Four Species of Plague Locust | Encyclopedia MDPI

Four Species of Plague Locust: History

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Subjects: Entomology

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South Africa has four species of recognized plague locusts that have caused economic damage to crops and rangeland grazing, with plagues of some of the species posing a serious threat to agricultural production in South Africa at different times in recorded history over the past 380 years. The plague locusts in order of current economic importance in South Africa are the brown locust, Locustana pardalina (Walker), the African migratory locust, Locusta migratoria migratorioides, (Reiche and Fairmaire), the red locust, Nomadacris septemfasciata (Serville) and the southern African desert locust, Schistocerca gregaria flaviventris (Burmeister).

South Africa
locust outbreak dynamics
brown locust
red locust
African migratory locust
southern African desert locust

1. The Brown Locust, Locustana pardalina

The brown locust, Locustana pardalina (Walker), is the most economically important plague locust species in South Africa. This locust is indigenous to the semi-arid Karoo areas of South Africa and parts of southern Botswana and southern Namibia, with a recognized outbreak area stretching over 250,000 km² of mainly sheep-grazing country across the Nama Karoo biome [6,7]. Outbreaks occur very regularly, with gregarious swarming populations being controlled somewhere or in the Karoo in approximately 90% of the years throughout the entire twentieth century [4]. However, large-scale population upsurges, leading to uncontrollable plague cycles, occur perhaps once per decade, which require a massive chemical control campaign to bring under control [4].

The rainfall gradient across the outbreak area of the brown locust in the semi-arid Karoo ranges from approximately 380–400 mm in the southeast to only 100–130 mm per annum in the arid northwest, with most of the rain falling as thundershowers in late summer and autumn. However, the rainfall patterns are typically erratic and are very patchy in distribution, and extended droughts are common [4]. The relationship between rainfall and the periodicity of brown locust outbreaks has been extensively studied, but no conclusive correlations have been found to date that could aid an early warning system [4]. The brown locust is primarily a grass feeder, and outbreaks pose a direct threat to the sheep grazing rangeland in the Karoo and to cereal crops grown under irrigation within the Karoo, while escaping swarms pose a threat to the main commercial cereal cropping areas in the Free State and North West Provinces [4].

2. Brown Locust Control Strategy

It is widely accepted that the incipient outbreaks of the solitary phase are impossible to control over a vast area of the Karoo [7,13,16], so it is more practical and economically viable to wait until the locusts aggregate and gregarize and then target gregarious hopper bands and the young adult swarms. The brown locust control strategy that has been in operation since 1907 is upsurge elimination to combat outbreaks at source in the Karoo before migrating swarms can exit the Karoo and threaten the main cereal crop production areas in the Free State and North West Provinces and then further afield. The history of brown locust control in South Africa, along with a detailed review of the strategy and operational tactics employed during control campaigns, was given by Price [4]. Landowners and farmers are legally required to report the presence of gregarious locust populations on their land, while the Government is legally responsible for the costs of undertaking the control action. In a large-scale outbreak season, tens of thousands of hopper bands and thousands of adult swarms are individually tracked down and controlled using fast-acting synthetic pyrethroid insecticides applied from a ground-based knapsack and vehicle-mounted ULV sprayers, using an army of temporarily appointed locust officers and labor assistants [4].

3. Threat of the Brown Locust to Agriculture in South Africa

The brown locust produces regular outbreaks in the Karoo, and the large-scale upsurges, if left uncontrolled, certainly pose a threat to agricultural production and the food security of South Africa and the rest of southern Africa. However, upsurges are usually contained within the Karoo with great effort and expense using relatively unsophisticated control tactics and operations. Over the past 80 years, apart from short-term swarm escapes, there have been no significant or wide-scale crop losses due to brown locust infestations outside the Karoo. The ground control tactics utilizing the temporarily employed local farming community whenever there are outbreaks, known as the ‘Commando system’, can be very efficient and cost-effective when dealing with small outbreaks, especially in the eastern Karoo where there is a high population of resident farmers [4]. However, the changing demographics and the depopulation of many farms in the more remote areas of the Karoo, especially in the Upper Central and western Karoo, Great Karoo, and Bushmanland, have negatively impacted the reporting and reaction response to outbreaks in these vast areas so that swarms regularly escape control in these areas [4]. The locust control capacity clearly becomes overwhelmed in these remote areas during the large-scale upsurges by the thousands of locust targets that require control. In addition, the ever-increasing costs of insecticides, spray equipment, labor, and transport cause a budget crisis whenever there are large outbreaks [4]. The long-term sustainability of the current brown locust control system in South Africa is therefore in doubt, and the locust reporting and campaign management system needs to be urgently modernized to utilize available technologies for GIS mapping and for the prediction and real-time monitoring of outbreaks. International support may be necessary in the future to contain the potential menace of the brown locust. Ongoing climate change is also predicted to Increase the frequency of erratic climatic events, possibly leading to more intense drought cycles and floods. The brown locust is known to respond closely to erratic rainfall events in the Karoo, and therefore, the frequency and intensity of outbreaks are likely to be impacted by climate change in the future.

