Locusts are those species of grasshoppers which under certain favourable circumstances congregate and move together in the nymphal and adult stages and then fly in swarms of dense masses to distant areas, causing immense destruction to the vegetation on the way. The word Locust literarily means “plague”, indicating the high destructive capabilities of these insects to the crops in epidemic form in all continents.
THE DESERT LOCUST, Schistocerca gregaria (Forsk.)
Host: Locusts are polyphagous insects that are capable of feeding on almost anything that is green. They can defoliate all crops and almost all kinds of trees in plantations and forestry. However, they show preference towards cereals and vegetable crops and avoid certain plants such as, neem, jamun, sheesam, milkweed etc.
Damage: Leaves and soft shoots are eaten from the margin inwards. Swarms usually defoliate the crops completely. Damage can be devastating over a wide area. At times the losses have been estimated to be about dollars 100 million in a year as estimated by FAO. Adults can eat equal to their own body weight in a day and nymph can eat 5-7 times their body weight. Thus a swarm of one sq. km that weighs 300 tons can destroy equal amount of vegetation. Average swarms are 10 sq km each and the largest swarms have been recorded to cover an area of 300 sq km. One swarm killed by aerial spray weighed 2447 tons.
Life history: Locusts breed in sandy areas in deserts, where soil is loose enough to hold egg pods and abundant Cyperus weed to serve as food for the nymphs. Occasional abundant rainfall brings about a boom in the growth of Cyperus weed and provides ideal conditions for locusts to breed.
Eggs are laid in sand about 10 cm deep by thrusting the ovipositor. Eggs are laid in a frothy mass which hardens to form a tubular egg pod. Each egg pod contains 70-150 eggs. Each egg is 1.2-1.5 mm long and 0.7-0.8 mm wide and whitish in colour. Fecundity is 4-5 egg pods per female or 500-1000 eggs per female. The abundance of locust eggs can be gauged by the fact that in Turkey 430 tons of eggs were collected in 3 months operation by digging the soil. Eggs are incubated by the sun rays heating the sandy soil. The incubation period is 15-30 days or 2-4 weeks according to the temperature.
First instar nymph is vermiform and wriggles up from the egg pod to the surface, where it moults soon to become a hopper. There are 5 instars of the nymphs which take 4-10 weeks to attain adulthood. Nymphs spend the night on the tall bushes and trees and in the morning they come down to congregate and bask in the sun, generally orienting themselves at right angle to the sun rays.
As their body temperature rises they form large bands and start marching to long distances and defoliate all vegetation on the way. Small marching bands join together and form large swarms that continue marching and feeding even when they have become adults. Flight is triggered by a few individuals initially and then the whole swarm becomes air borne and starts migrating along the wind direction. Swarms are generally rolling type, i.e. locusts flying in front settle on the vegetation, feed and destroy it and join the swarm at the back. This way the swarm continues to migrate and at the same time feed on the vegetation on the way.
Adults have solitary and gregarious phases which differ in colour and behaviour. Male is 40-50 mm long while the female measures 50-60 mm. They are pale yellow or brownish, with fore wings greenish-yellow, translucent with many brown spots. There is a peg-like process between the bases of fore legs and thorax is not hairy ventrally. A blunt median prosternal process is present. Eye stripes are 6-7 in number. Hind wings are hyaline to light yellow in younger stages, becoming bright yellow on maturity. There are several generations in a year.
Biotic theory of periodicity of locusts
Locust swarms do not appear every year but at irregular intervals, sometimes after a gap of several years. This is called periodicity of locust activity of which at least 15 cycles have been recorded in India since 1812. Perhaps in some years when environmental conditions become suitable for their rapid growth and reproduction they transform into Gregarious phase through an intermediate, Transient phase. All the three phases differ markedly in their morphology and habits.
The Biotic theory of periodicity of locust cycles was propounded by Dr. S. Pradhan (1967), who was at that time head of the division of Entomology at the Indian Agricultural Research Institute, New Delhi. Locusts breed in semi desert areas where sandy soil is suitable for thrusting the eggs up to a depth of 15 cm. While locusts lead a marginal existence in these areas, their vertebrate predators, namely, lizards, snakes, birds, shrews, hedgehogs, moles etc. find it difficult to sustain themselves in the extreme conditions of deserts and semi deserts and hence gradually move out to the more tolerant areas on the periphery.
