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Locusts and Uvarov’s phase theory of 1921

Discover Uvarov’s 1921 locust phase theory, experiments on behaviour, and the development of field observations protecting crops and grazing land.

1921: Uvarov’s theory; nineteenth-century observations to mechanistic research · The Caucasus, London, southern Africa and international research networks · Science & experiments

In 1921 Boris Uvarov proposed an explanation that placed apparently different insects within the history of one species. A locust could occur in a solitarious or a gregarious phase, depending on its circumstances. Phase theory redirected attention from the place where a swarm arrived towards the conditions in which its members had previously lived. For agriculture, it connected species identification, ecological research and the observation of developing threats.

Farmers knew the damage before scientists explained phases

Farmers and pastoralists experienced locust damage long before Uvarov’s explanation. Writing to Darwin on 23 October 1868, James Philip Mansel Weale described an invasion near Bedford in southern Africa. He distinguished heavily damaged pasture from places where fewer insects settled, noting weather, vegetation and behaviour after landing. His observations document practical and natural-history knowledge without themselves explaining phase transformation.

In 1915 photography documented another agricultural catastrophe. An American Colony album from Jerusalem recorded the invasion, damage to vegetation and efforts against locusts. Images of a fig tree before and after feeding made losses visible without a chart of harvest statistics. Authorities also ordered egg collection. The archive records organisation and pressure on local people during a wartime food crisis. A photograph can reveal severe damage at a particular location, however, without measuring losses throughout the region or attributing every aspect of food shortage to the insects.

A classification problem became a biological question

Uvarov’s paper of June 1921 combined a revision of the genus Locusta with a new interpretation of periodic outbreaks and migration. Conflicting species identifications, he argued, obstructed comparison of information about biology and control in different countries. The forms called Locusta danica and Locusta migratoria were to be understood within one species rather than as separate species. Misclassification could therefore detach observations made before an outbreak from the apparently different insects involved in the later invasion.

Field experience helped lead to this interpretation, including Uvarov’s work in the Caucasus during 1911–1914 and reports of breeding observations by other investigators. Comparisons involved parents, offspring, intermediate forms and anatomical similarities. The proposition was not that any grasshopper could suddenly become a locust. It was that environmental circumstances could alter a coordinated set of characteristics within particular species. Differences previously organised into separate taxonomic categories required an explanation involving the lives and surroundings of the insects themselves.

Rearing experiments tested the effects of crowding

By 1929 Augustus Imms described solitarious and gregarious phases as extremes linked by transitional forms. His account discussed observations from Russia and Madagascar and efforts to develop shared terminology. Colour and body proportions differed, but so did behaviour. Naming the phases did not settle their cause: researchers needed to separate parental origin from the circumstances in which offspring developed.

In 1932 Jacobus Faure published extensive South African experiments. In a reported series involving the brown locust, Locustana pardalina, offspring were raised either in isolation or under crowded conditions. Isolated insects generally developed solitarious characteristics, whereas crowded insects tended towards gregarious ones, regardless of the parental phase; crosses tested the same reasoning. Rearing conditions could thus be distinguished from genealogy. The result did not mean that genes were irrelevant. It showed plasticity that could not be explained simply by inheritance of two supposedly fixed forms.

From physical contact to a change in behaviour

A further question concerned how a locust detected the presence of other locusts. Stephen Simpson and colleagues reported in 2001 that repeated touching of the outer hind femur promoted gregarious behaviour in the desert locust, Schistocerca gregaria. Behaviour was subsequently assessed in a test arena. Instead of treating crowding as one indivisible condition, the experiment isolated a particular stimulus. It moved investigation from an overall association with density towards a mechanism through which an individual received information.

In 2009 Michael Anstey and colleagues linked the onset of gregarious behaviour in this species to serotonin. A later analysis by Rogers and colleagues in 2014 distinguished increased movement from attraction to other insects and found coordinated changes in both. Faster walking alone would not establish gregarization. Behaviour can also change more quickly than colour or body proportions. These findings concern particular experiments and a particular species; they do not imply that every feature of an insect is transformed instantly by a single encounter.

