Locust
Locusts are orthopteran insects in the family Acrididae, distinguished from typical grasshoppers by their capacity for density-dependent phase polyphenism, which shifts them from a solitary, cryptic phase to a gregarious, swarming phase under high population densities.[1][2] This behavioral and morphological transformation enables the formation of vast migratory swarms, where individuals exhibit heightened activity, altered coloration, and increased appetite for vegetation.[3][4] In the gregarious phase, locust swarms can span thousands of square kilometers, consuming up to twice their body weight in plant matter daily per individual and stripping landscapes of foliage, leading to profound agricultural losses and threats to food security.[5][6] Major species include the desert locust (Schistocerca gregaria), prone to explosive outbreaks across arid regions of Africa, the Arabian Peninsula, and South Asia, and the migratory locust (Locusta migratoria), historically responsible for plagues in temperate zones of Eurasia.[2][7] These outbreaks, driven by environmental factors like rainfall fostering breeding, have inflicted billions in economic damage, as seen in the 2019–2021 desert locust upsurge that ravaged crops in the Horn of Africa and beyond.[6][8] Management relies on early detection, aerial spraying of insecticides, and international coordination to suppress hopper bands before they mature into flying swarms, underscoring locusts' role as one of the most destructive migratory pests despite their otherwise unremarkable solitary existence.[5][9]Definition and Characteristics
Phase Polyphenism and Behavioral Shifts
Locusts display phase polyphenism, a form of density-dependent phenotypic plasticity enabling reversible transitions between a solitary phase and a gregarious phase within the same species. This phenomenon, first systematically described by Boris Uvarov in 1921, explains how non-swarming, grasshopper-like individuals can transform into swarming forms under high population densities.[10] In the solitary phase, characteristic of low-density conditions, locusts exhibit avoidance behavior toward conspecifics, reduced locomotor activity, and a tendency to remain sedentary. Solitarious individuals actively flee from others upon encounter, spend more time resting, and display lower rates of walking and grooming.[3][7] These behaviors minimize aggregation and align with cryptic coloration and solitary foraging, reducing visibility to predators.[10] Conversely, the gregarious phase emerges at high densities and features attraction to conspecifics, heightened activity, and cohesive group formation. Gregarious locusts orient toward others, increase walking and grooming frequency by up to several fold, and reduce resting time, facilitating synchronized marching in nymphs and flight in adults.[3][7] Nymphal bands exhibit aligned movement, with individuals maintaining contact through hind-leg kicking and tactile interactions, promoting swarm cohesion.[11] The behavioral shift from solitary to gregarious is primarily triggered by tactile stimulation from crowding, which activates neural pathways leading to rapid gregarization within 1-4 hours in species like the desert locust Schistocerca gregaria.[12] Physical contact, such as hind-leg touches, elicits serotonin release in the central nervous system, elevating levels up to ninefold in thoracic ganglia and inducing attraction, hyperactivity, and reduced aversion.[3][13] Serotonin injection into isolated solitarious locusts replicates these changes, confirming its causal role independent of prolonged crowding.[14] The reverse gregarious-to-solitary transition occurs under prolonged isolation, taking days to weeks, with serotonin signaling promoting withdrawal-like behaviors while dopamine modulates incomplete reversals.[15][16] These shifts underscore a causal link between environmental density cues and neurochemical modulation, enabling adaptive responses to population pressures without genetic change.[17]Morphological and Physiological Adaptations