But in desert areas once in a decade or so, there is sufficient rainfall that triggers the growth of sedge grass and other weeds and locust gets a chance to realise its full biotic potential and its population shoots up in the absence of predators that are present outside the breeding areas and cannot move in fast enough to contain its population. Unchecked by the natural enemies, locust moves from the solitary phase to migratory phase and develops into large congregations and eventually to swarms that migrate out of the breeding areas causing immense destruction. Locust swarms having gone out of the desert areas, scattered population of adults and nymphs is left behind which then persists as solitary phase till the arrival of another phase of tolerant environmental conditions.
Distribution: Mainly in the desert and semi desert areas of Africa, extending through Middle East to India and Pakistan. Swarms invade areas of Africa north of equator, European countries up to Spain and Turkey, Russia and in the east up to India.
THE MIGRATORY LOCUST, Locusta migratoria (R. & F.)
The Indian subspecies is known as Locusta migratoria migratoria, whereas the African subspecies is called Locusta migratoria migratoroides.
Host: They are polyphagous insects showing some preference for Gramineae both wild and cultivated.
Damage: Solitary forms cause negligible damage but swarms completely defoliate crops.
Life history: Eggs are laid in pods in the soil. Fecundity is 4-5 pods per female. Incubation period varies between 10 and 25 days depending on temperature. Nymphs are like those of the desert locust. Adults are slightly smaller than the desert locust, male being 3-4 cm long and female 4-5 cm. Body is pale-yellow with fine dark lateral stripes along the abdomen. Hind wings are hyaline in the younger stages but become yellow in adult. Otherwise the life cycle and periodicity are similar to the desert locust. It breeds during spring in Baluchistan and the resultant adults migrate to the desert areas of India where they breed in summer.
Distribution: Africa, Europe, Asia east of Pakistan and Australia.
THE BOMBAY LOCUST, Patanga succincta L.
Host: This is a polyphagous pest. The species is found among tall sedge grass (Cyperus tuberosus).
Damage: Damages crops by defoliation.
Life history: Life cycle is similar to the previous two species with the following differences: Egg incubation period is 34 days in July-August. Nymphal instars 7-9 and the nymphal period is 56 days. Adults that emerge in September-October undergo diapause and do not mature till June-July. Therefore there is only one generation per year in this species. Adult is attenuate, with an acute ventral tubercle. There are prominent yellowish stripes laterally on pronotum and on the elytra, which also have oblique dark markings.
Distribution: The species is distributed in India, Sri Lanka and Malaya. In India its distribution extends from Gujarat to Madras and has also been recorded from Rajasthan and Madhya Pradesh.
As the locusts breed in the inaccessible areas of deserts and migrate long distances to cause damage in countries where they do not breed, it is necessary to undertake international cooperative measures and constant surveillance to keep them under check. In view of this fact, United Nation’s special fund for desert locust was established by FAO in 1959 and International Locust Research Centre was set up in London in 1960.
In India Central Anti Locust Organisation (CALO) was established in 1939 to keep a constant vigil by undertaking periodic surveys and issue warnings. Now this organisation is called Locust Warning Organisation (LWO). It has centres in every state called State Anti Locust Organisation (SALO), which keep a vigil on locust breeding in their areas.
Prevention of swarm building in the breeding areas: This is a very important area taken care of by FAO. Constant surveillance is kept at the known breeding sites and quick action is taken when large scale breeding and swarm building is recorded.
Control at the egg stage. Eggs are destroyed by digging, ploughing, flooding and by chemical treatment of the soil. Digging exposes the eggs to direct sunrays and heat and to the predators and checks the population right in the beginning.
Control at the hopper stage. As the hoppers start congregating and basking in the sun, they can be controlled by burning them using flame-throwers. Trenching can be dug around the marching hoppers so that they fall in them and die. Aerial spraying or dusting or barrier spraying the breeding areas with Dieldrin, Aldrin, BHC, Carbaryl or Methyl Parathion kills the hoppers before they can build up into a sizable population to develop into swarms.
Control at adult stage. Control of adults in the breeding areas demands same measures as for the nymphs but they can be easily killed by spraying on them in the night when they are resting on the vegetation.
Annihilation of locust swarms and protection of crops. Generally it is impossible to save the crops from marauding swarms but some relief can be obtained by undertaking the following measures.
Air to air spraying on migrating swarms by using aeroplanes kills a large number of them but the number is so large that enough are left to cause damage to the crops.
Repellents such as 1% suspension of neem seed kernel oil if sprayed on the crop provide protection for three weeks.
Spraying insecticides on resting swarms in the night or early in the morning before take off kills large number of locusts. However, organising these control measures generally becomes difficult as the locusts can migrate very fast and in unexpected directions.