Polyphagous pests are those which feed on plants belonging to diverse taxonomic groups. Generally, these pests multiply in large number are voracious feeders and cause considerable damage to agricultural crops. The most important polyphagous pests are termites, grasshoppers, hairy caterpillars, cutworms, etc. Other polyphagous pests include aphids, whiteflies, white grubs, mealy-bugs, tobacco caterpillar, gram caterpillar, etc.
These polyphagous pests damage a number of crops and cause substantial losses. The magnitude of damage and loss caused by some of the polyphagous pests such as locust, is gigantic and beyond imagination as they have the capacity to cause starvation due to their voracious feeding ability. On an average, a small locust swarm eats as much food in one day as about 10 elephants, 25 camels or 2500 people.
1. Locusts:
The larger grasshoppers which form swarms are called locusts and there are three species of the family Acrididae found in India.
(i) Desert locust, Schistocerca gregaria (Forskal)
(ii) Bombay locust, Patanga succincta Linnaeus
(iii) Migratory locust, Locusta migratoria (Linnaeus).
Of these, the desert locust is the most important. There are very few records of swarms involving other locusts, particularly of the migratory locusts. The first and second species are important in Maharashtra, Gujarat and Rajasthan but the desert locust is of all India importance. In fact, it is an international pest and efforts are being made constantly to control it through the International Locust Control Organization of the Food and Agriculture Organization.
(i) Desert locust, Schistocerca gregaria (Forskal) (Orthoptera: Acrididea):
The desert locust is found in two phases, i.e. the solitary phase and the gregarious phase. Individuals having characteristics in between the two are often placed under the transient phase. The characteristics of the first two phases are different from each other, particularly in the colour of their nymphs.
The nymphs of the gregarious phase are yellow or pink, with distinct black markings, whereas the colour of the solitary hoppers varies according to the colour of the surrounding vegetation. The gregarious adults are pink on emergence, gradually turning grey and ultimately yellow, when sexually mature. The adults-of the solitary phase remain greenish grey throughout their life.
(ii) Bombay Locust, Patanga Succincta Linnaeus (Orthoptera: Acrididae):
The Bombay locust is present in India, Sri Lanka and Malaysia. In India, its activities are generally confined to the area extending from Gujarat in the north to Tamil Nadu in the south; though in certain years its swarms have reached as far as Bengal. The adult is smaller in size than the desert locust and it has an acute ventral tubercle. There are conspicuous stripes laterally, on pronotum and elytra. The hind wings become rose coloured at base after the insects have spent one month as fledgings. The elytra have oblique dark markings.
Life-Cycle:
In the desert areas of India, the adults of this locust species are found during autumn, winter or spring but none during summer. They are generally met with amongst a tall sedge (Cyperus tuberosus), locally known as chiya. Studies have shown that the adults which fledge during September-October undergo diapause and do not mature till June-July next year. Thus, there is only one brood in a year. During July and August, the minimum incubation and nymphal periods recorded were 34 and 56 days, respectively. The hoppers pass through 7-9 instars before becoming adults.
Damage:
Both adults and hoppers normally feed on wild grasses but when the locust plague occurs they inflict heavy damage to crops. Unlike the desert and the migratory locusts, hoppers of this species do not congregate to form bands but remain scattered among crops or grasses.
(iii) Migratory Locust, Locusta Migratoria (Linnaeus) (Orthoptera: Acrididae):
The geographical distribution of the migratory locust is Europe, Africa, Pakistan, Eastern and Southern Asia and Australia. In India, it develops swarms in certain years; swarms were observed in 1876 in Madras (Tamil Nadu) and in June 1954 in Mysore (Karnataka).
The main distinguishing characteristics of migratory locust from Bombay locust are that the ventral tubercle and eye-stripes are absent in the former. The hind wings are hyaline in the young forms, but become yellow subsequently.
