Locust

A locust is any species of short-horned grasshopper in the family Acrididae (order Orthoptera, suborder Caelifera) that exhibits pronounced density-dependent phase polyphenism—a reversible form of phenotypic plasticity in which identical genotypes switch between two contrasting behavioral, physiological, and morphological phenotypes in response to local population crowding: a cryptic, mutually repulsive solitarious phase (solitaria) at low population densities and a conspicuously pigmented, cohesive, swarm-forming gregarious phase (gregaria) at high densities (Uvarov, 1921; Pener & Simpson, 2009). Unlike ordinary grasshoppers, locusts do not constitute a single monophyletic clade; rather, phase polyphenism has evolved convergently at least six times across diverse subfamilies of Acrididae (including Cyrtacanthacridinae, Oedipodinae, Gomphocerinae, Calliptaminae, and Melanoplinae). During continental plagues, a single adult desert locust (Schistocerca gregaria) swarm can span over 1,000 km², contain 40 to 80 billion individuals, travel 100 to 150 km per day via insect flight on prevailing winds, and consume as much green vegetation in 24 hours as 35,000 people.

Prior to the early twentieth century, taxonomists classified solitary grasshoppers and swarming plague locusts as entirely separate biological species—for example, describing the solitary green morph of the Eurasian migratory locust as Locusta danica and its swarming brown morph as Locusta migratoria. In 1921, Russian-British entomologist Sir Boris P. Uvarov published his landmark Phase Theory, demonstrating through field observations and laboratory rearing that L. danica and L. migratoria are reversible density-induced phases (solitaria, transiens, and gregaria) of a single species.

Of the approximately 11,000 described species in family Acrididae, fewer than 20 express full behavioral and morphological phase polyphenism:

A central discovery of modern entomology and neuroethology is that the behavioral transition from mutual repulsion (solitaria) to mutual attraction and hyperactive locomotion (gregaria) occurs on a much faster timescale than morphological and color changes: a solitarious desert locust (S. gregaria) crowded with conspecifics becomes behaviorally gregarious within 2 to 4 hours, whereas full cuticular color and morphometric remodeling requires several nymphal molts and transgenerational maternal priming (Pener & Simpson, 2009).

Dual Tactile vs. Visual–Olfactory Sensory Pathways

Working at the University of Oxford and University of Cambridge, Stephen J. Simpson, Stephen M. Rogers, Malcolm Burrows, and colleagues (2001–2009) mapped the peripheral sensory inputs that trigger gregarization in Schistocerca gregaria:

1. The Hind-Femur Mechanosensory Pathway: Repeated physical jostling of the outer surface of the metathoracic (hind) femur—specifically stimulating trichoid sensilla innervated by metathoracic nerve 5—is both necessary and sufficient to induce complete behavioral gregarization within 4 hours, whereas tactile stimulation of the abdomen, thorax, prothoracic legs, or antennae fails to induce the switch (Simpson et al., 2001). This mechanosensory specificity reflects ecological crowding around patchy desert shrubs, where jostling locusts repeatedly bump one another's large saltatorial hind femora.
2. The Cephalic Visual + Olfactory Pathway: Independently of physical contact, simultaneous exposure to the sight and odor of conspecifics also triggers behavioral gregarization via descending interneurons from the brain's ommatidia and antennal lobes, though neither visual cues alone nor olfactory cues alone suffice without the other (Roessingh et al., 1998; Rogers et al., 2003).

The Thoracic Serotonin (5-HT) Surge and Dopamine/Hormonal Cascade

In 2009, Michael L. Anstey, Stephen M. Rogers, Swidbert R. Ott, Malcolm Burrows, and Stephen J. Simpson demonstrated that both the hind-femur mechanosensory pathway and the cephalic visual–olfactory pathway converge onto a transient neurochemical pulse of serotonin (5-hydroxytryptamine, 5-HT) strictly localized within the thoracic ganglia:

• During the first 1 to 4 hours of crowding, thoracic 5-HT titers rise ~3-fold in strict linear correlation with the degree of behavioral gregarization.
• Pharmacological blockade of 5-HT receptor signaling (via ketanserin or methiothepin) or inhibition of serotonin synthesis (via α-methyltryptophan) completely prevents crowded locusts from gregarizing, whereas injecting 5-HT or its agonists into the thoracic hemocoel of isolated solitarious locusts rapidly induces gregarious behavior without any social contact (Anstey et al., 2009).
• In the migratory locust (Locusta migratoria), Le Kang and colleagues (2011–2014) showed that catecholamine dopamine signaling (via the pale, henna, and D1-like receptor pathway) operates alongside serotonin to drive phase transitions and cuticular melanization.

