Locusts are specialized short-horned grasshoppers, but unlike most of the 11,000 grasshopper species, they possess a remarkable capacity for sudden, mass transformation. The Desert Locust (Schistocerca gregaria) is the most infamous, capable of switching from a harmless, solitary insect into a massive, synchronized horde. These swarms are enormous collective events, sometimes covering hundreds of square kilometers and containing billions of individuals. Swarming is a coordinated movement driven by profound internal and external changes that transform the insect’s biology and behavior.
The Two Phases of Locust Life
Locusts exhibit phase polyphenism, meaning they exist in two drastically different forms—the solitary phase and the gregarious phase—despite sharing the same genetic makeup. Solitary-phase locusts are cryptic, possessing colors like green or sandy brown that allow them to blend into their environment. Behaviorally, they are shy, avoid contact with others, and are active only at night.
The gregarious phase involves wholesale changes in appearance and activity. Their coloring shifts dramatically to bright, aposematic patterns, often yellow with black markings, which serve as a warning to predators. This phase also involves physical changes, including larger flight muscles and a body shape optimized for long-distance travel.
Gregarious locusts become highly social and actively attracted to other locusts, moving together in large, dense groups during the day. This transformation also affects their physiology, including a faster rate of development and a metabolic shift to fuel the energy demands of mass migration. These differences were so pronounced that scientists once believed the two forms were distinct species until the biological link was established in 1921.
The Density Trigger
The switch to the gregarious phase is driven by a sudden increase in population density, often initiated by environmental conditions. This typically follows rare, localized rainfall in arid regions, which causes a burst of vegetation and successful breeding. When the food supply dwindles as the area dries out, surviving insects are forced to congregate around remaining green patches, causing a rapid rise in density.
This crowding results in frequent physical contact among individuals, which acts as the direct mechanical stimulus for the phase change. Specifically, repeated tactile stimulation of the sensory hairs on the hind legs (femora) is sufficient to trigger the transformation. Experiments have shown that just a few hours of this stimulation can initiate the behavioral shift.
The mechanical stimulus causes a rapid neurochemical response within the central nervous system. This response is driven by the release of the neurotransmitter serotonin, which increases significantly in the thoracic ganglia. This surge in serotonin acts as the internal switch, quickly converting the insect’s behavior from aversion to attraction toward other locusts. This mechanism links the environmental pressure of crowding to the behavioral change, accelerating the formation of a swarm.
The Mechanics of Migration and Coordination
Once the behavioral shift is complete, coordinated movement begins, with the method of travel depending on the locust’s life stage. Juvenile, flightless locusts (hoppers or nymphs) move across the ground in dense columns called marching bands. These bands are highly directional, with individuals aligning their movement based on visual cues from their neighbors, creating a self-organized collective motion.
Adult locusts form flying swarms capable of covering vast distances, sometimes moving up to 150 kilometers in a single day. Movement is maintained through chemical communication, primarily using aggregation pheromones. One compound, 4-vinylanisole (4VA), acts as a powerful attractant, maintaining the swarm’s cohesion.
Collective movement is also influenced by environmental factors, particularly wind and temperature. Swarms often travel downwind, exploiting air currents for long-distance migration. As density increases, locusts also release phenylacetonitrile (PAN), which serves as an anti-cannibalism pheromone, preventing individuals from eating each other.
Economic and Ecological Impact
The consequences of a locust swarm are severe, primarily due to the insects’ voracious consumption habits and the sheer scale of the group. A single adult Desert Locust consumes approximately its own weight in fresh vegetation daily. Due to the immense numbers, a medium-sized swarm covering one square kilometer can consume the same amount of food as about 35,000 human adults in a single day.
This destructive capacity translates directly into catastrophic crop failure and pasture destruction. The rapid decimation of foliage severely threatens food security, especially in vulnerable areas of Africa, the Middle East, and parts of Asia. The loss of crops and pastureland leads to massive economic damage, disrupting livelihoods and creating ecological effects through the widespread removal of vegetation.