Life-Cycle:
It breeds during spring in Baluchistan (Pakistan) and the resultant adults migrate into the desert areas of India as individuals. They breed there during summer and again in September- October. Under the laboratory conditions, the eggs of this locust species hatch in 12 days during July-August. The hoppers moult 5-6 times during the nymphal period of 13 days. Under favourable environmental conditions, overlapping generations of this locust are met with. Unlike the Bombay locust, the adults of this species do not undergo diapause.
Damage:
During swarming period, both adults and hoppers feed on all kinds of vegetation, destroying all crops. It may cause famine in the areas of its abundance.
2. Grasshoppers:
(i) Sporadic Grasshopper, Oxya Nitidula Walker (Orthoptera: Acrididae):
Sporadic grasshopper is widely distributed in the Orient and is a pest of sporadic occurrence. In Punjab, it is serious in the low-lying areas of Amritsar, Gurdaspur and Hoshiarpur which are flooded occasionally. Damage is done by nymphs and adults who feed on various kharif crops, such as maize, sorghum, rice, sugarcane, etc.
The nymphs are shiny green and have a broad, black band extending from the base of the eyes to the wing pads. The legs are greenish-yellow and the full-grown nymphs measure about 15-25 mm in length. The adults are similar in body colour but are larger (about 25-30 mm) and have green wings.
Life Cycle:
The pest is active from March to December and passes winter in egg stage in the soil. The over-wintered eggs hatch in March and the emerging nymphs feed on berseem (Trifolium alexandrinum), khabbal grass (Cynodon dactylori) and other available crops. They moult 6 or 7 times by June and are transformed into winged adults in 60-64 days.
The adults also feed on various kharif crops and live for 40-49 days. They lay brown eggs in clusters on the underside of leaves of various food plants, particularly sugarcane. A female may lay 29-143 eggs, which hatch in 16-24 days. The nymphs of the second generation developing during July-August are transformed into winged adults in 46-57 days.
These adults live for about 60-71 days and lay eggs in the soil instead of on plants. Some of these eggs hatch in October and give rise to third-generation nymphs which never complete their development and eventually die of cold in December. A majority of eggs, however, do not hatch and remain in the soil till next March. Thus, the pest completes two broods in a year.
Damage:
The nymphs and adults feed on berseem, maize, sorghum, bajra and sugarcane. When there is a serious infestation, the plants may be completely defoliated and the crops, such as berseem, may be wiped out.
Management:
Dust the crop with fenvalerate 0.4 per cent @ 25 kg per ha.
(ii) Surface Grasshopper, Chrotogonus Trachypterus (Blanchard) (Orthoptera: Acrididae):
This species of surface grasshoppers is widely distributed in the Orient and Africa. In India, it is common in the north whereas C. oxypterus occurs in the southern regions. The pests are very common in the field and assume serious proportions in certain years.
The former causes considerable damage to the germinating cotton in April-May and the germinating wheat in November. The grasshoppers are polyphagous and, besides cotton and wheat, they feed on a number of other cultivated crops such as berseem (Trifolium alexandrinum), sugarcane, kharif fodders, barley, etc.
Damage is done by the nymphs as well as the adults. The adults are stoutly built, about 20 mm long and 8 mm broad, with a rough brownish appearance. There are two longitudinal rows of black dots on the underside of the white body. The nymphs are similar in body form but are smaller and have wing pads instead of wings.
Life-Cycle:
C. trachypterus remains active throughout the year but its activity is considerably reduced during winter when only eggs and adults are found. A female may lay 36- 434 eggs in 2-15 egg pods, at a depth of 3-6 cm in the soil. There is generally 16-46.eggs in a pod. In summer, the eggs hatch in 19-33 days whereas in winter they hatch in about 5 months.
The developing nymphs feed on grasses or other soft foods. They moult 5-7 times and are full-fed in 40- 70 days. Adults are long-lived, the female for about three months and the male about 1.5 months. There are two complete generations in a year and sometimes the eggs laid in October also hatch giving rise to a third partial brood, which dies in the nymphal stage during the winter.