In the gregarious phase, locust nymphs develop conspicuous black patterning on a yellow or orange background, contrasting with the cryptic green or brown coloration of the solitarious phase, which aids in camouflage and reduces predation risk in low-density environments.[18] This pigmentation shift, prominent in species like the desert locust (Schistocerca gregaria), emerges rapidly after crowding and is regulated by neuropeptides such as [His⁷]-corazonin.[19] Adults in the gregarious phase exhibit brighter, aposematic hues, enhancing visibility during swarms.[20] Morphological differences extend to body proportions and appendages: gregarious-phase adults of S. gregaria possess larger heads, longer relative wing lengths suited for sustained migration, and more robust hind femora for powerful jumps, compared to the shorter wings and proportionally larger hind legs (higher femur-to-head width ratio) in solitarious individuals.[21] Eye size is smaller in gregarious locusts, with altered ommatidial structure and pigment distribution that may optimize visual processing for dense aggregations.[21] Solitarious females often exceed gregarious counterparts in overall body size and ovariole number across species like Locusta migratoria and S. gregaria, supporting higher per-female egg production in isolation despite slower maturation.[22] Physiologically, the transition involves elevated serotonin levels in the central nervous system, rising within hours of tactile stimulation from crowding and persisting to sustain gregarization; injections of serotonin induce phase-like behavioral and morphological shifts even in isolation.[23] [24] This neurotransmitter surge alters multiple systems, including rapid changes in six key brain chemicals toward gregarious profiles within four hours.[24] Metabolic adaptations favor the gregarious phase with upregulated lipid and carbohydrate pathways, enabling higher energy demands for flight and rapid maturation—gregarious S. gregaria adults mature faster than solitarious ones, though the reverse occurs in L. migratoria.[18] [22] Juvenile hormone titers modulate some traits but do not primarily drive the polyphenism.[22]Taxonomy and Diversity
Principal Species
The principal species of locusts are those orthopterans in the family Acrididae exhibiting pronounced phase polyphenism, transitioning from solitary to gregarious forms that enable massive swarms capable of devastating crops and vegetation over vast areas. These species are distinguished by their capacity for long-distance migration and rapid population explosions under favorable environmental conditions, leading to recurrent plagues documented throughout history.[25] Foremost among them is the desert locust (Schistocerca gregaria), recognized as the most destructive migratory pest worldwide due to its ability to form swarms covering up to 29 million square kilometers during outbreaks, affecting 20% of the world's land surface across Africa, the Arabian Peninsula, and Southwest Asia.[25][26] In its recession phase, it occupies approximately 6 million square miles of arid and semi-arid habitats, but gregarious phases triggered by rainfall and vegetation growth propagate plagues that can persist for years, as seen in the 2019–2021 upsurge impacting East Africa and Yemen.[27][28] The migratory locust (Locusta migratoria) ranks as the most widely distributed locust species, spanning grasslands and wetlands across Africa, Eurasia, and into northern Australia, with distinct subspecies adapted to regional climates.[29][30] This species has caused significant historical outbreaks, such as in China and Russia, where swarms have migrated thousands of kilometers, consuming equivalent to the daily food intake of tens of thousands of people per swarm.[31] Other principal species include the Australian plague locust (Chortoicetes terminifera), endemic to inland Australia and capable of forming bands that cover up to 50 square kilometers, leading to plagues that threaten agricultural production in arid zones.[5] In Africa, the red locust (Nomadacris septemfasciata) periodically emerges from breeding sites in eastern and southern regions, forming swarms that migrate northward.[32] These species collectively underscore the global threat posed by locusts, with control efforts coordinated by organizations like the FAO targeting early detection to mitigate escalation to plague status.[25]| Species | Scientific Name | Primary Regions | Key Characteristics and Impacts |
|---|---|---|---|
| Desert Locust | Schistocerca gregaria | Africa, Middle East, SW Asia | Most destructive; swarms up to 29 million km²; 2019–2021 plague affected 23 countries.[28] |
| Migratory Locust | Locusta migratoria | Africa, Asia, Australia, Europe | Widest distribution; historical plagues in Eurasia; high reproductive rate.[29] |
| Australian Plague Locust | Chortoicetes terminifera | Inland Australia | Inland outbreaks; bands up to 50 km²; impacts crops in dry seasons.[5] |
Global Distribution Patterns
