just as often as they respond positively. For locusts specifically, low-N environments are actually beneficial because they supply high energy rates that support migration. Therefore, intensive land use, such as continuous grazing or cropping, and elevated ambient CO₂levels that decrease the protein:carbohydrate ratios of plants are predicted to broadly promote locust outbreaks. 

Many theoretical treatments of foraging use energy as currency, with carbohydrates and lipids considered interchangeable as energy sources. However, herbivores must often synthesize lipids from carbohydrates since they are in short supply in plants, theoretically increasing the cost of growth. We tested whether a generalist insect herbivore (Locusta migratoria) can improve its growth efficiency by consuming lipids, and whether these locusts have a preferred caloric intake ratio of carbohydrate to lipid (C : L). Locusts fed pairs of isocaloric, isoprotein diets differing in C and L consistently selected a 2C : 1L target. Locusts reared on isocaloric, isoprotein 3C : 0L diets attained similar final body masses and lipid contents to locusts fed the 2C : 1L diet, but they ate more and had a ~12% higher metabolic rate, indicating an energetic cost for lipogenesis. These results demonstrate that some animals can selectively regulate carbohydrate-to-lipid intake and that consumption of dietary lipids can improve growth efficiency. 

In contrast to predictions from nitrogen limitation theory, recent studies have shown that herbivorous migratory insects tend to be carbohydrate (not protein) limited, likely due to increased energy demands, leading them to preferentially feed on high carbohydrate plants. However, additional factors such as mechanical and chemical defenses can also influence host plant choice and nutrient accessibility. In this study, we investigated the effects of plant protein and carbohydrate availability on plant selection and performance for a migratory generalist herbivore, the Australian plague locust, Chortoicetes terminifera. We manipulated the protein and carbohydrate content of seedling wheat ( Triticum aestivum L. ) by increasing the protein:carbohydrate ratio using nitrogen (N) fertilizer, and manipulated the physical structure of the plants by grinding and breaking down cell walls after drying the plants. Using a full factorial design, we ran both choice and no-choice experiments to measure preference and performance. We confirmed locust preference for plants with a lower protein-carbohydrate ratio (unfertilized plants). Unlike previous studies with mature wild grass species, we found that intact plants supported better performance than dried and ground plants, suggesting that cell wall removal may only improve performance for tougher or more carbohydrate-rich plants. These results add to the growing body of evidence suggesting that several migratory herbivorous species perform better on plants with a lower protein:carbohydrate ratio. 

Locusts are a very serious problem for agriculture and for the livelihoods of populations around the world [...] 

ABSTRACT Migration allows animals to track favorable environments and avoid harmful conditions. However, migration is energetically costly, so migrating animals must prepare themselves by increasing their energy stores. Despite the importance of locust migratory swarms, we still understand little about the physiology of locust migration. During long-distance flight, locusts rely on lipid oxidation, despite the fact that lipids are relatively rare in their leaf-based diets. Therefore, locusts and other insect herbivores synthesize and store lipid from ingested carbohydrates, which are also important for initial flight. These data suggest that diets high in carbohydrate should increase lipid stores and the capacity for migratory flight in locusts. As predicted, locust lipid stores and flight performance increased with an increase in the relative carbohydrate content in their food. However, locust flight termination was not associated with complete lipid depletion. We propose potential testable mechanisms that might explain how macronutrient consumption can affect flight endurance. 

Locust outbreaks have long been a very serious problem for agriculture and livelihoods in many countries globally. This article is an introduction to a Special Issue of the journal Agronomy devoted to the management of these pests. Although not exhaustive, the nineteen articles herein cover a variety of species, many regions of the world and many aspects of pest locust management and research in the early 21st century. This book is a source of information and reflection, as well as a resource, to support new areas of investigation and practice contributing to the process of developing sustainable solutions for locust invasions.