What Causes Locust Migration and Swarms?

Locust migration is a dramatic insect phenomenon with historical accounts of swarms dating to ancient times. These events are not random but are complex biological responses to specific environmental cues. The transformation from a harmless insect into a member of a devastating swarm is an example of phenotypic plasticity, where an organism’s characteristics change in response to its environment. This process turns solitary creatures into a collective force.

The Solitary to Gregarious Transformation

Locusts normally exist in a solitary phase, living isolated lives and avoiding one another. In this state, they are harmless and resemble ordinary grasshoppers. This behavior changes when heavy rainfall in arid regions leads to an abundance of vegetation.

This new plant life provides ample food, causing populations to grow and concentrate in dwindling patches of greenery. This forced crowding is the primary trigger for the shift to the gregarious phase. The constant physical contact, particularly the stimulation of their hind legs, initiates a cascade of internal changes.

A primary element in this behavioral shift is the neurotransmitter serotonin. Within hours of being crowded, serotonin levels in a locust’s nervous system increase, altering their behavior from avoidance to attraction. This change is also accompanied by physical alterations, as their bodies become more compact and their coloration changes to a conspicuous yellow and black.

Anatomy of a Locust Swarm

Once gregarious, locusts aggregate to form immense swarms that can cover areas from less than one to several hundred square kilometers. Within each square kilometer, there can be between 40 and 80 million locusts, with some swarms containing billions of individuals.

The movement of these swarms is determined by the prevailing winds, allowing them to cover vast distances with minimal energy expenditure. A swarm’s average speed is about 15 to 20 kilometers per hour. This allows them to travel 5 to 130 kilometers or more in a single day.

Locusts can stay airborne for extended periods, enabling them to cross large bodies of water. For instance, swarms regularly cross the Red Sea, a distance of about 300 kilometers. Documented cases show swarms traveling from West Africa to the Caribbean, a journey of 5,000 kilometers that took about ten days.

Impact on Agriculture and Ecosystems

The arrival of a locust swarm has severe consequences for agriculture. A single adult locust can consume its own weight in fresh food, about two grams, per day. Multiplied by the billions of individuals in a swarm, the amount of vegetation consumed is staggering. A swarm covering just one square kilometer can eat the same amount of food in one day as approximately 35,000 people.

The primary impact is on food crops, with swarms destroying entire fields of cereals, fruits, and vegetables within hours. During the 2019-2021 locust upsurge in East Africa, hundreds of thousands of hectares of cropland were damaged, leading to significant cereal losses. These losses threaten the food security and livelihoods of millions in vulnerable regions.

Beyond cultivated crops, locust swarms also decimate natural vegetation, including grasslands and fodder for livestock. This impacts pastoralist communities that depend on these resources. The stripping of vegetation can also lead to soil erosion and disrupt the balance of arid ecosystems.

Global Hotspots and Migration Cycles

Desert locust plagues are most common in the arid and semi-arid regions of Africa, the Middle East, and Southwest Asia. During quiet periods, known as recessions, locusts are confined to an area of about 16 million square kilometers across 30 countries. During plagues, they can spread to 29 million square kilometers, affecting up to 60 countries.

Migration routes are tied to seasonal wind patterns and weather systems. Swarms are carried on the wind to areas where recent rainfall has created favorable conditions for breeding. This allows the locusts to find moist soil to lay their eggs and fresh vegetation for the emerging nymphs, known as hoppers.

The locust life cycle of eggs, wingless hoppers, and winged adults allows plagues to persist across generations and vast geographical areas. As one generation matures, it flies to new areas, breeds, and gives rise to the next wave of swarms. This creates a continuous cycle of outbreaks that can last for years, with major hotspots in Northwest Africa, the Horn of Africa, the Arabian Peninsula, and the Indo-Pakistan border area.