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close this book Locust handbook
close this folder 3. Other African locusts
View the document African migratory locust-Locusts migratoria migratorioides
View the document Other subspecies of Locusta migratoria
View the document Red locust-Nomadacris septemfasciata
View the document Brown locust-Locustana pardalina
View the document Tree locusts-Anacridium melanorhodon melanorhodon, Anacridium melanorhodon arabafrum, Anacridium wernerellum, Anacridium aegyptium

African migratory locust-Locusts migratoria migratorioides

The distribution of this locust is shown in Fig. 100. Its main breeding area is in the flood plains of the Middle Niger in West Africa and it is the escape of swarms from this outbreak area that are known to lead to plagues. Elsewhere throughout the distribution area populations of solitarious phase Migratory Locust may be found. These solitary populations are generally regarded as residual populations left over after the dying out of plagues. There are other areas, such as parts of Sudan, Ethiopia and Angola, where swarms of Locusta have formed, but none are as meteorologically favourable as the Niger flood plains and none has yet led to the start of a plague. It is considered that Locusta is actually indigenous in the Lake Chad basin.


Fig. 100. The distribution of the African Migratory Locust.

 

Life cycle

The life cycle of the Migratory Locust is similar to that of the Desert Locust. The time spent in each stage varies from season to season in different parts of the distribution area according to environmental factors; temperature appears to be the most important factor. There can be 4-5 generations per year in the Niger delta but this falls to three in the Chad basin area and to between one and two generations in South Africa. Elsewhere in the invasion area there are two generations per year.

Immature adults

Solitarious adults differ from gregarious ones in shape and colour. They have an arched pronotum and are green or brown in colour. The gregarious adults have a much flatter pronotum and are a yellowish-brown or greyish colour with darker markings.

Maturation. The first signs of egg development in solitaries usually appear a few days after fledging and males can copulate four days after fledging. In the laboratory solitarious females start to lay about 10 days, and gregarious females 2.5-3 weeks, after fledging. Egg laying may start as soon as this in the field, but much more information is needed. It is known, however, that there is no delay in maturation even though laying does not always occur as the eggs can be resorbed. Resorption means that yolk and other material already deposited in the developing eggs are removed from these eggs which, therefore, get smaller and typically become red and are known as corps rouges. In the African Migratory Locust the presence of a double row of red spots in the ovaries (they are very small and it is best to use a hand lens when looking for them) usually means that the female has resorbed her eggs.

Resorption can be caused by low temperatures and also occasionally by a lack of food or prolonged flight. Corpora lutea are also found in the ovaries. They are another kind of resorption body and their presence indicates that the female has laid. They too are sometimes reddish but they are usually colourless or yellow; they are more conspicuous in the Desert Locust than in the African Migratory Locust.

Mature adults

As solitarious locusts mature they become somewhat darker, particularly the abdomen of the females. In swarms the general colour becomes duller and the males become partly yellow (Plate 4).

Egg laying. Egg laying usually takes place in soil which has been moistened either by rain or by recent flooding but can also occur in dry soil. Reports during the last plague indicate that typical laying sites for swarms include burnt patches of grassland with a flush of new green grass, open areas near swamps, cleared farmland or bare areas near villages where cattle have been herded.

After the eggs are laid and the female has withdrawn her abdomen she scrapes the soil over the hole and pats the surface. The Desert Locust has never been seen to do this.

The eggs are roughly similar to those of the Desert Locust but their arrangement within the pod is different in that they are arranged in rows (compare Fig. 15 with Fig. 101). The eggs are surrounded by a thin but tough film of froth to which sand particles adhere.

The number of pods laid by a single female varies considerably as does the interval between successive layings. During the winter months in the outbreak area locusts have been found to lay 1-3 pods each at intervals of about 20 days, while in the summer they may lay 2-4 egg pods each at intervals of only 3-4 days. Solitary locusts lay about 65 eggs (range 551101 per pod which falls to 39 eggs per pod for gregarious females.

Incubation period

For successful development Locusta eggs must absorb water from the soil in the early stages, and provided there is adequate water in the soil the incubation period is dependent upon temperature. In the outbreak area of the Niger flood plains the incubation period is generally 10-20 days in the summer and 20-40 days in the winter. Eggs laid in dry soil, or in soil that dried out rapidly after laying, may take over 100 days to hatch.

Hoppers

An important aspect in which the life cycle of the African Migratory Locust differs from that of the Desert Locust is in the number of instars. In both phases of the African Migratory Locust there are normally five instars but in the solitarious phase under very dry conditions there may be six or even seven instars.

