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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

3. Other African locusts

There are three other plague species of locust in Africa south of the Sahara: the African Migratory Locust, the Red Locust and the Brown Locust. All these species have one important difference in common from the Desert Locust. Whereas the Desert Locust can form large populations leading to plagues in several geographically separate parts of its distribution, the other African species have been shown to have more restricted outbreak areas. This knowledge has been used to prevent further plagues in the first two species by siting control organisations in or near the outbreak areas. These organisations have kept populations small and restricted. Figure 7 compares the annual fluctuations in the number of countries infested from 18871 970.

While chemical control of the Brown Locust has not prevented swarming or reduced the frequency of outbreaks it has, nevertheless, reduced the area subject to invasion and breeding and the intensity of swarming and crop damage.

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.

 

Other subspecies of Locusta migratoria

Figure 108 shows the widespread distribution of this species. Nine subspecies have so far been distinguished. The Locusta found in Arabia and India is very similar to the African subspecies but forms swarms much less frequently and except for India is a relatively minor pest. In Madagascar Locusta is regarded as the most important agricultural pest, especially of rice and sugarcane. There can be four generations in a year: one in the dry season and three in the rainy season. Development is continuous so that all stages can be found at the same time. Plagues last 1-3 years (last plague was 1960-1961) and follow periods of above average rainfall. Recession periods are much longer. The main outbreak area is in the southwest corner of the island where Locusta is concentrated by the North East Trade Winds into marshy areas suitable for breeding.

Locusta m. manilensis is described in Chapter 5 and the other subspecies are outside the area covered by the handbook.

 


Fig. 108. Distribution of the subspecies of the Migratory Locust.

 

Red locust-Nomadacris septemfasciata

Figure 109 shows the outbreak areas and invasion area of the Red Locust. Small numbers of this locust are present in grasslands up to 2000 m over much of Africa south of the Sahara. It is also found in the Cape Verde Islands, Madagascar, Mauritius and Reunion.

The Red Locust is similar to Cyrtacanthacris tatarica tatarica and Ornithacris spp. Plate 2 shows how they can be distinguished.

Life cycle

The immature adult is a brown colour which gets deeper and redder with age with a distinctive yellow band running along the head, pronotum and central edges of the folded wings. There is only one generation per year unlike the Desert Locust and Migratory Locust. Adults mature and lay at the beginning of the rainy season which for most of the distribution area is in November-December. Females of both phases usually lay twice but the egg pods of solitarious ones contain 20-195 eggs while those of gregarious females contain 20-100 eggs.

The average time taken for the eggs to hatch is 30 days, ranging from 18 days in Mozambique to 54 days in cool areas in South Africa. Hoppers go through 6-8 instars and the average development period is 60 days, ranging from 37 days in Madagascar to 78 days in Natal in South Africa. Solitarious hoppers have more instars than the gregarious ones. Fledging begins in February and continues until May. The adults then remain immature for some six months until the start of the next rainy season. This is true even in areas like southwest Uganda where there is no marked dry season. Breeding can occur all year round but there is still a six-month period as an immature so that there is one generation per year.

North of the equator, Red Locust populations in the Lake Chad Basin and Niger delta flood plains mature and lay between April and August depending on the beginning of the rains. Hoppers are found from mid-July to October.


Fig. 109. The distribution of the Red Locust.

 

Behaviour

Hoppers

The most detailed studies have been carried out in the Rukwa Valley where the locust populations are never wholly solitarious. The habitat is a mixture of tall and short grasses, sedges and patches of open ground. At the end of the dry season most of the grasses are burned so that when the rains start and laying begins large areas are available for oviposition. Laying is restricted in areas of unburned grassland.

Newly hatched hoppers disperse but regroup in the second and third instars. Bands are most likely to be found in the tall grass and mixed short and tall grass habitats. The hoppers like to roost in the tall grass but feed on soft leaves with a high moisture content. Bands do not move very far in dense vegetation but in open country can march up to 700 m/day.

Adults

Adults fledging from hoppers in bands remain in cohesive groups. Swarms are formed by locusts congregating in tall vegetation to roost and by restriction of the habitat during the dry season usually caused by burning of the grassland. Gregarious behaviour is most fully developed in dry years when the vegetation becomes patchy earlier. Flight behaviour changes from wandering to a migratory pattern and swarms leave the outbreak area.

Flight occurs during the day at temperatures above 26ºC; swarms do not travel far, rarely more than 20-30 km/day. The direction of displacement is generally downwind.

