| Locust handbook |
|3. Other African locusts|
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.
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.
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 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 26ēC; 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.
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.
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 20ēN 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 km˛ was infested. Thereafter the plague became more restricted until it declined in 1945. A total of 16 generations was estimated to have been involved.