The desert locust (Schistocerca gregaria), one of the most economically significant species, occupies recession areas primarily in the arid and semi-arid deserts spanning northern Africa (from Mauritania to Sudan and Ethiopia), the Arabian Peninsula, and extends eastward to Pakistan and northwest India, with potential for irruptive expansions into eastern Africa, South Asia, and beyond during outbreaks driven by favorable climatic conditions.[25][33] These patterns reflect a dependency on ephemeral breeding sites in wadi beds and coastal plains where sporadic rainfall triggers gregarization and swarm formation, enabling long-distance flights that can cover thousands of kilometers.[6] The migratory locust (Locusta migratoria), exhibiting the widest natural range among locust species, is distributed across the Old World from sea level to elevations exceeding 4,000 meters in Central Asian mountains, encompassing sub-Saharan Africa, southern Europe, vast expanses of Asia (including China, India, and the Russian Far East), and northern Australia, with subspecies like L. m. migratorioides predominant in African savannas and L. m. migratoria in Eurasian steppes.[34][35] Its distribution correlates with temperate grasslands and floodplains where monsoon cycles and riverine flooding facilitate hopper band formation and adult migrations, though human-modified landscapes have fragmented some traditional breeding habitats.[36] Other principal species show more restricted patterns: the Australian plague locust (Chortoicetes terminifera) is endemic to inland Australia, with outbreaks linked to cyclonic rainfall in arid interiors; the redlocust (Nomadacris septemfasciata) recurs in eastern and southern African wetlands; and the Italian locust (Calliptamus italicus) prevails in Central Asian steppes and Mediterranean fringes.[37][38] Globally, locust distributions cluster in 10-20% of arid zones worldwide, with 21 recognized species concentrated in the Afrotropical, Palearctic, and Australasian realms, underscoring a causal link between climate variability—such as El Niño-induced wet phases—and episodic range expansions beyond permanent recession zones.[39][37] While modeling predicts climate-driven shifts, such as poleward extensions in suitability under warming scenarios, empirical records emphasize historical stability in core habitats punctuated by plague dispersals rather than permanent poleward migration.[40][41]Evolutionary History
Phylogenetic Origins
Locusts represent polyphenic forms of select grasshopper species within the family Acrididae, which belongs to the superfamily Acridoidea and the suborder Caelifera of the order Orthoptera.[42][43] Orthoptera as a whole traces its origins to the late Carboniferous period, approximately 300 million years ago, with early fossils exhibiting orthopteran-like ovipositors and mandibles indicative of herbivorous habits in Paleozoic forests.[44] However, the Acrididae family, encompassing over 6,700 species including locust progenitors, emerged later during the Paleocene epoch of the Cenozoic era, around 59.3 million years ago, with molecular clock estimates and biogeographic analyses pinpointing a South American origin for its common ancestor.[43][45] Phylogenetic reconstructions using mitochondrial and nuclear genes, as well as ultraconserved elements, confirm Acrididae's monophyly within Acridoidea and highlight its diversification through multiple radiations, initially in Gondwanan landmasses before global dispersal.[46][47] The family's expansion coincided with post-Cretaceous ecological opportunities, such as the proliferation of angiosperm grasslands, favoring short-horned grasshoppers adapted for jumping and stridulation.[43] Fossil evidence for Acrididae is limited but includes indeterminate locust-like wings from the Early Oligocene (approximately 30 million years ago) in Iranian sediments, suggesting that locust morphologies were established by the late Paleogene.[48] The swarming (gregarious) phase characteristic of locusts evolved convergently across at least six Acrididae subfamilies, implying independent origins rather than a single ancestral trait.[49] For instance, in the genus Schistocerca, which includes the desert locust (S. gregaria), molecular phylogenies indicate S. gregaria as an early-diverging lineage, with biogeographic debates centering on trans-Atlantic dispersal: some analyses support an African origin followed by westward migration of swarming ancestors around 5-7 million years ago, while others place it within a New World clade, suggesting eastward return migration.[42][48] These conflicting hypotheses underscore the role of vicariance and rare long-distance flights in shaping Acrididae phylogeny, with genomic comparisons revealing adaptations in metabolic and mitochondrial genes linked to migratory evolution.[50]Evolution of Swarming Traits