The duration of each instar is, as in the Desert Locust, dependent on environmental factors of which the most important is temperature. In the laboratory hoppers developed faster in humid conditions (38 days) than in dry conditions (57 days). Records from the last plague in both East and West Africa show that the whole hopper period lasts 30-60 days for the five instars. In the Middle Niger area hopper development lasts only 24-35 days.

Gregarious Migratory Locust hoppers have a very distinct brown and black colour pattern. Solitarious hoppers are grey in the first instar but the colour may vary in later instars: there can then be a very wide range of colours including green, grey, buff, brown, red and black, often resembling the background colour of their habitat, e.g. black hoppers on burnt grassland, and various shades of green according to the tone of the vegetation.

Table 12 gives an indication of the size and weight of the various hopper instars.

TABLE 12

 

Average length

Average weight

 

(mm)

(mg)

First instar

7-8

14

Second instar

15

33-38

Third instar

19-20

190-220

Fourth instar

26-27

430-500

Fifth instar

34-37

1000-1220

 

Behaviour

The behaviour of the African Migratory Locust is similar in many respects to that of the Desert Locust, described above. In order to avoid undue repetition the emphasis in this chapter is therefore upon the differences in behaviour between the two species.

As the last plague of the African Migratory Locust ended in 1941 most present-day observers have seen neither swarms nor hopper bands produced by swarm laying. The only hopper bands seen since the last plague have been produced as a result of local concentration, multiplication and gregarisation, but not on a scale sufficient to initiate a new plague.

Hoppers

Migratory Locust hopper bands are sometimes much denser than those of the Desert Locust. For example, there are records of hopper bands piling up to 15 cm deep. During the first instar the hoppers tend to form very dense ground groups or small bands on bare patches of soil or on stones. In the later instars bands march and the daily pattern of behaviour is similar to that of the Desert Locust.

Hopper bands tend to march downwind, but canalisation along gullies, tracks etc. is common. Bands, particularly large ones, often cross obstacles such as stones and the same direction of march may be maintained for long periods. There are records of bands marching 24 km during the whole period of hopper life.

Hopper bands can maintain cohesion in the very dense grasslands of the flood plain but move less under these circumstances, or when there is no sun or temperatures are low.

Adults

Swarms of the African Migratory Locust can be very dense and the densest locust swarm ever recorded photographically was a Migratory Locust swarm in West Africa during the last plague (Fig. 102).

The daily pattern of behaviour is similar to that of the Desert Locust as described on page 31 except that swarms often settle at mid-day but feed little. The main feeding occurs in the evening when they settle for the night roost. Migratory Locusts mainly eat grasses or low vegetation and cereal crops are often damaged.

Swarms generally fly in stratiform shape and tend to avoid obstacles such as prominent topographic features or lines of trees. Swarms of African Migratory Locusts seem to be somewhat less active than those of the Desert Locust, perhaps because they tend to fly close to the ground, but large-scale migrations do occur and are described below.

Seasonal migrations of swarms

While the details of swarm movement during the last plague have not yet been fully worked out it appears that, in general, swarms of the African Migratory Locust, like those of the Desert Locust, move downwind.

The spread of the last Migratory Locust plague, which is illustrated in Fig. 103 shows the progressive increase in the area invaded by swarms and Fig. 104 shows the number of months of infestation by swarms and hopper bands in each affected country expressed as a percentage of the 157-month period (June 1928-June 1941) for which the plague lasted. By mid-1932 swarms occurred nearly 7000 km from the outbreak area in Mali. This plague clearly spread in an easterly and then southerly direction over a number of generations. This progressive type of spread can be contrasted with the seasonal north-south migration of the Desert Locust in West Africa, northward after the summer breeding and southward after the following spring breeding, and similarly back and forth between the summer and spring breeding areas of the Eastern Region (Figs 35-37).


Fig. 103. The initial spread of swarms of the African Migratory Locust during the 1928-1941 plague.


Fig. 104. The number of months of infestation by the African Migratory Locust in each country expressed as a percentage of the 157-month period of the 1928-1941 plague.

In addition to the eastward and southward spread in Africa there are regular small-scale migrations in areas such as Nigeria where swarms were reported every month between December 1929 and January 1940. During the winter months from December to February and March swarms and hopper bands were found in the southern part of the country whilst in the summer months from July to September they occurred in the north. It would appear that swarms move northwards on southwesterly winds with the northward extent of the ITCZ from February to September. There is then a corresponding southward displacement of swarms as northerly winds spread over Nigeria following the ITCZ as it retreats towards the south from October to December.

In the event of another plague arising in Mali it is not possible to say whether it would develop in exactly the same way.