The mechanisms leading to migration from the outbreak areas are not fully known. It is unlikely that food availability is important. The level of gregarisation of adults is important as are numbers in excess of 5-10 million locusts in a swarm. The escape of a few swarms from the outbreak areas does not always lead to a plague; these swarms frequently seem to disappear.

Red Locusts in West Africa behave in a broadly similar way. In Madagascar breeding occurs in the extreme southwest of the island from November to March and the ensuing adults disperse northward and northeastward during the dry season from April to October. Swarms form at the end of the dry season during outbreaks; the last one occurred in 19601961.

Plagues, outbreak areas and seasonal movements

There have been three plagues documented since the middle of the nineteenth century, i.e. 1847-1854, 1891-1920 and 1930-1944. Detailed studies of the last plague suggested strongly that they start from restricted outbreak areas. The most important of these are the Mweru wa Ntipa marshes in Zambia and the Rukwa valley in Tanzania. The ideal environment for an outbreak area is either a treeless grassland with poor drainage or seasonally flooded plains and valleys.

Other likely outbreak areas are: the Malagarasi and Wembere plains in central Tanzania; the plains north of Lake Chilwa in southern Malawi; the Busi-Gorongosa plains in Mozambique; the Kafue marshes in Zambia; the Niger flood plain and the Lake Chad basin in West Africa. Swarms have formed in some of these areas but so far none has led to the formation of a widespread plague.

 


Fig. 110. Diagrammatic representation of the main directions of swarm displacements and of swarm retention areas (shaded). In most years of the 1930-1944 plague of Red Locust swarms were displaced in the directions indicated in the months shown. In all years of the plague some swarms persisted in the retention areas throughout the dry season. (Symmons, P. (1964). The dynamics of the most recent plague of the Red Locust, Nomadacris septemfasciata (Serville) with special reference to the importance of climate and weather. Ph.D. thesis, Bristol.)

The last plague began after populations increased over three generations from 1927-1930 probably in the Mweru wa Ntipa and also the Rukwa valley. When swarms left these initial sites they eventually reached two areas: the first, the Shire and lower Zambesi valleys in Malawi and Zimbabwe and the second, an area to the west of Lake Victoria in northwest Tanzania and southwest Uganda (Fig. 1 10). In both these areas swarms successfully survived as immature adults until the next breeding season. Elsewhere either dry season survival or breeding success was low. Figure 1 10 shows the retention areas and the seasonal movements out of them.

The general movements of young immature swarms are westwards from April to June towards Angola and Zaire with some swarms moving northwest towards the retention area in Uganda and others northward along the coastal area of Tanzania to Kenya. By August, when swarms are beginning to mature and the dry season coming to an end, movements are more likely to be southward. Swarms move through Botswana and Zimbabwe to the Transvaal and Natal provinces of South Africa and into Swaziland. These movements continue until October-November and the beginning of the rains.

It is important to note that the movements are general shifts of population and included swarms which had bred in areas along the displacement routes. Further, although there is an apparent 'return' movement of swarms it is thought unlikely that the swarms reach those areas where they or their forbears were produced. Thus many of the swarms died without breeding or reached areas where breeding was not very successful. Similar movements the following year were dependent on new swarms from the retention areas where breeding was successful. Once the main area in Zimbabwe, Mozambique and Malawi became clear of Red Locust in 1943 the plague rapidly declined.


Fig. 111. Frequency of breeding of Red Locust swarms in different areas during the 1927-1945 plague (Morant, V. 11947). Migrations and breeding of the Red Locust (Nomadacris septemfasciata Serville) in Africa, 1927-1945. Anti-Locust Memoir No. 2: 60pp.)

Figure 111 shows the frequency of breeding during the 1927-1945 plague. Although Natal is shown as a high frequency area and was regularly invaded by swarms from the north, the breeding success was very low. There was a virtual absence of any swarms leaving this area. Other movements took swarms to places where breeding was unsuccessful such as the striking northward movements along the Nile to about 20ºN and along the Somali Peninsula to the Gulf of Aden coast. Elsewhere there were occasional invasions of Gabon from northwest Angola and of Namibia from Zimbabwe.

The maximum spread of swarms occurred in 1934-1935 when 7,000,000 km² was infested. Thereafter the plague became more restricted until it declined in 1945. A total of 16 generations was estimated to have been involved.

 

Brown locust-Locustana pardalina


Fig. 112. The distribution of the Brown Locust.

