Pests of vegetables

Дата канвертавання25.04.2016
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GENERAL INTRODUCTION: Insects are among the most abundant and successful of terrestrial animals. On one hand they not only damage or destroy crops and domestic animals but also transmit diseases of plants and animals. On the other hand they may be valuable in providing beneficiary products and pollinating crops and in controlling other species of noxious insects and weeds.
Insect pest of crops become a threat to the farmer when their population level exceed a certain level at which they can inflict economic loss. This is termed as the economic threshold level, which is defined as the population level of the pest at which the damage caused by the pest in terms of yield is equal to the cost of controlling the pest. This is the threshold at which control measures should be applied.
Generally when considering crop pests and their control the following aspects should be considered.

a) State of the pest population (i.e. number of individuals present)

b) Nature of damage done to the crop

c) Value of damage as assessed by human society

It should be borne in mind that there exists some sort of 'ecological balance' in nature whereby the pest population may be kept in check by natural enemies like parasitoids and predators. Free and excessive use of pesticides can not only upset this natural balance resulting in an upsurge of pest population but also bring about other side effects like resistance development, pollution, poisoning etc.
To sum up: constant monitoring of the pest population and the severity of the damage caused to the crop is necessary to see whether a pesticide control is essential. Be far-sighted and encourage natural and cultural control as far as practicable and try to minimize pesticide use as much as possible.


CUTWORM (Agrotis segetum, A.ipsilon, Euxoa spp)
Agrotis segetum is the commonest and most widely distributed. It is also called greasy cutworm because of the greasy appearance of the full-grown caterpillar. They are known as cutworms because they cut and fell to the ground either the whole plant or part of it. They are confined to the top few centimetres of surface soil. Seedlings of tomato, chilli, cabbage and maize are affected; it may also attack root crops like carrot, turnip and potatoes. It is a pest of sporadic importance and occasionally the damage can be serious.

DESCRIPTION: The adult moth is medium sized with a wingspan of 30-40 mm. The fore-wing is grey-brown with a dark brown or black kidney-shaped marking. During the day they hide under the leaves, in cracks and crevices. They are often seen flying about from the time it is dusk till it is quite dark. The caterpillar is clay-coloured, 30-40 mm long when fully grown with faint dark lines along the sides of the body. The pupa is smooth shiny brown and 20-25 mm long.


Eggs are laid in a series of batches or clusters of about 30 eggs usually on the leaves and stems of weeds or crop plants. Each female can lay up to about 1000 eggs. Each egg is round, dome-shaped and sculptured with radiating ribs of a greenish-white tinge. The hatching takes place within two to six days or more and the young larvae begin to feed on their eggshell as their first meal and fall to the ground at the slightest disturbance and hide under fallen leaves. There they feed on the epidermis of fallen leaves or the green leaves touching the ground during the early stages but as they develop they move down into the soil and assume the cutworm habit. They remain in the soil during the day and come to the surface at night to feed. The full-grown larvae have the habit of coiling up at the slightest disturbance. After 4 to 5 weeks of larval life, it enters deep into the soil and pupates in an earthen chamber. The pupa is about 20 mm in length and the stage lasts for 10 to 30 days depending on the climate.

DAMAGE: Larvae spend the day hiding in the litter or in the soil and may feed on plant root and tubers. When in large swarms, they may even come out during the day. During the night they come to the soil surface and feed on plant stems, cutting through young stems close to the ground. Sometimes they partially drag part of the plant and this is a sure index of the place of hiding. The destructive peculiarity of the pest is that it destroys much more than what it actually eats. If left uncontrolled, they will fell many young plants in matter of few days. As seedlings mature and stems become hard, it cannot fell the plant.

Generally cutworms are quite difficult to control, by the time infestations are apparent the susceptible stages of the larvae are often past, and damage may be already quite serious. And in most areas the sporadic nature of cutworm population outbreaks makes preventive treatments rather futile.

Deep ploughing - Will bring larvae and pupae to the soil surface for exposure to predators.
Flooding the field for several days before planting may be feasible for some crops grown on paddy terraces. It should however be borne in mind that flooding the field after growing a crop may adversely affect the plants.
Weed destruction - Weeds are often preferred sites for oviposition and food for the first instar larvae - so keeping land weed free for several weeks before planting will reduce the number of larvae in the soil. The weeds on edges of the field should be removed.
Closer plant spacing: This method is pracitised by farmers growing chillies. They transplant chilli seedlings at a closer distance to compensate for the loss of any seedlings. It really does not matter much because the final spacing would remain adequate.

