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close this book Locust handbook
close this folder 2. Desert Locust-Schistocerca gregaria
View the document Anatomy of a locust
View the document Life cycle
View the document Behaviour in relation to habitat
View the document Seasonal movements and breeding areas of desert locust during plagues and recessions
View the document Recession periods, outbreaks and the origin of plagues

Behaviour in relation to habitat

Different species of locusts and grasshoppers behave in different ways. The behaviour of a single species also changes with age and the size and density of the population. Behaviour is affected by a wide range of external factors which characterise the habitat and among which the weather is dominant.

Knowledge of locust behaviour and of the factors which determine it are essential for an efficient locust officer.

In this section the behaviour of the Desert Locust in the hopper and adult stages is described. Particular reference is made to those features of behaviour which are of direct importance in locust control operations.

Hoppers

When hoppers occur in large numbers-as the result of breeding by a swarm or dense concentrations of locusts-they gather together in bands, which move about as distinct units (Fig. 25). The behaviour of the individual hoppers in bands and that of the bands themselves are best considered separately.

Behaviour of individual hoppers in bands

Hoppers have several kinds of general activity. There is a more or less regular pattern of daily behaviour which is summarised in Table 3.

Roosting. At night and in the early morning hoppers will be found roosting {Fig. 26). This means that the hoppers are off the ground, resting on plants, bushes or stones. Roosting also occurs during the middle of the day when the temperature exceeds about 36ºC.

Ground grouping. When hoppers are concentrated in dense groups on the ground and are mainly stationary they are said to be in ground groups (Fig. 27). These are seen in the morning when the hoppers come down from the bushes and again in the evening before they roost for the night. Ground grouping may occasionally occur at other times of day.

Marching. Hoppers usually spend the greater part of the day marching (Fig. 28). This means that they are moving together by either jumping or walking in a definite direction. They stimulate each other and the whole band moves from one place to another. The net distance covered is called the displacement.

Feeding. The main feeding period occurs when the hoppers go up into the bushes for their evening roost, but they also feed when marching, stopping briefly to eat low vegetation in their line of march. These two types of feeding are important to note because the success of certain control operations depends upon them.

Many kinds of plants are eaten by hoppers but they seem to have preferences when there is a choice of plants. Note in your own area those which seem to be specially liked by hoppers, and also record any common plants which you never see them eating. This will be useful information when applying insecticide. On some plants hoppers prefer the flowers or fruit while on others they prefer the leaves and on some they eat both. They have on occasions also been seen eating bark.

The readiness with which a plant is eaten depends on the presence of other food plants at the site. A plant which is readily eaten in one area may be avoided in another because of the presence there of other plants which are even more palatable. Furthermore, the appetite of hoppers for a given species of plant may change with their age and their physiological condition. For instance, as hoppers grow older they may eat tough plants which they avoid when young, and thirsty hoppers may eat fleshy, watery plants which would normally not attract them.

Feeding mainly occurs in the evening but sometimes it is seen on a considerable scale in the middle part of the morning. Very little occurs during the hotter, middle part of the day. Hoppers eat more during the middle part of each instar than at the beginning or the end. Just before moulting they feed either very little or not at all. For the first three moults this non-feeding period lasts about one day, but for the two later moults it may be 2-4 days. This is important when bait or vegetation poisoned by insecticides is being relied on to kill them when they eat it, since it means that at certain times the hoppers will not eat enough to pick up a lethal dose of insecticide. It is therefore necessary to learn to recognise these periods by observation and postpone control work until the appetite of the hoppers returns, or to use persistent insecticides which will remain effective for several days at least.

Daily pattern of behaviour of hoppers in bands. Table 3 shows the daily pattern of behaviour usually seen in the East African region. If you find differences in your own area make notes of them and think about why they are different. Organise your control work to fit in with the hopper behaviour.


Table 3

 

Behaviour of hopper bands

Hopper bands vary in size from a few square metres to several square kilometres, depending upon the number and density of the hoppers they contain (Fig. 29). The number of bands and the proportion of large and small ones depend on the amount and pattern of egg laying in the egg field from which they have arisen. Bands often join together to form larger ones, but sometimes they divide into smaller bands.

Hopper bands tend to occur in groups and this is important from a control point of view. It means that if one band is discovered it is probable that there will be others nearby.

Change in size. The joining and splitting of bands clearly causes changes in the numbers of hoppers in them, and sometimes bands of different ages join together so that hoppers of very different instars occur in the same band.

