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More about locusts

WHAT ARE LOCUSTS


Locusts and grasshoppers are acridids, i.e. short-horned insects belonging to a Family called Acrididae in the Orthoptera Order, which also gathers crickets and long-horned grasshoppers. Insects belonging to the Orthoptera Order can be readily identified by their large hind legs which enable them to jump.

                        

Short-horned grasshoppers                             Long-horned grasshoppers                                    Crickets


Locusts differ from grasshoppers by their capacity to react to high densities. Under crowded conditions, their behaviour, morphology, appearance, physiology, habits and ecology change progressively (over several generations), a shift known as phase change. When phase change occurs from a solitary to a gregarious phase, locusts behave no more individually but form eventually dense bands of hoppers and swarms of adults.

This phase change occurs after suitable ecological conditions (sufficient vegetation for food and roosting, appropriate soil moisture for laying) have allowed successful breeding generations, low natural mortality and number increase.


The modifications accompanying the phase change are as follows:

  • Behavior

In solitary phase (low numbers and densities), locusts behave as individuals, much like grasshoppers. In gregarious phase, they form dense and highly mobile (marching) bands of hoppers and flying swarms of adults (winged locusts), which behave as an entity.

  • Biology

This gregarious behavior is reinforced by a synchronization of the biological events: mating, egg-laying, hatching, fledging. Thus, eggs hatch at the same moment from dense egg-beds and the newly appeared hoppers form immediately primary bands; after fledging, the immature adults will form swarms.

  • Morphology

Some modifications in morphology, which come together with phase change, provide locusts with a better capacity to fly over long distances (better ration between wing surface and body weight). There can be other changes in shape (pronotum for instance) and in colour (general body pattern). In the solitary phase, locusts exhibit sexual dimorphism with females being larger in size than males; the differences in size between the sexes become less pronounced and may sometimes disappear in the gregarious phase.

  

Migratory Locust: Difference in the shape of the adult pronotum between solitary (a) and gregarious (b) phase.


Desert Locust: Difference in the colour of solitary (green) and gregarious (black with orange spots) hoppers.

  • Physiology

Generally, gregarious females lay fewer eggs than the solitary ones but these eggs are bigger and more resistant.

  • Habits

Gregarious locusts are able to change their ecological and food habits and therefore to settle and breed in a wide range of habitats. A direct consequence concerns the distribution areas with the invasion one (gregarious populations) much bigger than the recession one (solitary populations).
The most famous example of such capacity to migrate and settle a wide range of habitats is given by the Desert Locust [Schistocerca gregaria (Forskål 1775)], which is present in about 30 countries during recession periods (about 16 million square kilometers) and can extend over or into parts of 60 countries during plagues (about 29 million square kilometers), as indicated on the map.

LOCUSTS IN CAUCASUS AND CENTRAL ASIA


An important specificity of these 3 locusts in the considered area (CCA) is that they are univoltine, i.e. they have only one generation per year.


As for other locusts and grasshoppers, there are three successive development stages: egg, nymph and adult. The nymph (or hopper) stage can be further divided into growth stages called instars. The eggs are almost always laid in the ground; after hatching, the nymphs (locusts without wings) develop into instars separate by moults and eventually become immature adults after fledging (the last moult). As an example, the following diagram shows the life cycle of the Australian Plague Locust [Chortoicetes terminifera (Walker 1870)] which has five instars during its nymph stage. In such diagrams, the times provided for development correspond to optimum conditions and are only approximate. 

 

Life cycle of the Australian Plague Locust
From Department of Agriculture, Fisheries and Forestry, Australian Government

In CCA, the unique generation develops in spring and summer, from egg hatching (in early spring) to last egg-laying and death of adults, in late summer. The eggs over-winter in the ground. Consequently, the three species are present and likely to provoke infestations at about the same period, which can complicate implementation of monitoring and control operations because their habitats rarely coincide.

When in solitary phase, the three species have preferred traditional habitats: CIT can be considered as the locust of fallows and waste lands and DMA of semi-arid steppe and semi-desert while LMI is mainly found on river and lake banks. This has obvious consequences on their diet. When they are in gregarious phase, the three species can eat a wide range of natural and cultivated plants and become more polyphagous; their hopper bands and adult swarms are therefore devastating for agriculture and forestry. 

Specific data sheets are available for the three main locust species present in Caucasus and Central Asia (CCA): the Italian Locust Calliptamus italicus (Linnaeus 1758) -CIT, the Moroccan Locust Dociostaurus maroccanus (Thunberg 1815) -DMA, and the Migratory Locust Locusta migratoria migratoria (Linnaeus 1758) -LMI.