The Brown Locust is illustrated in Plate 4. Figure 112 shows the outbreak area and the invasion area for this locust. Between plagues solitary Brown Locusts can be found throughout the Karoo area of South Africa, some 250,000 km². This is a dry region where the rainfall is erratic and generally less than 300 mm annually. The vegetation is made up of dwarf grasses which are the main food of the locust, and many bare patches which become suitable basking and egg-laying sites.

Life cycle

As its name suggests, the immature adult is brown, but there are occasionally green forms. In seasons with good rainfall there can be three generations but in droughts the eggs can remain dormant for up to 15 months. Laying takes place in well-drained, loose soil and the eggs hatch after 10-20 days; sometimes there is a 1-3 month diapause. Both diapause and non-diapause eggs can be laid in the same pod but generally solitarious locusts are more likely to lay eggs which diapause than gregarious locusts. Females can lay up to five pods but the average is nearer two; each pod contains 10-82 eggs.

Solitary hoppers can be brown, green or grey; gregarious hoppers are black with orange markings in the later instars. There are 4-5 hopper instars for males and five for females. Development periods vary from 21-38 days for solitarious hoppers. Gregarious hoppers take longer, at least 42 days, because they are significantly larger than the other phase. The size difference between the two phases is greater than in any other species.

In the Karoo Brown Locusts overwinter in the egg stage (1-3 months). Hatching begins in September after the first rains and continues in October with fledging in December. In years of good rainfall further laying can occur in January to produce a second generation in March; this generation in turn can lay again, probably in April.

Behaviour

Hoppers

Solitary hoppers move away from other hoppers while gregarious hoppers group as with other locust species. Research has suggested, however, that within a single egg pod hoppers hatch with different behavioural characteristics. For example, reared under similar conditions, hoppers from the top of the egg pod are more likely to have gregarious characteristics than those at the bottom of the egg pod.

Bands of Brown Locust hoppers usually march in very long narrow columns and can cover up to 2 km during their life time, although up to 40 km has been recorded. Densities of hoppers in bands sometimes reach 3000-4000 hoppers/km².

Adults

Gregarious behaviour takes at least two generations to develop. Adults fledging from grouping hoppers fly in loose swarm formation and lay together some 15-50 km from the hatching site. The following generation produces more strongly gregarious characteristics and swarms (up to 43 km²) can fly hundreds of kilometres in a generally downwind direction.

Gregarisation occurs when the numbers of solitary locusts begin to rise over a wide area. As more contacts are made between insects phase transformation begins. This is further encouraged by good rainfall especially if it follows an early summer drought the previous season.

Plagues, upsurges and seasonal movements

Studies of the Brown Locust this century have suggested that periods of 7-11 years of great swarming activity are separated by recessions of similar length.

Neither chemical control nor exceptional periods of rainfall have effectively disrupted this periodicity of swarming and recession. This has led to the suggestion that there are other factors influencing changes in numbers and behaviour, such as an individual locust's response to density. Locusts not sensitive to density will not be active movers and if numbers increase locally these locusts will not move away but become part of a transient population. Such a population will provide a target for predators and will be reduced. Locusts sensitive to density will initially move away from contact with other insects. They will thus be difficult to find and will not be a prey to natural predators. Eventually the proportion of these adults in the total population will predominate. When overall numbers reach a critical level phase transformation takes place quickly and highly mobile swarms are formed. At the end of an active swarming phase it is thought that the density-sensitive adults are killed or die out leaving the remaining solitary population with a majority of locusts not sensitive to density. There must then be a period of build-up of locusts which are density sensitive before there will be another swarming period.

When swarms do form they can be displaced by the winds northward, northwestward and northeastward away from the recession area in the Karoo. This movement takes place in February (early summer) but later in the summer movements have an easterly trend both north and south. Control has been relatively effective so there have been few invasions outside South Africa in the last 50 years. Botswana and Namibia are the countries most likely to be invaded. Invasions of Mozambique, Angola and Zambia are not well documented. In 1924 swarms from northern Botswana invaded Zimbabwe. Breeding occurred in the extreme west in January and the resulting swarms moved eastward throughout the country from late April. By June maturation had occurred and laying took place. The eggs hatched in September when the rains came and the ensuing adults flew westward into Botswana in December.

 

Tree locusts-Anacridium melanorhodon melanorhodon, Anacridium melanorhodon arabafrum, Anacridium wernerellum, Anacridium aegyptium

 


Fig. 113. The distribution of Tree Locusts.

Figure 113 shows the areas where Anacridium species are found in Africa and Arabia. Anacridium melanorhodon melanorhodon, A. melanorhodon arabafrum and A. wernerellum are illustrated in Plate 3.