Swarming traits in locusts, primarily manifested as phase polyphenism, represent a form of density-dependent phenotypic plasticity enabling reversible shifts between solitarious (avoidance of conspecifics) and gregarious (attraction and cohesion) behaviors. Phylogenetic analyses of Acrididae reveal that this polyphenism has evolved convergently multiple times, with locust species forming a polyphyletic assemblage across at least six subfamilies, indicating independent origins rather than descent from a single swarming ancestor. For example, morphological phylogenies of the Cyrtacanthacridinae subfamily demonstrate that phase polyphenism arose separately within this lineage, with behavioral, morphological, and coloration components potentially evolving semi-independently.[51][52] In specific genera like Schistocerca, reconstructions suggest an ancestral swarming condition with plastic reaction norms for behavior and pigmentation, from which non-swarming grasshopper forms diverged secondarily through loss of density responsiveness. This pattern implies that the genetic and sensory machinery for gregarization—such as tactile hind-leg stimulation and visual cues triggering serotonin release—may have been exapted from pre-existing avoidance mechanisms in high-density scenarios, favoring cohesion for migration in resource-scarce environments. Gregarious phases enhance survival by enabling bands of nymphs and adult swarms to traverse landscapes, exploiting ephemeral vegetation surges following irregular rainfall in arid zones, where solitarious phases predominate under stable, low-density conditions.[42][2][3] Comparative genomic studies further illuminate selective pressures, showing intensified evolution in metabolic and mitochondrial pathways among migratory locust species, which underpin the elevated energy demands of prolonged flight, lipid mobilization, and heightened fecundity during swarms—traits absent or reduced in non-swarming relatives. These adaptations align with first-principles expectations for unstable habitats, where swarming permits rapid population irruptions to capitalize on transient booms, offsetting predation risks through dilution effects and collective foraging efficiency, though it incurs costs like nutritional depletion of host plants. Overall, the repeated evolution of these traits underscores their causal role in enabling locusts to persist in marginal ecosystems prone to boom-bust dynamics.[50][53]Biology and Ecology
Life Cycle and Reproduction
Locusts undergo incomplete metamorphosis, progressing through egg, nymph (hopper), and adult stages without a pupal phase.[54] The total life cycle duration varies by species, environmental conditions, and phase state, typically spanning 2-6 months for the desert locust (Schistocerca gregaria), with eggs hatching in 10-65 days, nymphal development lasting 24-95 days (average 36 days), and adults living 2.5-5 months after fledging around 40-50 days post-egg laying.[54] [25]Female locusts reproduce sexually, mating after reaching adulthood and laying eggs in soil pods formed by their ovipositor, which digs a narrow chamber 5-15 cm deep in moist, sandy substrates.[55] [56] Each pod contains 50-100 eggs encased in a frothy secretion that hardens into a protective plug, facilitating synchronized hatching by providing an escape route for nymphs while deterring predators and desiccation.[55] [54] In the migratory locust (Locusta migratoria), clutches reach up to 80 eggs per pod in soft, wet sand, with aggregated laying in gregarious phases possibly guided by pheromones or site scarcity.[56] Egg development requires soil temperatures above 20°C and adequate moisture, with diapause possible under suboptimal conditions to delay hatching until favorable weather.[54] Nymphs hatch synchronously, often in mid-morning peaks, and pass through 5-6 instars as wingless hoppers that resemble miniature adults but lack functional wings until the final molt.[54] [57] These instars last 20-40 days in warm conditions (e.g., 6 weeks for desert locust hoppers), during which nymphs feed voraciously and exhibit phase polyphenism: solitarious nymphs avoid crowds, while gregarious ones form bands that amplify serotonin-driven gregarization, influencing faster development and swarming potential.[25] [54] Adults fledge with fully developed wings for migration, maturing sexually in 2-4 weeks (up to 1 month minimum for desert locusts), after which copulation occurs and females produce multiple pods over their lifespan, potentially increasing populations sevenfold per generation in outbreak conditions.[54] [25] Gregarious-phase adults lay fewer but larger eggs due to prolonged oogenesis and oosorption, enhancing hatchling viability in dense swarms, whereas solitarious females prioritize higher clutch numbers over size.[58] [59] Overall generation turnover accelerates in warm, recession-free environments, enabling plague dynamics.[25]