Learn about the pests of wheat and its control.
1. Wheat Aphid, Sitobion Miscanthi (Takahashi) (Hemiptera: Aphididae):
Wheat aphid attacks wheat, barley, oats, etc., and is widely distributed in India. Like other aphids, the nymphs and adults suck the sap from plants, particularly from their ears. The insects are green, inert, louse like and appear on young leaves or ears in large numbers during the cold and cloudy weather. The nymphs and the females look alike, except that the latter are larger. The winged forms appear only in early summer.
Like other winter aphids, the wheat aphid breeds at fast rate during the cold weather and reaches the height of its population in February-March when the ears are ripening. The females give birth to young ones and are capable of reproducing without mating. During the active breeding season, there are no males and the rate of reproduction is very high.
When the wheat crop is ripe and the summer is approaching, the winged forms of both males and females are produced and they migrate to other plants like doob grass (Cynodon dactylon). It is not known how the pest passes the summer and the monsoon season. In October-November, the aphids again appear on wheat. If available, barley is preferred to wheat. The losses due to aphids have been reported upto 36 per cent.
These plant lice suck sap from the ears and tender leaves, and decrease yield of the crop. The damage is particularly severe in years of cold and cloudy weather. A heavily manured, well- irrigated and succulent crop will harbour the pest for a longer period and suffer greater damage.
Spray 100 ml of imidacloprid 200SL or 50g of thiamethoxam 25 WG or 30 g of clothianidin 50WDG or 375 ml of dimethoate 30EC or oxydemeton methyl 25EC in 250 litres of water per ha. Control aphids at ear head stage at economic threshold level of 5 aphids/ear head. Since the aphids appear first on the borders of the crop, spray only the infected strip to check their further spread.
2. Armyworm, Mythimna Separata (Walker) (Lepidoptera: Noctuidae):
The armyworm is a pest of graminaceous crops all over the world. In India, it is a sporadic pest of wheat, sugarcane, maize, jowar, bajra and other graminaceous crops. It has gained prominence as a pest of wheat only recently, particularly after the introduction of Mexican varieties. The larvae feed voraciously and migrate from one field to another.
The adult moths of armyworm are pale brown. They live for 1-9 days and lay eggs singly in rows or in clusters on dry or fresh plants or on the soil. The eggs are round, light green, when freshly laid, and turn pale yellow and finally black. In the Punjab, they hatch in 4-11 days from March to May, and in 19 days in December-January. Freshly emerged larvae are very active, dull white and later turn green. In the spring, the larval stage is completed in 13-14 days, but in the winter it is prolonged to 88-100 days.
In the pre-pupal stage, the insect spins a cocoon. The pre-pupal stage lasts 1-11 days during January to May. Pupation usually takes place in the soil at a depth of 0.5-5 cm, but it may also occur under dry leaves among the stubble or fresh tillers. Generally, the larvae before pupation seem to select sites near the water-channels.
The pupal stage is completed in 9-13 days in May and 36-48 days in the winter months. The survival of the pupae depends on the soil moisture. In one study made in the Punjab, the maximum population of caterpillars on wheat was found in March. From May to February in the next year, the population remains rather low and the insects feed on maize, sugarcane, baru grass (Sorghum halepense) and other crops. The population build-up starts in the beginning of March and increases with the rise of temperature in the spring.
The freshly emerged larvae spin threads from which they suspend themselves in the air and then with the help of air currents reach from one plant to another. In the early stages, they feed on tender leaves in the central whorl of the plant. As they grow, they are able to feed on older leaves also and skeletonize them totally. The grown-up caterpillars throw out faecal pellets, which are quite prominent.
In the case of a severe attack by the armyworms, whole leaves, including the mid-rib, are consumed and the field looks as if grazed by cattle. The pest may also eat away ears, including the awns and immature grains. The yield losses upto 42 per cent have been reported from Ludhiana in Punjab.
(i) The pest can be suppressed by collecting and destroying the caterpillars.
(ii) Spray 500 ml of dichlorvos 85SL or 3 kg of carbaryl 50WP or one litre of quinalphos 25EC in 250 litres of water per ha.
3. Ghujhia Weevil, Tanymecus Indicus Faust (Coleoptera: Curculionidae):
Ghujhia weevil is widely distributed in the Indian Sub-continent and is a sporadic pest of considerable importance, feeding on germinating rabi crops, particularly wheat, barley, gram and mustard.
The damage is caused by the adult weevils only and they cut the germinating seedlings at the ground levels. Weevils are earthen grey and measure about 6.8 mm in length and 2.4 mm in width. Their fore wings are oblong and hind wings are more or less triangular, but they cannot fly. Their mouthparts are light brown and are practically concealed beneath the head.
The pest is active from June to December and passes the rest of the year as a grub or pupa in the soil. Weevils emerging in June mature sexually sometime in October. They mate frequently and lay 6-76 eggs in 5-11 installments in the soil under clods or in crevices in the ground.
The eggs hatch in 6-7 weeks and the young grubs enter the soil where they feed, probably on soil humus. They are full grown in 10-18 days and pupate in earthen chambers at a depth of 15-60 cm. The pupal stage lasts 7-9 weeks, and the adults emerge next year in June or July. The pest has only one generation in a year.