Handpicking of full grown larva is a practical method for small holdings. However, constant vigilance during early mornings is required to be able to spot any fresh damage so as to locate the cutworm.
Baiting techniques:

In the fields where cutworm problem is expected to be very severe, baiting can be done with a mixture of rice bran 1 kg + Fenvalerate(Sumicidin) 7.5 ml + sugar 100 gms (10 % of bran) with 1100 ml of water. This mixture may be applied in each planting hole at the time of transplanting the seedlings. Other suggested chemical control methods are:

Chemical Trade name Application rate

Chlorpyrifos Durmet 4 ml of 20% EC/1 lit. water

The spray should be directed along the plant rows, aiming at run-off to the soil below.
PIERIS BRASSICAE (Large white butterfly)
CROPS ATTACKED: Brassicas (cabbage, cauliflower, mustard)

The adult is a large white butterfly with a wing span of 5 – 7 cm. The forewings have black tips. In females, the upper side of the forewings has two black spots, while in the males the spots are located on the underside of the wings. The caterpillars when fully grown are about 25 mm long with three longitudinal yellow stripes and many black spots.

LIFE CYCLE: The eggs are orange coloured and are laid in batches of 20-50 on the underside of the leaves. They hatch in 8 -10 days. The larvae on hatching, feed in groups for 25 - 30 days. Pupation takes place on a vertical surface on the plant itself or they may pupate on a tree trunk or a wall. Pupation lasts for about 2-3 weeks in summer but may also last throughout the winter season if pupation started late in the season.
DAMAGE: Larvae feed in groups, initially on the underside of the leaf surface as a result of which leaves of brassicas may be completely defoliated.
CONTROL: The adult butterfly is quite characteristic and can be seen easily in the field, while laying eggs. The eggs are easily visible and can be handpicked and destroyed. However, care must be taken not to pick the eggs of the ladybirds, which are also orange coloured and of the same size. Closer examination will reveal the ladybird eggs to be smooth, whereas the eggs of Pieris brassicae are with longitudinal ridges. During the earlier stages when larvae feed in groups, they can be collected and destroyed or can even be fed to poultry birds.

In cases of severe attack, the following chemicals are recommended.

Chemical Application rate

Cypermethrin 0.5 ml /1 lit. water

Chlorpyrifos 4 ml /1 lit. water
CROPS ATTACKED: Brassicas (Cabbage, Cauliflower etc.)
DESCRIPTION: It is a tiny greyish green insect, 2–3 mm long, and usually covered with a heavy coating of ash-coloured mealy wax (powder) that also is found spread throughout the aphid colony. On account of the waxy colonies it forms, B. brassicae is unlikely to be mistaken for any other aphid. The antenna is much shorter than the body.
DAMAGE: Heavy attack on young plants may retard their growth. The plants also become covered with wax and cast skins, and moulds may grow on the honeydew produced by the aphids, thereby affecting the quality. The aphid is also a vector of virus diseases.
LIFE HISTORY: Reproduction in B. brassicae is mostly parthenogenetic (asexual – without mating) and viviparous (producing young ones directly instead of eggs). This sort of reproduction occurs throughout the summer and in places where winter is mild. In colder regions, sexual reproduction may also take place. Eggs are laid during November-December and probably hatch towards the end of March in Bhutan. There are four nymphal stages prior to the adult stage. In spring and summer, crowding of nymphs usually results in the production of alates or winged aphids. The lifecycle is completed in 11-45 days depending on the temperature. These aphids can double its population within a few days under favourable conditions. The insect is greyish green in colour and covered with white powder.

CONTROL: Parasites and predators are important in keeping the pest in check. The larvae of Syrphid flies (hover flies), and both young and adult of ladybird beetles prey on the aphids. These should be encouraged as much as possible by reducing the use of chemicals to the minimum. Because of the rapid build up of population, chemicals if used have to be applied before before the build up of heavy infestations.

Chemical Application rate
Cypermethrin 0.5 ml /1 lit. water
Chlorpyrifos 4 ml /1 lit. water

CROPS ATTACKED: Cabbage, cauliflower and a range of other crops including maize.
LIFE CYCLE: Eggs are round and laid singly on the leaves. The caterpillars are very variable in colour but are green when young, later becoming pale brown. Pupation takes place in the soil.
DAMAGE: The larvae bore into the heart of cabbage eventually rendering it unfit for market. Although few in number as compared to Pieris brassicae, the resulting damage is more severe in this case than the other.
CONTROL: Control should be carried out early and before the larvae have a chance to burrow into the heart of the cabbage. Recommended chemicals are:
Chemical Application rate

