The crops grown in India are attacked by a large number of different pests, some of which are specific to a particular crop or feed or related species of a particular family, or to crops grown in particular season. However, their are other insects, such as termites, armyworms, cutworms, grasshoppers and locusts which attack a large variety of crops in different seasons. Out of them, Locusts occupy a unique position because of their swarming habit and migrations to distant areas irrespective of international boundaries. An account of these group of insects, specially the desert locust, which is regarded as 'Farmer's enemy No.1', is given seperatly.

India has been subjected, from time immemorial, to locust invasions which cause immense damage to crops and trees, and sometimes resulting in severe fodder faminies and considerable loss of livestock.

Locusts are closely allied to short-horned grasshoppers from which they differ markedly in certain respects. Both are anatomically very similar, feed on vegetations, lay eggs in the soil and pass through several instars before they become adults. Whereas grasshoppers feed on specific crops only, locusts devour almost all plants and vegetation, and even feed upon dead and dying locusts, i.e. they are omnivourous. Unlike grasshoppers, locusts exists in two phases : (i) the solitary phase and (ii) the gregarious phase. In solitary phase, they are very active, live and breed as scattered individuals, and are relatively harmless. In the gregarious phase, they are very active, acquire the habit of congregation and forming bands, breed rapidly, and in adult stage fly long distance during migration even overseas. Climate, weather, soil, vegetation and their mass destruction organised by man cause the transformation of the gregarious phase into the solitary phase.

There are eleven main species of locust, of which three, viz. the Bombay locust, the migratory locust and the desert locust, are found in India. The first two species live and breed in the country and are found scattered all over western India from Rajasthan in the North to Karnataka and Tamil Nadu in the south. They rarely form swarms. They mainly feed on plants of the grass family and occasionally when they become gregarious, do considerable damage to jowar, bajra and other millets.

The desert locust in the most destructive intenational pest, its distribution extending from India in the east to Africa in the west, passing through pakistan, Afganistan, Iran, Iraq, southern USSR, Arabia and in the north, east and west of Africa. In this vast region, the locust lives and breeds endemically, forming swarms and undertaking long flights to distant places. The locust invasions occur in cycles, the duaration of a cycle as well as of the period between two cycles being variable.

In the solitary phase, the locust is more or less harmless, whereas in the gregarious phase, when its hoppers congregate and march in bands, destroying all the vegetation in their path and give rise to swarms, it is very destructive. The hoppers of the solitary phase are green throughout their life, but the hoppers of the gregarious phase are black in the first two phases of development and their dark color turns greenish yellow and orange in the later stages. The adult locust of the solitary phase remains grey, but the adult is pink at first, but turns grey and yellow as it matures.

There are generally two breeding seasons in a year: (i) winter-spring, and (ii) summer-monsoon. In regions where the rainfall is received in winter and early spring, e.g. in sout-east Arabia, southern Iran and Baluchistan, the locust breeds in spring. In India and other countries where the rainfall is received during the summer months, breeding takes place at that time of the year. The swarms produced during winter and spring in the middle east and Baluchistan usually fly eastwards into pakistan and India, in summer. With the advent of monsoon in this area, they lay eggs from June to September. The swarms resulting from this breeding return during autumn to the areas of winter rainfall when conditions in the summer rainfall area become dry and unsuitable for breeding. A number of swarms developed in the country fly also to the north, south and east, and thus invade all parts of India and damage the kharif crops.

Sometimes, breeding takes place in winter, specially in Punjab and Haryana when they damage the rabi crops as well.

The female of the gregarious phase lays eggs in clusters of 40-120 in a hole in moist soil to a depth of 75 to 150 mm and covers them with a liquid material that soon hardens to form a protective covering. Young hoppers hatch from the eggs in two or three weeks in summer, and in three to four weeks in autumn and spring. There are five stages in the development of hoppers. At each stage they cast off their skin every sixth or seventh day as they progressively become bigger. As long as they are hoppers, they have only rudimentary wings. After the fifth moulting, they develop wings from the wing-pads present the later hopper stages. During summer and monsoon, the hoppers complete their development in four to five weeks;and during autumn and spring in six to eight weeks. As the hoppers moult, they undergo a changer in color and size. In early stages, they are almost wholly black, and in the later stages, they are black, with orange and yellow patterns. When they acquire wings, they are pink and ultimately turn yellow, when fully mature.