The adults feed on leaves and tender shoots of the host plants. The damage is particularly serious during October-November, when the rabi crops are germinating. Weevils cut the seedlings at the ground level and often the crop has to be resown.
Dust carbaryl or malathion 5 per cent @ 25 kg per ha.
4. Gram Pod Borer, Helicoverpa Armigera (Olivier) (Lepidoptera: Noctuidae):
The gram pod borer attacks wheat at maturity. It feeds on the grains in the earheads. The damage is more where wheat follows cotton.
Spray 3 kg of carbaryl 50WP or 2.0 litres of quinalphos 25EC in 250 litres of water/ha.
5. Pink Stem Borer, Sesamia Inferens (Walker) (Lepidoptera: Noctuidae):
The larvae bore into the young plants and feed inside. As a result, the plants turn yellow, dry and ultimately die.
Spray 2 litres of quinalphos 25EC in 250 litres of water/ha.
6. Termites, Odontotermes Obesus (Rambur) and Microtermes Obesi Holmgren (Isoptera: Termitidae):
Termites damage the wheat crop soon after sowing and near maturity. The damaged plants dry up completely and are easily pulled out. The plants damaged at later stages give rise to white ears.
Treat the seed @ 4 ml of chlorpyriphos 20EC or 6 ml of fipronil 5SC or 3 ml of imidacloprid 600FS per kg of seed. Spread 100 kg of seed in thin layers on cemented floor, tarpaulin or plastic sheet and spray 400 ml of chlorpyriphos 20EC or 600 ml of fipronil 5SC or 300 ml of imidacloprid 600FS in 2.5 litres of water.
7. Molya Nematode, Heterodera Avenae Wollen Weber (Tylenchida: Heteroderidae):
This nematode is widely distributed in Europe and Australia and has recently been recorded in Rajasthan, Haryana and the Punjab. It infests wheat, barley, oats and rye, and the attacked plants remain stunted and give a shrivelled unhealthy appearance.
This nematode passes unfavourable season in the form of cysts, mostly in the soil. A cyst consists of the dead body of a female, containing a large number of eggs. When the conditions are favourable, eggs hatch within the cysts and the larvae are set free into the soil in the second stage of growth.
The larvae may invade any underground part of a susceptible plant but most of them enter it at or near the root tips. After moving a short distance through the cortex, they assume a position, more or less parallel to the main axis of the root, with the head away from the tip.
The male increases in girth, until the width is equal to about l/5th of its length and during this period it undergoes the second and third moultings. The body begins to elongate and becomes folded or coiled within the cuticle during the third stage. After assuming the final cylindrical shape, it moults for the fourth time and becomes an adult.
The female does not undergo such metamorphosis, but after the second and third moultings it continues to increase in girth until it becomes ovate. It then undergoes the fourth or final moulting and emerges as a full grown adult. After mating, the eggs mature inside the body of the female and it dies, the body being converted into a cyst.
Although the nematodes cause very little mechanical injury to the roots, yet their presence stimulates the formation of branched rootlets. The main root remains short or bunchy, bearing small galls. In case of severe infestation, the seedlings may fail to come out of the soil and even if they grow a little, the infested plants remain stunted. The plants that escape the early damage produce short stalks and ears, yielding a poor harvest.
8. Wheat Gall Nematode, Anguina Tricici (Steinback) (Tylenchida: Tylenchidae):
The ear-cockle or mamni disease is caused by the nematode throughout the wheat-growing parts of the world. If the black rounded mamni galls are soaked in water overnight, the coat softens and a large number of larvae are set free. They can be observed under a microscope as wriggling thread-like creatures. Besides wheat, the nematode produces galls in rye, spelt and emer, but oats and barley are immune to its attack. The nematode is also the carrier of the bacterial yellow slime ear-rot (tundu disease) caused by Corynebacterium tritici.
Under natural conditions, the dry galls either fall to the ground from the ripe ears or they are harvested and find their way to the stores along with the healthy produce. The galls may remain dry for long periods and yet the larvae within remain viable. A gall may contain from 800 to 30,000 larvae which revive and become active when the gall is moistened.
When wheat is sown, the galls become soft on imbibing moisture and the larvae are set free into the soil. From there, they reach the host plants, if available within a distance of one-third of a metre. They rise up the plant and find a site for feeding as free parasites on the young leaves and the growing-points. Later on, as the plants approach the earing stage, they penetrate into the primordia of the flower-buds and form the galls instead of normal seed.
In the developing galls, the larvae mature into males and females, as the case may be. A single gall at this stage may contain 40 females and an equal number of males. They mate within the gall and the gravid females lay a large number of eggs. The young larvae on emerging from the eggs develop up to the second stage and then become dormant. They remain in that state in the dry galls till the next sowing season. There is only one generation in a year.
Affected plants are more or less stunted and their leaves are wrinkled, rolled or twisted. A variable number of grains in an infested ear may produce galls. The diseased ears are shorter and thicker than the healthy ones and the glumes are spread farther apart.
(i) The wheat gall nematode can be controlled by separating the galls from the wheat seed by floating them on water in a tub. The galls, being lighter, float on the surface and may be skimmed off. The seed should then be dried before sowing.
(ii) The pest can also be suppressed by sowing clean seed in uninfested soil. Only one year’s fallowing is sufficient to eradicate this nematode from the fields.