Gregariousness and behavioural changes
Gregariousness is not a disease in the classical sense, but a natural biological phenomenon specific to the desert locust. This insect can exist in two very different forms: the 'solitary' form, discreet and dispersed, and the 'gregarious'...
What is it?
Gregariousness is not a disease in the classical sense, but a natural biological phenomenon specific to the desert locust. This insect can exist in two very different forms: the 'solitary' form, discreet and dispersed, and the 'gregarious' form, which gathers in larval bands and then in flying swarms capable of covering very long distances. This shift from one state to the other, called phase transformation, is triggered by particular environmental conditions and represents one of the mechanisms at the origin of acridian invasions that threaten agriculture and livestock.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The observable changes are not symptoms of disease but modifications in appearance and behaviour. Locusts become more active, group together spontaneously, and lose their usual mutual avoidance behaviour. A more pronounced body colouration is seen, often darker or more contrasted in young stages (nymphs), whereas solitary forms are duller and more discreet. Body proportions may also change slightly, with variations in the relative size of the head and thorax depending on population density. Feeding behaviour also changes: gregarious individuals move in groups to feed, which increases pressure on crops and natural vegetation.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The main trigger is repeated physical contact between individuals, particularly at the level of the hind legs and antennae, when population density increases sharply in a restricted area. This repeated tactile stimulation activates hormonal and neurochemical changes that shift insects from the solitary phase to the gregarious phase within a few generations, or even more rapidly in young stages. Favourable rains followed by abundant vegetation allow massive reproduction; when this vegetation later becomes scarce, locusts concentrate on areas still green, which increases density and promotes gregariousness.
Desert and semi-desert zones of North Africa, the Sahel and the Arabian Peninsula are recognised phase transformation areas. Periods of abundant rainfall following prolonged drought create ideal conditions for rapid reproduction and population concentration. Lack of early monitoring in remote or unstable zones delays detection of emerging outbreaks and increases the risk of evolution towards an invasion.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
There is no individual veterinary diagnosis for a locust, but rather a field assessment conducted by agents specializing in plant protection or entomologists.
This assessment is based on direct observation of population densities, group behavior, and coloration of individuals, supplemented by satellite monitoring of vegetation and rainfall in at-risk areas.
Treatment
Managing gregariousness is part of locust control and not individual medical treatment.
It may include the use of specific insecticides applied by plant protection services, targeting larval bands or newly forming swarms before they disperse.
Biological methods, such as certain entomopathogenic fungi, are also used as part of integrated pest management to limit reliance on chemical products.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies essentially on regular monitoring of known breeding zones, notably by specialised teams that patrol desert areas after rainfall. Regional and international organisations, such as the FAO, coordinate early warning networks in order to detect the first signs of concentration and act before the formation of bands or swarms. In Tunisia and neighbouring countries, cross-border collaboration is essential since swarms recognise no administrative boundaries.
Possible complications
Without rapid intervention, phase transformation can lead to the formation of migratory swarms capable of destroying vast areas of crops and pasture along their path.
These invasions have major economic consequences for agriculture and livestock, with indirect effects on food security in affected regions.
When to see a veterinarian
Observation of unusual groupings of locusts in the same area, especially following rainfall.
Presence of larvae with bright or contrasting colors moving in compact groups.
Report of group flying behavior or bands moving together on the ground.
Any suspicion should be reported promptly to local agricultural or plant protection services.
Prognosis
When detected early, the phenomenon can be contained locally through targeted interventions, preventing swarm formation.
If the concentration progresses without monitoring or action, the evolution toward large-scale invasion becomes difficult to control and may spread across multiple countries.
Frequently asked questions
Is gregariousness a disease of the locust?
No, this is not a disease. It is a natural change in behavior and appearance that the species can adopt depending on environmental conditions and population density.
Why do locusts become gregarious?
Repeated physical contact between individuals, favored by high population density following heavy rainfall, triggers hormonal changes that drive locusts to swarm rather than remain solitary.
Can a gregarious transformation already underway be stopped?
Yes, early intervention by plant protection services, targeting larval bands before they become flying swarms, often makes it possible to limit their spread.
What to do if you observe an abnormal gathering of locusts in Tunisia? Contact the agricultural authorities immediately, particularly the Plant Protection and Control Service (Service de la Protection et du Contrôle des Végétaux). Report the location, size of the swarm, and any direction of movement. Document the sighting with photos if possible. This information is crucial for monitoring and controlling potential locust invasions, which pose a serious threat to crops and food security in the region.
It is recommended to report the observation to local agricultural authorities or plant protection services, which will be able to assess the situation and coordinate a response if necessary.
References
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 31 August 2026
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