4. The African Migratory Locust, Locusta migratoria migratorioides

The African migratory locust, Locusta migratoria migratorioides (Reiche and Fairmaire 1850) is widely distributed throughout the grassland areas of Africa, south of the Sahara. This tropical subspecies of Locusta is multivoltine, breeding continuously and without a reproductive or egg diapause [23,24]. The main outbreak area for the African migratory locust (AML) is found in the floodplain grasslands of the Middle Niger River in Mali [1,25], from where swarms developed in 1928 that led to the last great plague cycle that invaded most of Africa’s south of the Sahara between 1928 and 1941 [26]. However, regular swarming on a smaller scale has been reported from at least 12 secondary breeding areas in different areas of Africa [1], but none of these outbreaks has been large enough to initiate a plague cycle. In southern Africa, swarming populations of AML have been regularly reported from the South and Western Provinces of Zambia, especially in the Zambezi Valley and Simalaha plains from 2019 to 2021, the Southeast Lowveld of Zimbabwe (Hippo Valley, Chipinge), the Shire Valley in Malawi, the eastern Caprivi grasslands of Namibia and associated grasslands along the Chobe River in northern Botswana, the Okavango Delta, and around Lake Xai in Botswana and at Simunye in eSwatini (John Kateru, IRLCO-CSA, pers. com.). Most of these outbreaks have occurred in natural grasslands and adjacent subsistence and smallholder maize and sorghum farming areas, with serious damage to maize and sorghum crop production regularly reported. Control operations have been undertaken by the International Red Locust Control Service for Central and Southern Africa (IRLCO-CSA) and by local Government migratory pest control officers.

5. Threat of the African Migratory Locust to Agriculture in South Africa

By the time the high-density AML populations are achieved in autumn (March–May), the maize crop is mostly ripe and drying out for harvest, and the AML does not usually pose a threat to most of the maize crop. However, some small-scale economic damage has been observed in late-planted maize where the developing cobs have been damaged, and the AML populations at high densities can also strip the maize leaves that would have been utilized by the farmers as winter fodder for stock animals [32]. The main economic damage is caused by newly planted wheat that is grown adjacent to standing maize fields. The high AML populations roosting in the dry maize crop in June and July make daily feeding forays into the adjacent newly emerging wheat crop boundary areas where widespread damage and crop loss have been reported on occasion [29]. In outbreak years, there are reports of farmers having to replant their wheat crops. The AML is difficult to control in the cereal crop environment, and outbreaks can arise relatively unnoticed, so the AML will continue to provide a low-level threat to crop production under current agricultural practices, but with only the occasional outbreak causing economic concern. Changes to agricultural practices to deny the AML an overwintering or dry season habitat have been advocated as a possible management tool for tropical Locusta [30].

6. The Red Locust, Nomadacris septemfasciata

The recognized outbreak areas of the red locust, Nomadacris septemfasciata (Serville), occur in eight relatively small, seasonally flooded, and remote grassland areas in central and east Africa [33,34,35]. A few other small-scale subsidiary outbreak areas are also recognized in eastern and southern Africa, where high-density populations are known to occur [35]. Likewise, outbreaks that produced hopper bands and swarms are also known to occur in Madagascar and in the Lake Chad and River Niger areas [1], but these areas are not considered as plague source areas. The biology and population dynamics of the red locust and the ecology of one of the main outbreak areas in the Rukwa Valley in Tanzania have been extensively studied [1,36,37,38,39,40]. Three major red locust plagues are recorded in the literature [1], with the last great plague lasting 18 years from 1927 to 1944 and invading most of Africa, south of the equator, except for the more arid southwestern areas [33,41]. Following this devastating plague cycle that inflicted serious damage to crops and pasture and threatened the food security of entire countries, it was clearly evident that individual countries could not tackle the threat of the red locust alone, which eventually led to the establishment of the International Red locust Control Service (IRLCS) in 1949. The IRLCS convention was signed by nearly every country that had suffered infestations by the red locust and had the primary objective of controlling locusts in their outbreak areas to prevent the recurrence of plagues. The history of the IRLCS and its subsequent development into the IRLC0-CSA in 1970 has been described in detail by Byaruhanga [34].