Damage:
Both nymphs and adults feed on leaves by cutting germinating plants of cotton, wheat, etc. particularly in areas adjoining the waste-lands. Often the damaged fields have to be resown. Unlike Oxya sp., the attack by this grasshopper is more serious in low-rainfall areas.
3. Hairy Caterpillars:
(i) Red Hairy Caterpillar, Amsacta Moorei (Butler) (Lepidoptera: Arctiidae):
The red hairy caterpillar or kutra is widely distributed in the Orient, including India. It is a polyphagous insect and feeds practically on all kinds of vegetation growing during the kharif season. Its attack is particularly serious on sunnhemp, maize, jowar (Sorghum vulgare), guara (Cyamopsis tetragonoloba), mung (Phaseolus aureus), moth (Phaseolus aconitifolius) and sesamum.
Damage is caused by full-grown caterpillars which measure about 25 mm in length. Their colour varies from reddish- amber to olive green and the body is covered with numerous long hairs arising from the fleshy tubercles. The moths are stoutly built and have white wings with black spots. The outer margins of the fore wings, the anterior margin of the thorax and the entire abdomen are scarlet red. There are black bands and dots on the abdomen.
Life-Cycle:
This pest is active from mid-June to the end of August and passes rest of the year in pupal stage in the soil. Moths from these pupae appear usually with the first shower of the monsoon. They are nocturnal in habit and lay light-yellow spherical eggs in clusters of 700-850 each on the under surface of the leaves of host plants.
A single female may lay up to 1,500 eggs, which hatch in 2-3 days. The young caterpillars feed gregariously and, as they grow older, they march in bands destroying field after field of various kharif crops. The caterpillars grow through six stages and complete their development in 15-23 days.
They enter the soil, shed their hair and make earthen cocoons at a depth of about 23 cm. Here they pupate and remain in this stage for many months till they emerge next year from the cocoons. In a given population, probably more than one generation is completed in a year.
Damage:
The young kutra caterpillars prefer to eat the growing points of plants. The older ones have no such discrimination and they feed voraciously on all vegetation resulting in disaster. Field after field is devastated by the moving army of caterpillars. In the years of severe infestation, there may be a complete failure of the kharif crops.
Management:
(i) The moths are strongly attracted to artificial light. Therefore, light traps of electric or petromax lamps placed just above a broad flat basin full of kerosenized water, should be put on the night following the first shower of the monsoon and continued throughout the period of emergence for about one month.
(ii) Young larvae are gregarious. They can be destroyed by pulling out the infested plants and burying them under-ground.
(iii) The grown up caterpillars may be destroyed by crushing them under feet or picking and putting them into kerosenoized water.
(iv) The pupae may be collected at the time of summer ploughing and destroyed,
(v) In case of serious attack, spray 1.25 litres of quinalphos 25EC or 250-500 ml of dichlorvos 100EC in 500 litres of water per ha.
(ii) Bihar Hairy Caterpillar, Spilarctia Obliqua (Walker) (Lepidoptera: Arctiidae):
The Bihar hairy caterpillar is a sporadic pest and is widely distributed in the Orient. In India, it is very serious in Bihar, Madhya Pradesh, Uttar Pradesh and the Punjab as a polyphagous pest, particularly of sesamum, mash (Phaseolus mungo), mung (P. aureus), linseed, mustard, sunflower and some vegetables.
Damage is caused by full-grown caterpillars which measure 40-45 mm in length and are profusely covered with long greyish hair. The moth measures about 50 mm across the wing spread. The head, thorax and under side of the body are dull yellow. The antennae and eyes are black.
Life-Cycle:
The pest breeds from March to April and again from July to November. It passes the hottest part of the summer (May-June) and winter (December to February) in the pupal stage amidst plant debris. Adults emerge from the overwintering larvae in March. The moths are nocturnal and they mate during the night.
