Learn about the pests that damage maize and its control.
1. Maize Borer, Chilo Partellus (Swinhoe) (Lepidoptera: Pyralidae):
It is the most destructive pest of maize and sorghum in Sri Lanka, India, Pakistan, Afghanistan, Uganda, Central and East Africa. It is found throughout India. This insect has also been recorded on bajra (Pennisetum typhoides), sugarcane, Sudan grass, baru (Sorghum halepense), sarkanda (Saccharum munja) and some other grasses.
Its caterpillars damage maize and sorghum by boring into the stems, cobs or ears. The grown up caterpillars are about 20-25 mm long and dirty greyish white, with black head and four brownish longitudinal stripes on the back. The adults are yellowish-grey moths, about 25 mm across the wings when spread.
Life-Cycle:
The insect breeds actively from March- April to October and for the rest of the year it remains in hibernation as a full-grown larva in maize and sorghum stubble, stalks or unshelled cobs. The larvae pupate sometime in March and emerge as moths in the end of that month or in early April.
They are active at night, when they mate and lay eggs on the underside of the leaves of various host plants, particularly the early crops of maize and sorghum grown as fodder. The eggs are flat, oval, yellowish and are laid in overlapping clusters each containing up to 20 eggs. A female lays over 300 eggs during its life-span of 2-12 days and the eggs hatch in 4-5 days in summer.
The young larvae first feed on the leaves, making a few shot holes and then bore their way downwards through the central whorl as it opens. More shot holes become visible, indicating an earlier attack and the plant also shows dead-hearts. In sorghum, the midribs are attacked by the young larvae, more than one plant being attacked by a larva.
The larva becomes full-fed in 14-28 days, passing through six stages and after making a hole in the stem pupates inside it. The life-cycle is completed in about 3 weeks and there are probably 5 generations in a year. The full-grown caterpillars of the last generation hibernate in stubble, stalks, etc., and remain there till the next spring.
Damage:
The freshly hatched larvae begin to feed on the tender leaves, making pin holes and leaf windowing. After feeding for few hours on leaves, they enter through the mid whorl into the shoot. The caterpillar attacks all parts of the plant except the roots. In case of younger plants, the growing point and the base of the central whorl are infested resulting in the drying up of the central whorl and forming the dead heart.
In case of older plants, the stem is riddled with. The attacked plants remain stunted in growth and produce no grain. More than one caterpillar is found in a plant. Maximum damage is caused in the month of August to maize while during September and October to sorghum. The pest remains active in the field from March to November.
In Rajasthan, Delhi and southern districts of Haryana, it is a very serious pest of sorghum, particularly of the grain crop. Damage to sorghum fodder is perhaps equally great, but it is not generally noticed. In the central and northern districts of the Punjab, it is the most serious pest of maize. From 25-40 per cent of the young plants are destroyed and around Solan (Himachal Pradesh) up to 90 per cent of the plants have been found infested. Sorghum and bajra are also attacked at the time of grain formation.
Control:
(i) The potential for carry over may be reduced by destroying the stubble, weeds and other alternate hosts of the stem borer by ploughing the field after harvest.
(ii) Removal and destruction of dead-hearts and destruction of infested plants showing early pin-hole damage has been found to be successful practice in reducing the pest incidence.
(iii) Destruction of crop residues and chopping of stems harbouring diapausing larvae could be very effective in reducing borer population.
(iv) Release Trichogramma chilonis Ishii @ one lakh per ha on 10-15 days old maize crop.
(v) Spray the crop 2-3 weeks after sowing or as soon as borer injury to the leaves is noticed with any of the following synthetic pyrethroids using 150 litres of water per ha- fenvalerate 20EC @ 100 ml/ha, cypermethrin 10EC @ 100 ml/ha or deltamethrin 2.8 EC @ 200 ml/ha. Usually, no additional spray is required after the spray with pyrethroids. Alternatively, the crop should be sprayed with any of the following insecticides: monocrotophos 36SL @ 275 ml/ha or carbaryl 50WP @ 250 g/ha.
2. European Corn Borer, Ostrinia Nubilalis (Hubner) (Lepidoptera: Pyraiidae):
It is a pest of maize in Europe, erstwhile USSR, UAR, Japan and other countries of the Far-East, Mexico and North America but it does not occur in India. Besides maize, it feeds extensively on sorghum, other millets, potato, bean, beet, celery, hemp, cowpea, soybean, aster, dahlias and some weeds. Its host range includes over 200 species of plants.