Hopper bands also change size without any alteration in the number of hoppers that they contain, partly through growth of the hoppers and partly because they are sometimes close together while at other times they are further apart.

Daily change in size due to changing behaviour. Hopper bands change their size during the day according to the behaviour of the hoppers. The size of a band when it is marching can be up to eight times that of the same band when it is roosting or in ground groups. The reduction in size at roosting time tends to be greater in an area with shrubby vegetation.

Change in size due to change in instar. Hopper bands get bigger as the hoppers in them grow older. A band of fifth-instar hoppers may be 20-30 times as large as a first-instar band containing the same number of hoppers. This means that bands are more concentrated in the early instars and are therefore easier and more economical to destroy.

Change in size due to fusion or splitting. Hopper bands may get larger owing to the fusion of two or more bands. Sometimes, however, bands split up and this is most likely to occur after moulting.

Movement of hopper bands

Speed and distance. Large bands move quicker and farther than small bands of the same instar. The thicker the vegetation the more slowly will bands move through it, and where there is much bare ground bands will move more quickly and therefore farther each day. Sunny weather, provided it does not get too hot, i.e. above 36ºC air temperature, favours marching, so that bands move quicker and farther each day under sunny conditions than in cloudy overcast weather.

Marching speeds of individual young hoppers may be as high or higher than those of old hoppers, but in general, older bands move faster and further. This is because by the time the fifth instar is reached in the same area the weather is usually sunnier and the vegetation has become drier and thinned out, and these conditions induce more rapid marching.

The following observations on the daily movement of hopper bands of various sizes and instars were made in East Africa.

TABLE 4

Size of band (night roosting area in square yards)

Instar single day (yards)

Displacement in a

282

1

267

150,000 (12.5 ha)

1

96 (88 m)

100

2

56

100

2

84

220

2

330

160,000 (13 ha}

2

579 1530 m)

70,000

3

274

165,000 (14 ha}

3

916 (838 ml

7400

5

382

9800

5

917

15,600

5

1216

One mile wide (260 ha)

5

1 mile (1610 m)

 

Direction of movement. Hopper bands often move in the same direction for several days at a time and sometimes in roughly the same direction throughout hopper life. They have often been observed moving generally downwind.

Very often all the bands in a quite large area move in the same general direction. This kind of behaviour is more likely in flat unbroken country. In hilly country and country broken up with many water channels there is likely to be less constant direction of movement. Sometimes hoppers move in long narrow bands along dry water channels and sunken roads. This is known as canalisation.

Shape of hopper bands

There is no regular shape. The shape depends on the behaviour of the hoppers and the type of country. Nearly always when the band is marching the hoppers are most dense at the leading edge, which is known as the front (Fig. 30). In large bands there is a high density for about 150 m back from the front; the density then decreases until at the trailing edge of the band the hoppers are very scattered.

It is useful to find the fronts of hopper bands when controlling them. If the leaders of each band stop to feed on bait or sprayed vegetation the ones following behind them are likely to do the same. To find the front, drive or walk in the direction in which most of the hoppers appear to be marching.

It is possible with experience to recognise the tracks of hoppers where they have recently marched over loose sand. This is quite useful in finding bands which might otherwise be missed. Hoppers may not be sighted but the tracks will indicate that they have passed by on that day or on the previous day.

Adults

Flying capabilities

One of the most striking features of the Desert Locust is its great mobility. Experiments have shown that locusts suspended in a windtunnel can go on flapping their wings non-stop for 6-17 h and there is evidence that the maximum period may be as long as 20 h. By gliding some of the time, locusts can stay in the air very much longer. This allows them to make long journeys over the sea. For example, locusts regularly cross the Red Sea, a distance of 300 km, and in October 1988, a large number crossed the Atlantic Ocean to the West Indies-a distance of 5000 km!

The muscles of a flying locust perform 10-20 times as much work in proportion to body size as those of a human being working at top speed. The energy needed is derived from fat stored inside the body, so that the flight endurance depends on the amount of fat present. The fat content of Desert Locusts, according to age, is shown in Table 5.

TABLE 5

Age

Total weight (g)

Weight of fat (mg)

Newly fledged

1-1.5

10-15

Second week after fledging

1.7-2

40-60

4-5 weeks after fledging

2-2.5

250-300

Very old locust about to die

Same as fledglings

 

The longest flights are made by immature locusts which do most feeding and store most fat. The Desert Locust has a flying speed of 16-19 km/in but the rate at which it moves relative to the ground depends on the wind as well as its own flying speed.