Over longer timescales (days to stadia), neuroendocrine factors orchestrate systemic morphological and chromatic remodeling:

• [His⁷]-Corazonin: A conserved neuropeptide synthesized by lateral neurosecretory cells of the brain and released via the corpora cardiaca induces dark melanin deposition in the insect cuticle and shortens the pronotum and hind femur relative to the forewing (F/C and E/F morphometric ratios).
• Juvenile Hormone (JH III): Secreted by the corpora allata, elevated juvenile hormone titers induce green background coloration in solitarious nymphs and regulate vitellogenesis and lipid mobilization in adult migrants.

Once behavioral gregarization is initiated, positive feedback loops mediated by volatile pheromones and collective motion lock locusts into dense aggregations:

• 4-Vinylanisole (4VA): In 2020, Xiaojiao Guo, Le Kang, and colleagues identified 4-vinylanisole (4VA, 4-methoxystyrene) as the universal aggregation pheromone of Locusta migratoria. Emitted from the hind legs when as few as 4 to 5 solitarious locusts congregate, 4VA is detected exclusively by basiconic sensilla on the antennae expressing the olfactory receptor OR35 (coupled to the co-receptor Orco), attracting both sexes and all nymphal instars across solitary and gregarious phases (Guo et al., 2020).
• Host-Plant Toxicity and Aposematism: Solitarious desert locust nymphs are uniformly green or sandy brown, blending cryptically into desert vegetation and avoiding toxic alkaloid-bearing plants such as Hyoscyamus muticus (henbane, containing scopolamine and hyoscyamine). Immediately upon gregarizing, nymphs switch their gustatory preference to actively ingest toxic Hyoscyamus leaves while developing striking aposematic warning coloration (black dorsal patterns on a bright yellow or orange background), advertising their chemical unpalatability to vertebrate predators (Sword et al., 2000).
• Cannibalism-Driven Collective Marching: Wingless gregarious nymphs form cohesive hopper bands containing millions of individuals moving in polarized fronts across kilometres of terrain. Nutritional geometry studies by Bazazi, Simpson, Couzin, and colleagues (2008) proved that collective marching is driven in large part by protein and salt deprivation combined with cannibalistic threat: locusts are vulnerable to being bitten from behind by protein-hungry conspecifics, and selectively denervating the abdominal mechanoreceptors reduces marching alignment and triggers widespread cannibalism.

Female desert locusts transmit phase state across generations via maternal epigenetic programming: females that experience crowding during oviposition secrete a low-molecular-weight hydrophilic gregarizing factor (alongside L-DOPA derivatives) into the accessory gland foam surrounding the subterranean egg pod, while loading phase-specific microRNAs and differential DNA methylation marks into the oocytes—causing hatchlings to emerge already dark-colored, synchronous, and behaviorally gregarious (Pener & Simpson, 2009).

At the continental scale, desert locust plagues originate in arid recession habitats following anomalous cyclonic rainfall events (such as Indian Ocean Dipole cyclones over the Rub' al Khali or Horn of Africa), which support consecutive generations of exponential breeding (outbreak → upsurge → plague) and concentrate airborne swarms along the Intertropical Convergence Zone (ITCZ). Contemporary locust integrated pest management, coordinated by the FAO Desert Locust Information Service (DLIS), combines satellite normalized difference vegetation index (NDVI) soil-moisture mapping with early preventive ground spraying of first-generation hopper bands using the host-specific entomopathogenic fungus Metarhizium acridum (Green Muscle) and chitin synthesis inhibitors (diflubenzuron, triflumuron), suppressing upsurges before transcontinental adult swarms take wing.

1. Uvarov, B. P. (1921). A revision of the genus Locusta, L. (= Pachytylus, Fieb.), with a new theory as to the periodicity and migrations of locusts. Bulletin of Entomological Research, 12(2), 135–163. https://doi.org/10.1017/S0007485300044989
2. Simpson, S. J., Despland, E., Hägele, B. F., & Dodgson, T. (2001). Elementary processes driving the establishment of locust gregarization. Proceedings of the National Academy of Sciences, 98(7), 3895–3897. https://doi.org/10.1073/pnas.071527998
3. Bazazi, S., Buhl, J., Hale, J. J., Anstey, M. L., Sword, G. A., Simpson, S. J., & Couzin, I. D. (2008). Collective motion and cannibalism in locust migratory bands. Current Biology, 18(10), 735–739. https://doi.org/10.1016/j.cub.2008.04.035
4. Anstey, M. L., Rogers, S. M., Ott, S. R., Burrows, M., & Simpson, S. J. (2009). Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts. Science, 323(5914), 627–630. https://doi.org/10.1126/science.1165939
5. Guo, X., Yu, Q., Chen, D., Wei, J., Yang, P., Yu, J., Wang, X., & Kang, L. (2020). 4-Vinylanisole is an aggregation pheromone in locusts. Nature, 584(7822), 584–588. https://doi.org/10.1038/s41586-020-2610-4