The eggs of this insect are flattened, scale-like and about 1 mm in diameter. They are white, when freshly laid, but change to pale yellow later on. The newly emerged larva is 1.5 mm long, has black head and pale yellow body, with rows of black or brown spots on the dorsal surface.
The full- grown larva is 25.4 mm long and flesh or cream-coloured, with a faintly spotted dorsum. The pupa is 12.7-19.05 mm long and light to dark reddish brown. The female has pale yellow to light brown wings with an expanse of 31.75 mm. The male is slender, and is slightly smaller and has darker wings than those of the female.
Life-Cycle:
This insect passes through 1-5 generations depending upon the climatic conditions. The full-grown larvae pass the winter in diapause inside tunnels made in the stalks, stubble and cobs. Before changing into pupae, the larvae make circular exits for the moths to escape in the late spring or early in summer.
The female lays about 400-600 eggs in clusters of 3-50 on the underside of the leaves. The eggs hatch in 3-12 days, depending upon the temperature. The newly emerged larvae of the first generation wander and feed sparingly on the surface of the leaves in a whorl.
In the third and fourth instars, they feed on the sheath and the mid-rib. Later in the fifth and sixth instars, they bore into the stalk. All the larvae do not survive owing to various causes. More than one larva may often be found in a single plant. As many as 167 eggs and 42 larvae have been recorded in a single stalk and 42 larvae in one cob.
The larvae pass through 5 or 6 instars and become full- grown in 25-44 days. Pupation takes place within the tunnels made in the stalk, stubble or cobs. The pupal stage lasts 8-11 days. The female moth lives for 6-24 days, depending upon the weather.
Damage:
The attack by the first-generation larvae results in a severer damage than that by larvae of the second or the subsequent generations. As a result of the attack, the plant weakens and the cobs get stunted. Tunneling by larvae of the second generation weakens the grown up plants. This results in stalk breaking, ear-dropping and shrivelled ears.
Control:
The use of resistant or tolerant varieties of maize will minimize losses caused by this borer. The ploughing-under of the crop residues and sanitary measures result in the reduction of pest population, as the overwintering larvae are destroyed to a great extent.
3. Asian Maize Borer, Ostrinia Furnacalis (Guenee) (Lepidoptera; Pyralidae):
Only the larvae of this pest are destructive and they make tunnels into the stalks and the ears. The pest is widely distributed in Sri Lanka, China, Taiwan, parts of India, Bangladesh, Indonesia, Japan, Korea, Vietnam, Laos, Cambodia, Malaysia, Myanmar, Thailand and the Philippines. It feeds on maize, sorghum, millets, Indian hemp, hops, Artemisia and many wild grasses.
The grown up larva is about 20 mm in length with dark brown head and is pinkish on the upper side of the body. The female moth is yellow to light brown in colour, with a wing expanse of about 27 mm. The male is dark brown and has a tapering abdomen.
Life-Cycle:
The female moth lays 500-1500 eggs in flat masses on the underside of leaves. The eggs hatch in about 3 days and the newly hatched larvae feed within the leaf whorl, under a protective silken web. After about 9 days of feeding they migrate downwards and bore into the stem. They feed there and complete five instars within 18 days. Normally, pupation takes place inside the tunnels made in the stem but the larvae may also pupate inside the maize ear, in-between the ear sheaths or in the ear stalk.
The pupal stage lasts 6 days and one or more generations may be completed in a year depending upon the prevalent temperature and the host plant on which they-feed. In colder climates the caterpillars hibernate in the stalks or the cobs and over-winter in that stage of development. They pupate next summer when the crop is ready in the field.
Two parasitoids namely, Brachymeria obscurcita (Chalcididae) and Xanthopimpla stemmator (Thunberg) (Ichneumonidae) have been recorded on the pupae.
Damage:
Maize plants are usually attacked 3-4 weeks after sowing. The larvae first feed on leaf tissue, then tunnel into the midrib, and thereafter reach the tassels or bore into the stalks and the ears. The development of the plants, the production of tassels and of the ears may be affected. When the pest attack is towards the base of the stalk the plant collapses, dries up and dies. At some places there are heavy crop losses.