Locusts are cold-blooded animals and their activity is affected by air temperature and sunshine. If there is no sunshine long continuous flight usually takes place only above 23ºC. Rain and cloud generally decrease the amount of flying. In sunshine long continuous flight is possible when air temperatures are above 14ºC but flight decreases when the air becomes hotter than 40ºC. Locusts take off and land into the wind, but in very strong winds they do not take off at all but shelter behind rocks or vegetation.

Adults in swarms

The size of Desert Locust swarms ranges from less than one square kilometre to several hundred square kilometres. There are about 50 million locusts in each square kilometre of a medium-density swarm. The total number of locusts in a swarm varies from a few hundred millions to tens of thousands of millions. The volume density varies from one locust per thousand cubic metres to 10 locusts per cubic metre.

The shape of flying swarms can be stratiform or cumuliform (Fig. 31).


Fig. 31. Shapes of flying swarms of Desert Locusts.

Large swarms generally fly higher than small ones. According to the weather conditions it experiences, a swarm may appear at one time as a very large single swarm, and perhaps a few days later as several small swarms with scattered locusts between them.

Daily pattern of behaviour of swarming locusts

Desert Locusts in swarms fly by day and settle on vegetation at night, although they have sometimes been seen flying after dark. It is not easy to say exactly what their behaviour will be at any time as this depends on the weather, the type of country they are in and the state of the locusts themselves, but the general daily behaviour pattern for immature (pink) swarms in tropical Africa or Asia is as shown in Table 6.

TABLE 6

Time

Behaviour

Night (1 h after sunset to sunrise)

Settled on vegetation; the place where the swarm settles is called the roosting site.

Sunrise +0.5 h

Crawling about slowly on vegetation. A few locusts may jump or fly about on disturbance. During the next hour or so more locusts make short flights and many come to the ground.

Sunrise +2 h (about 0800 h)

Dense carpets of locusts now on the ground lying broadside on to the sun's rays to absorb radiant heat. Some still basking on trees and some flying between groups. During the next hour the amount of local flying increases with streams of locusts flying in different directions while still remaining within the area of the roosting site.

0900 to 1000 h

Majority of locusts become airborne. As the air temperature rises and convection starts the locusts rise higher into the air and the swarm begins to leave the roosting site; this is usually referred to as mass departure.

1000 h to sunset or just

Swarm in flight as a whole, but there are nearly always many locusts temporarily settling beneath after it the swarm, particularly at the leading edge; near the rear edge these take off again and follow the rest of the swarm. Thus the swarm moves in a rolling manner.

Sunset or soon afterwards

Settling in progress, very often followed by heavy feeding.

 

A locust swarm consists of streams of locusts. Figure 32 shows that all the locusts in a stream face in the same direction (or orientation). There will be other streams in the same swarm facing in many other directions.

Any locusts which fly or are blown outside the edge of the swarm turn back into it and as this is taking place all round the edge of a swarm it means that the different directions of flight by the various streams do not affect the direction of movement of the swarm as a whole. The net distance traversed by the swarm as a whole is called the displacement. The presence of streams of locusts flying in all directions within a swarm means the direction of movement of the swarm as a whole cannot be judged merely by watching which way individual locusts are facing or moving, or noting which way the stream which happens to be above at the time appears to be moving.

The direction of movement of a whole swarm can be judged roughly from the ground by seeing where it is centred at different times in relation to landmarks. A much better way of finding out which way it is moving is by obtaining successive fixes of its position from an aircraft.

Non-swarming populations

Solitarious Desert Locusts living at low densities start flying after dusk on warm evenings and continue flying during the early part of the night. Large-scale night flight normally requires a temperature of 23ºC or more at take-off. Some solitarious locusts seem to fly frequently, but others hardly at all. There is evidence that long-distance migrations, rather similar to those of swarms, can occur but as they occur at night, unlike those of swarms, they are not seen, and thus can lead to surprise infestations of locusts in areas previously clear. During the daytime solitarious locusts will fly only when they are disturbed, or flushed. When they are flushed they usually fly low and settle quickly. Sometimes they rise almost vertically, and then drop quickly like a stone, or they may be carried out of sight by the wind.