Seasonal breeding and migrations of solitarious African Migratory Locusts

Solitarious African Migratory Locusts are found over many widely separated areas of Africa but this account is limited to the regular seasonal breeding and migrations in and around the main outbreak area in Mali. The map of the outbreak area (Fig. 105) shows the flood plains which are subject to seasonal inundation and in which the vegetation consists of various types of grassland, and the surrounding semi-arid areas of Sahelian vegetation consisting of wooded steppe and woodland.

Rain in this outbreak area normally falls between June and September but the rains may begin as early as May and continue into October. The River Niger begins to rise because of the rain which falls in the highlands to the southwest, and each year it floods the surrounding country. The first areas to flood are in the south in August and the floods generally do not reach the north until November. The floods begin to recede in October at the southern end of the plains but last until February or even March in the north.

This flooding of the outbreak area enables locusts to breed there not only in the rainy season from June to September but also during the period October to March or April as the floods recede, whereas in the surrounding areas of the Sahel breeding is limited to the rainy season alone. In the flood plains (Fig. 106), where breeding can occur throughout the year, there is some overlap between generations but it is possible to recognise a general seasonal pattern of breeding and migration (Fig. 107).

At the time of the high floods in August and September most of the adults leave the flood plain for the surrounding Sahelian areas where they breed and fledging of the Sahelian generation begins in September. (It should be noted that the Sahelian generation can also be referred to as the last rains generation.)

 


Fig. 105. The Niger outbreak area of the African Migratory Locust.


Fig. 107. Seasonal breeding cycle of the African Migratory Locust in the outbreak area.

Adults of the new generation then begin to migrate from the Sahel back to the flood plains and to concentrate in those areas most recently exposed by the retreating flood waters. The locusts move gradually northwards behind the retreating floods and lay in their wake, thus producing the first retreating flood generation during the period October to January. During January to April some locusts breed again and a last retreating flood generation is produced in the northern plains by locusts resulting from earlier breeding in the south.

By the time the rains begin in May or June the floods have disappeared and a first rains generation results. Fledging occurs in July and August. A second rains generation occurs in the northern plains, laying begins in June and there is fledging from August onwards. Adults from both generations disperse widely in the surrounding Sahel to breed and produce the last rains generation-a total of five generations during the year (Fig. 107). If the annual rains are late or finish early there will probably be only two generations (annual total four).

Habitats of solitarious African Migratory Locusts

Solitarious African Migratory Locusts have been found in widely separated parts of Africa but the following account of their habitats is restricted to the main outbreak areas in and around the flood plains of the Middle Niger in Mali. The vegetation of the flood plains is mainly controlled by the rise and fall of the floods, the lower areas being under water the longest. Over the whole flood plain the soil generally consists of a silty clay with more clay in the lower areas, but sandy patches are widespread.

The higher levels (those flooded for the shortest time)

In this zone the vegetation consists of very dense grasses, up to 2 m high. The main species are Andropogon gayanus and Hyperthelia dissolute. Owing to the extremely dense nature of the vegetation this zone is not a good habitat for the African Migratory Locust and it is seldom found there.

In addition to the natural vegetation there are also cultivations of rice, and no locusts are found in these, but in the northern part of the flood plain, where the last flooding occurs, crops of sorghum can be grown and harvested on the rains before the area is flooded. These cultivations form a good habitat for locusts breeding during the rains.

The intermediate levels

The vegetation in this zone also consists of grasses, forming a mixture of dense stands and areas of more open patchy vegetation. The main grasses in the dense stands are Andropogon africanus and Vetiveria nigritana. In the more open areas the principal species are Eragrostis atrovirens, and Panicum anabaptistum and P. fluviicola.

The more open areas constitute the better locust habitat and the scattered patches of bare soil provide good laying sites. These sites are particularly suitable for laying when locusts which bred in the drier Sahel return in November and give rise to the first retreating flood generation. Parts of this intermediate zone are also used for the growing of rice but locusts are not found in the rice-growing areas.

The lower levels (those flooded for the longest time)

This zone is dominated by Echinochloa stagnina and Oryza barthli. These grasses give rise to a mass of floating vegetation which forms a mat on the ground when the floods recede. This zone is a poor habitat for the African Migratory Locust, both when the vegetation is floating and when it is dry. All three zones described above are burnt in the dry season between February and May, after which time the locusts concentrate in the fresh regrowth, particularly in the middle zone.

The Sahel

Part of the locust population flies out of the flood plain in August and September and breeds in the surrounding more arid areas of the Sahel.

The areas of principal interest lie to the east and west of the flood plain between 15ºN and 17ºN. Further north there are semi-desert areas and to the south there is woodland vegetation. The vegetation of the Sahel, which consists of wooded steppe, is dependent upon rainfall, unlike that of the flood plains where the rise and fall of the flood is more important. In the Sahel the soils vary from silty sands on fixed dunes to clay-filled depressions, and in some areas the underlying laterite layer is exposed.