Anacridium melanorhodon

This species is generally found in areas of short grass and scattered trees, the most common trees in such habitats being various species of Acacia, but it may also be found in treeless areas. Of the two subspecies, A. melanorhodon melanorhodon occurs in the western part of the distribution area and A. melanorhodon arabafrum in the eastern, but the two subspecies meet in Ethiopia and Sudan.

Anacridium wernerellum

This species is mainly found to the south of A. melanorhodon in areas where there are more trees, but the two species overlap considerably.

Anacridium aegyptium

This is the Egyptian Tree Locust. It occurs in countries around the Mediterranean and in the Middle East.

 

Life cycle

The life cycle of Tree Locusts is similar to that of the Desert Locust except that the males generally have six instars and the females usually 7-8 or, more rarely, 6 or 9. The extra instars are quite common in both swarming and non-swarming Tree Locusts.

There is little definite information on the duration of the various stages of the life cycle but Table 13 gives some idea of the time taken for egg and hopper development. Breeding occurs during the rainy season and there is usually only one generation each year, but as in other locusts development is influenced by the weather and food supply.

 

TABLE 13

 

A. melanorhodon melanorhodon

A. melanorhodon arabafrum

Incubation

15-65 days

27-48 days

Hopper development

48-69 days

63-141 days

Immature adults

Immature adults are grey, sometimes a little brownish, brighter in younger, and duller in older individuals; in the latter a pinkish tinge is present at the base of the hind wings. In swarming adults this pinkish colour appears quite early, a month or so after fledging. Non-swarming adults are somewhat browner than swarming ones and the pink coloration may not develop until maturation.

Mature adults

Maturation takes place at the start of the rainy season. Mature adults cannot be distinguished from immature adults by colour alone. The only certain way to recognise a mature female is by dissection. The presence of yellow yolk in the eggs, whatever their size, is a sure sign of maturity.

Copulation and laying are much the same as in the Desert Locust except that the copulating pairs are mostly in trees or bushes until egg laying begins. Laying usually takes place at night. The eggs are laid in moist, usually sandy, soil in the form of an egg pod similar to that of the Desert Locust. The average number of eggs in a pod is high, about 150. Whether swarming Tree Locusts, like Desert Locusts, lay fewer eggs than non-swarming ones is not known. Females can lay up to three pods.

Hoppers

Gregarious hoppers of Tree Locusts are yellow with black markings, the pattern being different in detail for each species; solitarious hoppers are green with white and black dots. It is not unusual to find solitariously coloured hoppers amongst gregariously coloured ones, and vice versa. Various intermediate colour forms are also quite common.

Behaviour

Hoppers

After hatching hoppers climb the nearest bush or tree and may form groups. Where laying has been concentrated groups may be found over many hectares. Feeding occurs at night.

Adults

Adults form swarms which are usually both smaller and less mobile than those of the Desert Locust. During the day adults normally roost in trees, and if disturbed they will fly from branch to branch or to a nearby tree. Such disturbance, however, does not cause mass departure. Swarms usually fly at night, a habit which has earned them their Arabic name, Sari el leil, which means the 'night wanderer'. Flight usually starts soon after sunset when mass departure from the roosting sites occurs. It is in stratiform formation.

There is little information on how far Tree Locust swarms can fly because none has ever been followed. They probably settle fairly soon and begin to feed. If trees and bushes are available they settle on them, but they settle on herbaceous crops if there are no taller plants, and eat them. Feeding on crops is most likely during the dry season.

Seasonal movements and breeding

Little is known about the seasonal movements of the various species of Tree Locust. There are records which suggest that swarms of A. melanorhodon melanorhodon can move considerable distances during the dry season. For instance, it seems likely that swarms gradually moved eastward from northern Nigeria into eastern Chad between February and June 1956 and individual locusts have been caught at sea up to 100 km off the West African coast.

Anacridium melanorhodon melanorhodon breeds in an area where there is a well-defined wet season between June and October, during which breeding occurs, and a long dry season lasting from November until May. Records show that this breeding area is smaller than the overall distribution area. There is only one generation per year. Anacridium melanorhodon arabafrum occurs in areas of East Africa and Arabia where the rainy seasons are much more complex and variable. Hoppers of this subspecies have been recorded for every month between October and May. It is probable that there is only one generation a year, but it is possible that in some areas there may occasionally be two. Anacridium wernerellum has one generation per year in Nigeria where it survives the dry season as an immature adult, but two generations have been observed in Tanzania.

Further studies in the field are needed to increase the sparse knowledge of the seasonal breeding and movements of Tree Locusts.