Cypermethrin 0.5 ml /1 lit. water

Chlorpyrifos 4 ml /lit. water
DAMAGE: Newly hatched larvae move to the underside of the leaf where they burrow into the leaf tissue. Older larva also feed on the lower surface of the leaf but eat all the tissues except for the upper epidermis which is left intact so that 'windows' appear through the leaf or they may eat right through the leaves.
LIFE CYCLE: The eggs are laid singly or in small groups on the leaf. The full grown larva is pale green or blue green and 10-12 mm long. Pupation takes place inside a silken cocoon located on the underside of the leaf close to the midrib. The adult is a small grey moth. When at rest, a diamond pattern of light brown marks is visible along the middle of the back and the tips of the wings are turned up at the end.
CONTROL: In case of severe infestations, spraying may have to be done at intervals throughout the season and if fresh damage appears. Synthetic pyrethorids may be particularly effective against this pest, but resistance quickly develops.
Chemical Trade name Application rate
Cypermethrin Ripcord 1 ml 10% EC/3 lit. water

DELIA PLATURA (Bean seed fly)


CROPS ATTACKED: Beans and Brassicas
DAMAGE: The larvae burrow into the roots collar region and cotyledons of seedlings, so killing them. The first sign of damage is wilting of the seedlings.
LIFE CYCLE: The eggs are laid in the soil and it is said that the flies are attracted to dung and decaying plant material. The larvae are white, about 4 mm long, with a blunt rear end and tapering to a point at the head end. Pupation takes place in the soil.
CONTROL: Insecticidal seed dressings are effective against this pest but in many countries resistance has been developed to insecticides. Pre-planting treatment with Gamma HCH or sprays of Gamma HCH may be carried out when infestations are serious.
Chemical Trade name Application rate
Gamma HCH Hilbeech 4-5 gm 50% WP/1 lit. water


TETRANYCHUS URTICAE (Two spotted spidermite)


CROPS ATTACKED: In Bhutan, recorded only from beans but is known to infest many crop species elsewhere.
DAMAGE: Feeding on the leaves causes them to become yellow and finely speckled. The leaves may finally wither and may be shed.
LIFE HISTORY: The female mite is yellowish green with two large dark spots. Males are also yellowish green but with numerous dark spots.
CONTROL: Sprays of Kelthane may be effective in controlling this species.
Chemical Trade name Application rate
Dicofol Kelthane 4 ml 18.5% EC/1 lit. water



DAMAGE: Leaves, flowers and pods are fed on by the caterpillars, but more serious damage is done in the pods where the seeds are destroyed.
LIFE HISTORY: Eggs are laid singly in flowers or buds. The full grown caterpillar is greenish with rows of black spots along the back and is about 15 mm long. Pupation takes place in the pod. The adult moth has brown, fore-wings with white spots, whilst the hind-wings are silvery grey with brown tips.
CONTROL: If infestation is serious, it can be controlled with foliar sprays.
Chemical Trade name Application rate
Cypermethrin Ripcord 0.5 ml 10% EC/1 lit. water
Fenvalerate Sumicidin 1 ml 20% EC/2 lit. water
Fenitrothion Folithion 1.5-2 ml 50% EC/1 lt. water



DAMAGE: Attacked plants are yellow and stunted and often many are dead. Stems just above and below soil level are thickened and usually cracked.
LIFE CYCLE: The eggs are white and laid singly in holes made on the upper surface of the young leaves, near the petiole end of the leaf. The larva is small and white in colour. It bores down inside the stem where it feeds just above and below the ground level. The leaves turn yellow and some develop longitudinal cracks. Pupation takes place in the stem where the larvae have been feeding. The adult is a tiny black fly.
CONTROL: Successive, overlapping crops of beans should be avoided. Crop residues should be destroyed and volunteer plants removed. Some control can be achieved by the use of synthetic pyrethroids.
Chemical Trade name Aplication rate

Cypermethrin Ripcord 0.5 ml 10% EC/1 lit. water



CROPS ATTACKED: Pea and Cabbage

DAMAGE: Larvae mine into the leaves

LIFE CYCLE: It is presumed that eggs are laid by the fly directly into the leaf. The larvae burrows under the upper epidermis and the mine gets wider as the larva grows. Pupation takes place within the mine with the end of the black pupal case protuding from the mine enabling the emerging fly to escape.
CONTROL: The pest does not seem to occur in sufficient numbers to be worth controlling. If infestation becomes heavy then the following chemical can be applied.
Chemical Trade name Application rate
Pirimiphos-methyl Actellic 2 ml 50% EC/1 lit. water

200-400 lit./acre

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