Both hoppers and adults feed on all types of vegetation, though there are some plants which they would not touch, or eat only with reluctance.

The theory has been recently propounded to explain the causes of periodic locust ourbreaks. Accoding to this theory, the actually adverse climatic conditions of the desert are, in fact, favourable to the locust, as they adversely affect its enemies (specially cold-blooded reptiles), which are responsible for not allowing the locust to breed outside the desert, where the conditions are otherwise very favourable. The desert environment becomes more inhospitable to the enemies of the locust during the minima of the sun-spot cycle (periods of drier and more intense heat), leading to a reduction in the population of the locust enemies which consequently results to an increase in locust population and swarm formation. During the maxima of the sun-spot cycle (period of less dry and less intense heat) on the other hand, the desert environment becomes favourable to the enemies of the locusts which migrate to the interior of the desert from the periphery and bring down the locust population, thus breaking the locust cycle.

This biotic theory has made it possible to visualize the possibilities of checking the start of the locust cycle in the early stages by creating suitable ecological niches in the locust-breeding areas, so that their enemies can migrate and survive the adverse conditions in such areas and continue to keep down the locust population.

There are two main methods of locust control, viz. (i) the use of poison baits, and (ii) dusting and spraying with insecticides. Poison baiting, though cheap, is not very practicable, dusting and spraying have been found to be very effective. In the egg stage ploughing or flooding the land has been practised in the past. This method is not much effective in as much as it destroys only about 30 percent of the eggs and renders subsequent control measures with insecticides difficult. Spraying the egg-infected, uncultivated land with Aldrin is very effective, as it leaves a poisonous film on the surface with which emerging hoppers come into contact and die. Once the hoppers have emerged, they can be effectively controlled by dusting or spraying or both, as the hopper stage is the most vulnerable. For dusting, BHC is the most effective insecticide. For young hoppers, 3 to 5% dust is sufficient to kill them; for advanced stage 5 to 7% dust is necessary; and adults need 10% BHC. Dusting or spraying may be done on the hoppers directly or it may be done on strips of ground, two to three metres wide, in the path of the advancing hoppers. Another method is to dig trenches 45 to 75 cm deep and 30 to 60 cm wide depending on the size of the hoppers, in front of the moving hoppers to which they may be driven and buried alive. The duating and burning of bushes in which hoppers rest can also be resorted to.

Adult locusts, flying in swarms, are most difficult to control. If the alight in any field or in a piece or ground try to kill as many of them as possible, by beating or cruching, specially in the morning when they are lazy. Burning them by setting fire to dry bushes or wastelands, or by flame throwers, if they are settled on green bushes, hedges or trees, if useful. This method works well if the locusts have settled for resting, mating or egg-laying. Poison baitspread at the rate of 20 to 50 kg per hectare are useful if locusts have settled to feed in cropped feeds. If the swarm consists of pink locusts, dust them with 10% BHC. Aerial spraying of the flying swarms and ground hoppers has to be resorted to with Aldrin solution in oil in the case of very serious infestation. However, this cannot be done when the swarms and hoppers are in the populated areas.

Despite the discovery of a large number of potent insecticides, which can be used against locust, both from the ground and air, the farmer has been finding himself to be only a helpless onlooker once the locust swarm settles in his field. Though part of the swarm may be killed, he invariably loses his crop.

Situation has materially changed recently after the discovery of the anti-feeding property of the neem kernels. It has been found that 0.1 % spension of neem kernels, sprayed on the crop affords protection against locust feeding for about 14 days. The cultivator can locally collect and preserve the neem seeds till required. By spraying his crops with neem kernel suspension, when the invasion of his fields, by hopper bands or swarms is immenent a cultivator can now ensure the safety of his crops. Simultaneously, the control operations can be carried out by adopting suitable methods.