As the red locust is univoltine species and only breeds consistently and successfully in its seasonally flooded grassland outbreak areas, which occupy a relatively small area of less than 1500th the size of the invasion areas [41], the red locust is therefore vulnerable to targeted chemical control operations in the outbreak areas. The last major plague ended when the first organochlorine insecticides became available in the 1940s [33]. Since then, the numerous insecticide interventions undertaken against hopper bands and against swarms escaping from outbreak areas at various times have contained outbreaks before any new plague cycles could get going [33,35]. However, population upsurges are often difficult to detect in the remote and often inaccessible outbreak areas, and once swarms have escaped from these source areas, they become highly mobile and are difficult to track down and control in the vast invasion area. The red locust occurs in the solitary phase in the subtropical grassland areas of South Africa in northern Zululand and in the Kruger National Park, with isolated specimens also recorded from cultivated areas in other parts of the country [42]. For example, the red locust is occasionally observed in maize and wheat crops on the Highveld, with isolated egg pods being found between the maize rows (Price, pers. obs.). However, no incipient swarming populations have been produced in South Africa outside of the recognized plague cycles.

Lounsbury [2] and Lea [18] described reports of red locust infestations in South Africa from 1840 to 1853 and then again from about 1888 to 1907, while the third plague invasion between 1933 and 1944 is described in great detail [18,43,44]. Apart from these three recognized plague invasions, Lounsbury [2] also reports earlier accounts of locusts ravaging the gardens of the Dutch settlers in the Cape Town colony in 1653, in 1687, and again in December 1746. It is probable that these invasions were also of the red locust as records state that the locusts ate the leaves of the trees, which is not typical feeding behavior of the indigenous brown locust (Price, pers. obs.). In addition, Lounsbury [2] states that even in brown locust plague years, the brown locust swarms did not reach the Atlantic coast of South Africa.

7. Threat of the Red Locust to Agriculture in South Africa

The northern Natal coastal area of South Africa was a reception area for migrating swarms of the red locust during the last plague cycle and became a major center for successful breeding and for the production of numerous swarms that drove the plague cycle in southern Africa over an extended period [41]. Although local incipient populations of the red locust in South Africa seem incapable of building up to swarming populations by themselves, the Natal coastlands are a prime breeding area for invading swarms, and the potential threat of the red locust was clearly seen during the brief swarm invasion in October 1996 from the Buzi-Gorongoza outbreak. South Africa, therefore, relies on the effective control of upsurges in the main outbreak areas in central and eastern Africa by the IRLCO-CSA and associated Government control campaigns in the outbreak countries in order to prevent new plagues from developing in southern Africa and the resultant invasions into South Africa.

8. The Southern African Desert Locust, Schistocerca gregaria flaviventris

The southern African desert locust, Schistocerca gregaria flaviventris (Burmeister, 1838), is recognized as a subspecies of the well-known plague desert locust, Schistocerca gregaria (Forsk, 1775) that ranges over a huge area of North Africa, the Arabian Peninsula, and into Southwest Asia [47]. The southern form of Schistocerca is geographically isolated in southern Africa by the tropics and a wide belt of bush or woodland [48], and although it can interbreed with the northern desert locust in cages, the offspring are infertile [48,49]. In addition, the behavior and morphometric measurements of the two subspecies are also very different [48]. Recent studies have indicated that the level of genetic diversity of S. g. flaviventris was moderately lower than in the northern subspecies, although the separate populations were genetically homogeneous, such as observed in the northern subspecies [50]. In addition, univariate and multivariate morphometric analysis of the northern and southern populations of the desert locust indicated morphological differences that suggest that the southern population may be categorized as an evolutionary dichotomy and may separate the two locust populations into two distinct species [51].

9. Threat of the Southern African Desert Locust to Agriculture in South Africa

Migrating adult swarms have occasionally been recorded in the past, invading irrigated farming areas along the Orange River and damaging crops such as citrus, cotton, tobacco, vines, and vegetables [53]. However, outbreaks that are big enough to pose an economic threat are very rare, and the threat of this locust to agriculture is considered low.

This entry is adapted from the peer-reviewed paper 10.3390/insects14110846

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