Direction of swarm displacement in relation to meteorology

By using aircraft to find the direction of displacement of individual swarms and knowing the wind direction at the same time it has been shown that swarms move in a downwind direction (Figs 33 and 34). The steadier the wind direction the steadier will be the direction taken by the swarms. In unsettled weather or in mountainous country with frequent changes of wind, swarm movement will be more irregular. Steady flight against very light winds has often been seen in mature swarmlets flying only a few feet above the ground. These observations have led people to believe that in these cases whole swarms are moving upwind, for short distances at least. No satisfactory proof of swarm displacement upwind has so far been provided. The speed at which swarms move varies greatly. Rates of movement measured have ranged from 1.5-16 km/in and swarms have been known to travel a few kilometres to over 100 km in a day and as much as 3500 km in a month. The speed at which swarms travel is often considerably less than the wind speed because all the locusts in a swarm are not flying all the time.

 


Fig. 33. Direction and speed of displacement of individual Desert Locust swarms in relation to the wind: eastern Africa 1951-1957. Out of the 42 best documented observations 26 swarm displacements were within 10º of downwind. (Rainey, R.C. 11963). Meteorology and the migration of Desert Locusts. Anti-Locust Memoir No. 7: 115pp ).

 


Fig. 34. Swarm displacements in a quasi-uniform wind field in Kenya, January 1954, during a period of predominantly northeasterly winds. During periods of effectively constant wind direction, swarms displace progressively and systematically downwind with a constancy exceeding 75%, and make displacements of 5-130 km a day (Rainey, R.C. (1963). Meteorology and the migration of Desert Locusts. Anti-Locust Memoir No. 7: 115pp.)

 

Since swarms travel consistently downwind they eventually arrive at a place where winds meet or, in other words, where masses of air converge. These places of convergence can be recognised on synoptic weather maps which can be used to find out where swarms are likely to be found. A synoptic map shows the weather over a given area at a given time, it can be used to estimate the wind and temperature at any place in the area for that time. Winds and temperature vary with height above the ground but information of most use to a local control organisation can be found on synoptic maps prepared for the surface (nominally 10 m above ground) 850 and 700 millibars (about 1.5 and 3 km above sea level). Cooperation between a country's locust control organisation and its meteorological service is therefore highly desirable. Converging air masses produce certain recognisable weather systems in different parts of the world at different times of the year and it has been possible to relate the position of locust swarms to these. For instance in the summer months Desert Locust swarms tend to congregate along the belt where the northerly and southerly winds meet.


Fig. 35. Spring breeding areas of the Desert Locust.


Fig. 36. Summer breeding areas of the Desert Locust.


Fig. 37. Winter breeding areas of the Desert Locust.


Fig. 38. Occurrence of the Desert Locust during recessions, February to June, from 1920 to 1948 and from 1963 to 1976. Spring generation hoppers.


Fig. 39. Occurrence of the Desert Locust during recessions, July to October, from 1920 to 1948 and from 1963 to 1976. Summer generation hoppers.


Fig. 40. Occurrence of the Desert Locust during recessions, November to January, from 1920 to 1948 and from 1963 to 1976. Winter generation hoppers.

This is called the inter-Tropical Convergence Zone (ITCZ) or sometimes the Inter-Tropical Front. From late September onwards swarms generally move northwards out of the ITCZ into North Africa and the Middle East, often during periods of warm southerly winds associated with the passage eastwards of depressions through the Mediterranean and the Near East. In East Africa swarms follow the seasonal displacement of the ITCZ southwards across the Somali peninsula between late September and February.

Areas of convergence of air masses are the areas where rain is most likely to fall. An important result of the way swarms move is that they eventually come into places where there is rain. This is essential to the survival of the Desert Locust, for it can only breed successfully where there is moist sold in which eggs can be laid and develop and suitable plants will grow and provide food for the young hoppers. Sometimes the rainfall is too little and this leads to heavy mortality in locust populations.

At present weather stations in the Desert Locust area are few and far between, so there is room for much improvement in the weather maps for some parts of the area. As weather information improves, it should be possible to have a fuller understanding of swarm movements and to forecast them better. This improvement will occur with the widespread use of information from weather satellites. The cloud pictures now available from geostationary satellites show the growth of rainstorms throughout the day and can be used with surface observations of clouds and rain to determine the time and extent of rainfall. Pictures from both geostationary and polar-orbiting satellites are particularly useful in helping to estimate possible rains in remote places.

Anti-locust organisations themselves should do all they can to provide weather information from places where there are no permanent weather stations.