Vegetation patterns and the use of grazing land

Average density across a large area does not describe every encounter between insects. In 1998 Matthew Collett and colleagues combined simulations with a desert-locust laboratory experiment. They varied the spatial concentration of resources while holding their overall amount constant. Clumped feeding resources promoted movement towards gregarious behaviour. Vegetation arrangement therefore mattered: the same number of insects could encounter one another more frequently when they used a small set of shared locations.

Research published by Arianne Cease and colleagues in 2012 examined Oedaleus asiaticus in northern Asian grasslands. Greater plant nitrogen and high-protein diets did not improve this insect’s performance. Heavily grazed sites combined lower plant nitrogen with higher insect abundance, and the authors related their findings to steppe degradation. This was not a universal formula for every locust outbreak. It demonstrated why species identity, food quality and land use could supplement counts of insects when interpreting agricultural environments and their potential to support damaging populations.

Shared terminology made international comparison possible

Phase theory also had international implications because migrating insects crossed administrative boundaries. Uvarov’s 1934 report described the Rome conference of 1931, where representatives of Britain, France and Italy adopted a shared direction for research. The Imperial Institute of Entomology in London was designated a coordinating centre. Nine countries were represented at the next conference, held in Paris in 1932. This collaboration developed within the governmental and imperial structures of its period, with research assembled across territories rather than confined to an individual farm.

Agreed names for species and phases helped connect reports that could previously have seemed to concern different insects. Recognition of transitional forms was especially useful because observation need not begin only when a large swarm appeared above a crop. Agreement among specialists nevertheless depended on fieldwork. Populations had to be found, their behaviour described and the date and place retained with the record. The connection between classification and usable evidence had already been explicit in the problem posed at the opening of Uvarov’s 1921 paper.

A field report became evidence for a forecast

FAO’s historical manuals of 2001 show the development of a detailed observational practice. A record included coordinates or a map position, date, soil moisture, vegetation condition, and the stage, abundance and behaviour of the insects. Foot transects appeared alongside the possibilities of aerial observation. A statement that locusts were present was insufficient: the record had to identify the population, its habitat and the basis on which it had been assessed. Location and timing made a biological description useful to people elsewhere.

A separate manual on information and forecasting described the movement of data through registration, checking, mapping and analysis alongside weather information. Forecasts were then updated, while earlier reports were preserved for comparison. Predicting breeding during a period of low numbers answered different questions from predicting the movement of existing swarms. This was the development of a specific crop-protection practice, combining observations of insects with conditions in the surrounding landscape. The usefulness of a report depended on keeping those pieces of evidence together throughout its journey.

One theory, several species and continuing investigation

The desert locust and the migratory locust, Locusta migratoria, are different species, although both are used to investigate phase-related processes. In 2020 Xiaojiao Guo and colleagues identified 4-vinylanisole as an aggregation pheromone in L. migratoria. Behavioural tests, investigations of olfactory receptors and field experiments contributed to that result. Chemical communication added another mechanism to the history of encounters between individuals. Identifying it did not by itself solve every outbreak or demonstrate identical effects across all locust species.

The desert-locust biology manual distinguished concentration, multiplication and gregarization. It also explained that most grasshoppers do not form typical swarms and that the boundaries between categories are not always sharp. Uvarov’s legacy was a way of asking about a developing process rather than a label for every abundant insect population. In agricultural history, phase theory connected the experience of farmers with testable hypotheses, measurements of behaviour and observations of habitat. It made the route towards a threat an object of investigation before the arrival of an immense cloud of insects.

Questions and answers

Are solitarious and gregarious locusts two species?

No. Phase theory concerns forms within a particular species. Locusta migratoria and Schistocerca gregaria are separate species, each requiring its own investigation.

Can every grasshopper become a swarming locust?

No. Typical phase changes and swarming concern particular species. Most grasshoppers do not form such swarms.

Does behaviour change before appearance?

In desert-locust experiments, behaviour changed rapidly while colour and body characteristics followed different time courses. Appearance alone can therefore give an incomplete picture of phase state.

Sources

History of agriculture: all topics