The female lays 412-1241 light-green, spherical eggs in clusters on the underside of leaves. The eggs hatch in 8-13 days and during the first two stages, the tiny caterpillars feed gregariously, but afterwards they disperse widely in search of food. They grow to maturity through 7 stages, within 4-8 weeks.
When full-grown, larva spins a loose silken cocoon in which pupation takes place in plant debris or in the soil. The pupal stage lasts 1-2 weeks in the active period and the moths live for about a week. The life-cycle is completed in 6-12 weeks and the pest passes through 3 or 4 broods in a year.
Damage:
The caterpillars eat leaves and soft portions of stems and branches. In severe infestation, the plants may be completely denuded of leaves.
Management:
(i) The young caterpillars can be killed easily by dusting the infested crop with malathion 5 per cent @ 25 kg/ha.
(ii) When they are full-grown, it is difficult to kill them and very high doses of the pesticides are needed. The chemical control measures are same as in case of red hairy caterpillar.
4. Cutworms:
The term cutworm or surface caterpillar is applied to the larvae of several species of noctuid moths which have, in common, the habit of biting through the stems of seedlings at ground level and eating the leaves or the entire seedlings. The various species of cutworms have a wide range of food plants, both cultivated and wild.
The majority of cutworms found in India fall under the genera Agrotis and Euxoa. Of the recorded species, only four are widespread in the plains and have considerable economic importance. Agrotis ipsilon (Hufnagel) is the commonest and Ochropleura flammatra (Denis & Schiffermuller) is occasionally abundant in India, the latter being particularly destructive in the north.
The moths of these species have the curious habit of hiding in sheltered spots or in thatched roofs of houses during cold weather. Agrotis segetum (Denis & Schiffermuller) and A. biconica Kollar are also quite common in India, being more so in the South. Their life-history seems to be very similar to that of the Agrotis spp.
(i) Gram Cutworm, Ochropleura Flammatra (Denis & Schiffermuller) (Lepidoptera: Noctuidae):
The gram cutworm is one of the most important pests of chickpea or gram in northern India, particularly in the Punjab and in the sub- Himalayan region. Its distribution extends to Europe, Syria, Persia, western Siberia and Pakistan. It is a polyphagous pest and besides gram, it feeds on the seedlings of many vegetables and other plants such as potato, cucurbits, peas, okra, wheat, piazi (Asphodelus tenuifalius), tobacco, opium, poppy, etc.
Only the caterpillars cause damage. The adult is a heavy-bodied, greyish- brown or wheat-coloured insect which measures 5.0-6.2 cm in wing expanse. Its fore wings have characteristic markings and smoky patches, two-thirds of the proximal areas being pale.
On each wing, there is a semi-circular spot below the pale area and a greyish-brown kidney-shaped spot towards the apical area. The caterpillars are dark grey or dull green and their skin is smooth and greasy. The full-grown caterpillar is 40-50 mm in length.
Life-Cycle:
This pest is active from October to April in the plains and it probably passes the summer in aestivation in the adult stage. Since it is active in the mountains during summer, it is sometimes thought that perhaps there is a migration of moths to and back from the mountains. The moths probably appear in the plains sometime in October, to lay yellowish-white eggs on the under surface of leaves, on shoots, stems, or in the soil, within 5-9 days of their short life-span of 7-13 days.
A female lays up to 980 eggs in its life-time. The eggs hatch in 4-7 days during summer and 10-14 days during winter. The larvae remain hidden in soil during the day time and feed at night on young shoots or underground tubers. They are full grown in 4-7 weeks and then make earthen cells in the soil for pupation.
The pupal stage lasts 12-15 days but during winter it extends up to 5 weeks. The life-cycle is completed in 7-11 weeks and there are generally two generations in a year. Judging from the sudden appearance and disappearance of this pest, it would seem that it is a migrant. This insect is a strong flier and is active only at night.
