Locust invasion and control in East Africa: Economic and environmental impacts, 1890-1960s
In the late nineteenth century, European travelers and explorers witnessed the devastation caused by locust swarms to the agricultural and pastoral economies in the East and the Horn of Africa.1 We refer to eyewitness accounts. In 1883, crossing the Maasai savanna steppes, the caravan of the explorer Joseph Thomson2 witnessed how locust swarms ruined the grazing lands. He stated: ‘A cloud of locust settled in the land and left not a blade of grass.’ In 1892, William Astor Chanler5, journeying through the semi- desert of northern Kenya, watched vast swarms of locusts in disbelief. He reported: ‘For hours the locusts had swept by us in millions, and it seemed that there was no end to them.’ Similarly, in 1895 in northern Somalia, the large caravan of Arthur Donaldson Smith4 witnessed how desert locust swarms, after stripping the land of its green vegetation, caused starvation for the inhabitants and their livestock. Traveling from the city of Harar on his way to Addis Ababa, Herbert Vivian5 described scenes of desert locust swarms thus: ‘I looked up and beheld a driving rain of locusts whirling at a terrific rate high in the air against the white clouds.... I could scarcely see a yard in front for many minutes.’ Such invasions by locusts had been going on since ancient times. For the pastoralists and farmers in East Africa, every visit by swarms of locusts inevitably resulted in economic ruin and hunger.
This chapter analyzes historical outbreaks and control of two types of locusts: the most widespread desert locust (Schistocerca gregaria Forsk) and the red locust (Nomacris septerfasciata Serville)—the latter has restricted breeding grounds in East Africa.6 Due to their nomadic habits, the locust swarms posed regional and international challenges, thus making collabora- tion by several countries obligatory. Due to the frequent upsurge of new generations of swarms, systematic research has been mainly of an experimental nature and short-term.7 This investigation was conducted in the context of African environmental crisis hypothesis often associated with indigenous land use. We will encourage readers to bear in their mind extent to which the locust swarms contributed to environmental crisis, though the fact was not acknowledged directly by researchers.
The discussions are structured as follows: (1) the ecology of locust swarms; (2) outbreak areas of locust plagues; (3) economic impacts on the agricultural and rangelands; (4) locust control programs, including application of poisoned arsenic bait and aerial sprays using pesticides; (5) experimental research; and (6) monitoring of locust swarms.
The ecology of locust swarms
Modern surveillance methods (1920—1960s) have established that the desert locust used convectional air currents in the north to south of the intertropi- cal convergence zone (ITCZ) to aid their migrations. Taking advantage of the movements of the ITCZ, the swarms moved from outbreak areas into other regions rich in food supplies.8 The most significant biological behavior of locusts is their ability to respond to changes in their populations and climatic conditions. When not swarming, the populations become scattered, with locusts existing as solitary individuals.9
Periods of heavy rainfall and abundant growth of vegetation trigger the synchronic breeding and swarming phases, while during dry years, the swarms are in recession. This implies that during the remission phase, gregarious populations are completely absent—until environmental conditions become favorable again.10 The conditions that trigger swarming include suitable soil moisture that is required for laying eggs.11
There are no accurate records of locust numbers involved in swarming, although biologists have made some reasonable estimates, based on the amount of land covered by the swarms and the numbers of locusts observed per small land units.12 In one estimate, about 150 million locusts per km2 had been recorded. Extrapolating this means that a swarm covering 1,600 km2 (which was common) would contain at least 150,000 million locusts, weighing a total of approximately 300,000 tons. Each kilometer of the infested area would have 100-1,000 tons of locusts.15 A locust plague occurs when many countries are infested by generations of locusts from the area of outbreak to the destination of the swarms.14
Outbreak areas of locust plagues
Both the desert and red locusts complete different phases of population growth, ranging from solitary to gregarious and transient phases, before the swarms take flight during the mobile phase.1’ During a single season, the swarms spread and reach many thousands of kilometers from their breeding grounds. Originating in West Africa, some swarms pass through Sudan into the Red Sea region and the Ethiopian highlands; or from the deserts of India and Pakistan, others move across Iran, the Middle East and cross the Red Sea into Somaliland and Ethiopia16—from there they cross into East Africa (Figure 9.1). Desert locusts and red locusts disclose different patterns of swarming.17 We begin by analyzing the desert locust breeding patterns and swarms, before going on to discuss those of the red locust.
Figure 9.1 Desert locusts’ migration routes from the outbreak areas in Indo-Pakistan and Arabia to the regions of East Africa.