The southern Sahel. The natural vegetation in this area consists of quite dense low woodland in which Pterocarpus spp. and Acacia spp. are the main trees. Seasonally waterlogged clay-filled depressions support dense Pterocarpus woodland to a height of 5-8 m. Clearings have been made in the natural woodland for the cultivation of bulrush millet (Pennisetum americanum). From time to time the fields are abandoned and annual grasses soon begin to colonise the bare ground. The main species are Cenchrus biflorus and Eragrostis tremula and when the fields are left longer Diheteropogon hagrupae becomes important together with the perennial, Andropogon gayanus.

The northern Sahel. The northern part of the Sahelian zone is drier and the tree cover is lower and less dense. There are numerous fixed dunes and undulating country is common. in this region there are natural open areas as well as man-made clearings for growing millet. The commonest trees in this area are Acacia spp., Balanites aegyptiaca and Euphorbia balsamifera. When the fields are abandoned annual grasses soon appear; as in the south these include Cenchrus biflorus and Eragrostis tremula, but in this drier zone Aristida spp. are also important.

When locusts invade the Sahel from the flood plains the most important habitats are the man-made clearings. Judging from the numbers of locusts found in both the northern and southern Sahel, the fields in current use and those which have been recently abandoned seem to provide equally good habitats and in the northern Sahel man-made clearings are more favoured than those which occur naturally.

Recession periods and outbreaks

The African Migratory Locust has been in recession since 1941 when the last swarms of the plague died out in Chad, Sudan and Zambia. This recession period has been far longer than any known for either the Desert Locust or the Red Locust and historical evidence suggests that there have only been two plagues this century (1891-1903 and 1928-1941). Hopper bands have, however, formed in 16 years between 1941 and 1971 in the Middle Niger outbreak area since the end of the plague. The most important upsurge in the main outbreak area occurred in 1951 when over 17,000 bands were chemically controlled and a possible new plague was prevented.

The last plague was traced to the flood plains of the Middle Niger in Mali from where swarms left at the start of the rainy season in 1928. Hopper bands occur more frequently during the cool dry season as the floods recede and are rarely found during the rainy season. They appear to result from the breeding of concentrations of parent locusts which have appeared in larger numbers than usual due to increased multiplication during the preceding rainy season. Such high rates of multiplication are associated with above-average rainfall and were a feature of both 1927 (the year preceding the plague) and 1950 (the year preceding the upsurge). Hopper bands produced on the retreating floods rarely give rise to swarms because they are too widely scattered and the fledglings disperse. Such fledgling populations are, however, sometimes larger than usual due to increased survival of gregarious individuals compared with solitarious ones. If they persist until the following rainy season, and if breeding is successful and multiplication occurs on a large scale, they may be potentially dangerous. This is again associated with above-average rainfall during the early part of the rainy season and occurred in 1928 (the start of the plague), 1951 (the year of the upsurge) and 1968 (a recent upsurge).

The distribution of the egg pods appears to be the primary factor involved in the formation of hopper bands because gregarisation mainly occurs during the first instar. The concentration of solitary hoppers or adults does not normally lead to any gregarious interaction, as it does in some other species of locust, unless densities are extremely high.

The processes involved in an outbreak are basically similar to those already described for the Desert Locust. However, because the African Migratory Locust has a relatively fixed annual cycle where a period of dispersal and population increase (rainy season) alternates with a period of concentration and decrease (retreating floods season), the processes of concentration, multiplication and gregarisation are not so much concurrent as consecutive events and are restricted to certain breeding seasons. They tend to lead to outbreaks at the start of the rainy season when conditions are most favourable for the spread of swarms.

Although the origin of the last plague has been traced to the main outbreak area in Mali there have been reports from other widely separated parts of Africa of Migratory Locusts multiplying to form large hopper bands and small swarms.

These small outbreaks have been reported from around Lake Chad, on the Red Sea coastal plains of Ethiopia and Sudan, the Gash, Gedar and Gezira areas of Sudan, the northern coastal plains of Somalia, Hippo Valley in Zimbabwe, northwest Angola and the Lake Ngami region of Botswana. Some of these populations have been large enough to require control measures; whether or not outbreaks in such areas could start a plague is not known. If resident solitarious populations of the African Migratory Locust become established in areas of new agricultural development in countries such as Sudan and Zimbabwe, there is always the possibility that under suitable conditions they may concentrate, multiply and gregarise and so create migratory swarms. It must be borne in mind that areas which have so far only produced unimportant outbreaks may be capable of initiating a new plague. It is therefore essential that a watch be kept on the establishment and growth of solitarious populations of the African Migratory Locust in these suspect areas.