Outbreaks of desert locusts

The population dynamics of the desert locust sometimes created a puzzle for researchers and analysts. Population outbreaks occurred during some years but were followed by recession. For example, the desert locust plagues that commenced in the 1940s came at a time when countries outside East Africa and the Middle East were considered free from immediate danger.18 During the period 1941-1947, swarms of desert locusts arrived on the coast of Somaliland after crossing the Red Sea. By 1944, numerous swarms were reported in western Somaliland and the adjacent Ogaden region of Ethiopia where they began breeding. In 1946, some of the swarms crossed the Gulf of Aden from Somaliland and eastern Ethiopia to breed in the Arabian Desert during the monsoon season.19 In 1948, excessive rainfall in the Arabian Desert—and deserts in Pakistan and Indian—created favorable breeding con- ditions. The large swarms then crossed into British Somaliland in 1949 and in the early part of 1950. From there, they spread into Sudan, eastern Ethiopia and northern Kenya.20 From December 1949 to April 1950, breeding took place on the coast of Saudi Arabia and in the Somali Protectorate.21

Then after the short rains in 1951, the swarms arrived on the borders of East Africa. Following successful breeding in winter and spring in the east, by the end of that year, a new plague was crossing into Somaliland and eastern Ethiopia on the heels of the earlier swarms.22 Heavy short rains throughout East Africa in late 1951 produced another event that puzzled observers, in that the swarms were spreading against prevailing northerly and north-easterly winds.23 By 1952, the center of locust activities had shifted from Indo-Pakistan to the Red Sea coast, the Ethiopian highlands, Somaliland, and the northern region of Kenya. In Eritrea, heavy locust infestation posed a serious threat to agriculture.24 Once again, the region that caused the greatest concern was the Somali Peninsula and the Somali region in eastern Ethiopia. From there, the swarms crossed into East Africa in plague proportions like those of the preceding year (1951).ъ

A series of swarms arrived simultaneously in the Horn of Africa in 1952, through 1953, scattering in various directions into the Sudan, Eritrea and East Africa.26 By 1953 it was becoming clear to the Desert Locust Survey Organization based in Nairobi that protecting the croplands in East Africa would require control of locusts in the more remote regions of the Horn of Africa—Ethiopia and Somaliland and the Arabian Peninsula, the Aden Protectorate and Indo-Pakistan.2, The island of Socotra (south of the Arabian Peninsula) was heavily infested, from where new generations of swarms crossed into Somalia.28 During the short and long rains in 1954, following successful breeding of the locusts in Arabia, another large swarm crossed the Red Sea and the Gulf of Aden into Eritrea, the Ethiopian highlands and the Somali Peninsula. This swarm was augmented by those that bred in the Danakil Desert (the Afar region of Ethiopia) and others that had successfully laid eggs in the Somali Peninsula during the short rains.29 This invasion by an estimated 50 different swarms of locusts spread into Kenya and overwhelmed preventive measures.30

The successful breeding of the desert locust in Eritrea, the Danakil and the Somali Peninsula resulted in large-scale, severe infestations (reported by some observers as ‘unprecedented’). The hopper bands varied in size from tens of thousands to two to three million insects.’1 The apprehension of government officials and the desert locust survey team is therefore understandable. Later, these swarms moved into drier regions such as Turkana and the Maasailand in East Africa where they continued to breed.32 Meanwhile, after the short rains of 1955, new locust generations invaded Kenya, Tanganyika and Uganda. Some of these locusts remained in the Somali Peninsula and produced a new generation during the long rains. In the short term, new generations of locusts invaded eastern Ethiopia and the Red Sea coast of Eritrea where they bred to re-invade the Somali Peninsula and finally Kenya and the rest of East Africa, thus repeating the cycle. The populations remained unstable, shifting across the region with different generations of swarms spreading as far as northwest Africa, and eastwards to India, where breeding followed successful monsoon rains. Thus, the Arabian Peninsula served as a crossroads for different swarms that originated from different regions.’3 Within East Africa, the red locusts caused great concern.

Outbreaks of red locusts

The red locust had its outbreak areas in the marshes of Lake Rukwa in Tanzania (Figure 9.2) and Lake Mwenu in Northern Rhodesia (present-day Zambia). Lake Rukwa lies in the southern East African Rift Valley. The lake is about 129 km long and covers an area of 402 km2 when not in flood. During wet years the area expands to 804 km2.’4 The alternating floods and droughts had a marked influence on the ecology of the edaphic grasslands, creating ideal conditions for breeding by the red locust.3’ The species had attracted research interest, partly because of its restricted habitat, and partly because of their periodic swarms. Their populations periodically expanded into huge swarms, alternating with disappearance at other times—only to reappear again.36 The cyclic events of the red locust swarms were related to the dynamics of the floods in the marshes. The shallow alkaline lakes fluctuated in size from one season to another. During the solitary phase, individual locusts existed in the marshes. After the water receded, breeding ensued in the soft mud—with the grass growth providing food for the hoppers and adults.’' The breeding did not occur in landscapes covered by trees,’8 or in flooded marshes.’9

The red locust swarms of 1927 spread over an area of 482 km2 and continued to threaten the region until 1945, bringing about huge financial losses in terms of agricultural production in Tanganyika.40 During the period 1935-1936, various generations of red locusts spread across Tanganyika.41 By 1951, experiments were conducted to estimate the population, using scouting

192 Vectors, pests and environmental change

Outbreak area of red locusts in the marshes of Lake Rukvva in Tanganyika

Figure 9.2 Outbreak area of red locusts in the marshes of Lake Rukvva in Tanganyika.

by land rovers and foot methods.42 During the period 1953-1956, scouts monitored the marshes for the flights of the non-swarming populations, in order to identify the sites which could become outbreak areas.45 Thirteen years of continued outbreaks were suddenly followed in 1956 by no record of any swarms at all. This period coincided with flooding of the marshes.44 Both the desert and the red locusts caused huge economic impacts.

 
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