The adult moths are reported to aestivate in the thatched roofs of houses in summer.
Damage:
This cutworm is a sporadic pest of major importance. In some years, 50 per cent of the gram crop may be destroyed. The caterpillars spend the day hiding near and about the plant bases. They remain in the top 5-10 cm of the soil near the plants that might have been cut the night before.
At night, they come out and become active, cutting down the young plants of gram, potato, vegetable seedlings, etc. just above or slightly below the surface of the soil. They seem to be very voracious eaters and they fell more plants than they can consume.
Management:
(i) The pest can be controlled with fenvalerate 0.4 per cent dust @ 25 kg per ha.
(ii) Alternatively drench the soil around the plant and the ridges with chlorpyriphos 20EC @ 2.5 litres in 1000 litres of water/ha, at the appearance of the pest.
(ii) Greasy Cutworm, Agrotis Ipsilon (Hufnagel) (Lepidoptera: Noctuidae):
This is a pest of worldwide occurrence and is found in America, Europe, North Africa, Syria, Japan, China, Indonesia, Australia, New Zealand, Hawaii, Sri Lanka, Myanmar and India. It has been reported from almost all the potato growing regions of northern India, forming a continuous belt from the Punjab in the west to Bengal in the east and Madhya Pradesh in the south.
It causes considerable damage to potato. In the Punjab, it is not as common as O. flammatra, but along with other cutworms, it causes much damage to the gram crop. Besides gram, this polyphagus pest has been reported feeding on potato, tobacco, peas, wheat, lentil, mustard, linseed, maize, sugarcane, cucurbits, bhang (Cannabis sativa L.), vegetable seedlings and several weeds.
The other cutworm, turnip moth, Agrotis segetum (Denis & Schiffermuller), is a European species, but it is also found in Asia and Africa. The caterpillars attack the roots and lower parts of stem of a wide range of plants, and can be particularly serious pest of root vegetables and cereals.
Damage is caused by the caterpillars only. The slightly yellowish caterpillar, on emergence, is 1.5 mm long with a shiny, black head and a black shield on the prothorax. The full-grown larva is about 42-45 mm long and is dark or dark-brown with a plump and greasy body. The adult moth measures about 25 mm from the head to the tip of the abdomen and looks dark or blackish with some greyish patches on the back and dark streaks on the fore wings.
Life-Cycle:
The pest is active from October to April and probably migrates to the mountains for further breeding during summer. The moths appear in the plains in October and come out at dusk and fly about until darkness sets in. They oviposit at night and lay creamy- white, dome-shaped eggs in clusters of about 30 each, either on the undersurface of the leaves of food plants or in the soil.
The number of eggs laid by a female varies from 199 to 344. Oviposition continues from 5 to 11 days and the duration of the egg stage varies from 2 days in summer to 8-13 days in winter. The newly hatched larvae feed on their egg-shells and move like a semilooper.
The larval stage varies from 30 to 34 days in February-April. The advanced-stage larvae may become cannibalistic. The caterpillars are found throughout the winter and become active at night when they cut off and fell the young plants. During the day, they hide in cracks and crevices in the soil. When full-grown, they make earthen chambers in the soil and pupate underground.
The pupal stage varies from 10 days in summer to 30 days in winter. The moths usually emerge at night. The life- cycle is completed in 48-77 days and generally three generations are completed in a year. It is a cold-weather pest and is active from October to March in the plains. It suddenly disappears with the onset of summer during April and is not traceable during the off-season, from April to August-September.
Damage:
The young larvae feed on the epidermis of the leaves. As they grow, their habit changes. During the day-time they live in cracks and holes in the ground and come out at night and fell the plants by cutting their stems, either below the surface or above the ground.
The cut branches are sometimes seen to have been dragged into the holes where the leaves are eaten at leisure. The larvae may also bore into cabbages. Damage to the rabi crops may go as high as 37 per cent and to the potato crop may be as high as 80 per cent.