What is a pest?
In the last century, even in the first few decades of the present century, to the scientists the word ‘pest’ usually signified the species of insects destructive to cultivated plants and stored grains. But with the advancement of knowledge on ecology, the definition of pest not only signifies the insect but other animals also which interfere with human activities. According to Clark etal, 1967 “Animal species destructive to the economic life, convenience, and happiness of human beings are pests”. Any species becomes a pest when it crosses the threshold of abundance and this lime it goes against the interest of the human and is regarded as pest. The science which deals with the life, ecology, damage and control of the pest, is known as pestology. Due to loss of ecological balance and due to cultivation of large areas for crop the insects cross their threshold levels and become the pest. They then leave then host plants and attack the crops as the crops appear to be an immense wealth of food to them. During the act of food procuring they damage the crops in diverse ways. Some deveour on the roots and leaves, some on stems some on seeds etc. as a result the yield falls down sharply. About 13-14% or one crores tons of food crops are damaged by insects.
Paddy is the most staple crop of our country and many insect pests cause great damage to this crop by attacking the roots, stem, leaves and even the young grains when they are in milkstage. More than fifty potential paddy pests are in records of which Tryporyza incertulus, Leptocorisa acuta, Spodoptera mauriti and Hispa armigera and dangerous pests of paddy crop. The ecology, behaviour, life history, damage and control measure of the most menacing paddy pests Tryporyza incertulas, Hispa armigera, and Leptocoriza varicornis, have been discussed in this article.
Those inseed pests which give in a particular area year after year and producing many generation to great harm to the crops, are known as major pests. As yellow stem borer (Tryporyza incertulas), swarming caterpillar (Spodoptera mauritia), Pamripoka (Dicladispa armigera) etc. are important major pests of paddy.
Some insects become pests temporarily due to environmental stress hut the damage caused by them are not so important. As green bug (Schizaphis graminum) becomes minor pest of paddy in some exceptional cases.
Tryporyza incertulus is commonly known as ‘majrapoka’ to the cultivators. It is a moth and belongs to the phylum-Arthropoda, Sub-phylum-Mandibulata, Class-Insecta, Sub-class-Pterygota, order-Lepidoptera, Family-Pyralidae, Genus-Tryporyza, species-incertulus. They are distributed all over India. They are popularly known as yellow stem borers.
Tryporyza is a monophagus pest and only attacks the paddy plants and devours the stem tissue. As adult moths they are harmless but very dangerous at larval stage. Actually the larva is called ‘Majrapoka’ by the farmers. The larvae bore into the stem and consume the central shoot portion. The shoot gradually becomes yellow and ultimately no grain formation results.
The adult male and female moths can easily be distinguished. They possess yellow wing but the female is larger comparatively. The anterior pair of wings of female only possess a black spot each. After copulation the female lays eggs in phases and the eggs numbering more than 600, remain under the lower surface of the leaves. The eggs are placed in groups and in each group there are 40-100 eggs. The egg mass in a group is covered by buff coloured hair. If an infected paddy leaf is examined the egg masses become clearly visible.
The eggs hatch within 6-8 days time and hatching of eggs depend upon the temperature and moisture. The larvae are covered with very fine hair and hence they are known as caterpillars. The baby caterpillars begin to eat the tissue of the leaves and advance towards the leaf apices. Some of the caterpillars hang by their saliva threads from the leaf apices and swing in air and they are driven to another fresh plant and infect them. Some; caterpillars crawl down the plants and reach the central stem, make small bores and migrate within the central tissue. That is why Tryporyza is called a stem borer. The larvae confined within the stem tissue, eat, the tissue and attain a length of 2 centimeters. The larval period extends from 4-5 weeks. A fullgrown larva has a faint yellow coloured body while the head is yellowish orange. The body is with chitinous head, 3 thoracic and 12 abdominal segments. Each thoracic and abdominal segment bears a black spot.
The matured larvae cease feeding and prepare themselves to be metamorphosed into pupa. But before that, they make holes on the stem for the exit of the imago. The larva form cocoons by their saliva and within the cocoon the larva pupate. Time for complete pupation is 10 days.
After 10 days of pupation the pupa is converted into adult or imago moth which makes its exit through the hole prepared by the larva. The moths are attracted by the ligth and prepare to give rise to new generation. 45 day’s time is required to complete the life cycle of Majra Poka (Tryporyza). It may he noted that after harvesting the stubbles left on the field may harbour larvae or pupa which pass undisturbed throughout the winter season without an metamorphosis. They metamorphose only when favourable conditions appear in the next rainy season.
Period of infection
Tryporyza infects Boro, Aus and Aman paddy. They infect Boro paddy from January to March, Aus and Aman from June to November. September is the peak period of infection.
Nature of damage
Tryporyza damages the crop in the larval stage. Due to the effect of baring and consumption of stemtissue the plant gradually turns yellow and ultimately dies. Sometimes they attack the spikelet and thus grain formation is prevented. They are regarded as major pest. The yellow stem borer damages the paddy plants in all stages of its growth. If the seedling is attacked then the seedling do not grow and gradually die. But before the arrival of spikelet if it is attacked by stem borer then the stem of the plant becomes yellow; a condition know as dead heart. But if the stem is attacked before the arrival of the spikelets then the spikelets become white and the grains become chauffy; a condition known as white ear head.
Among the paddy pest rice hispa occupy an important place. They are one type of beetle. In adult the body covering appears to be bluish black. Their skin is covered with small spines. They wander through out the crop fields in swarms. The nymph is very small. The adult acquire a length of 3 mm.
Like Tryporyza Hispa does not lay eggs in clusture instead they lay egg on the apex of the paddy leaves one by one. The eggs are white and granular. Depending on humidity and temperature the eggs hatch into larvae within 4-6 days time. These larvae are known as grubs. Grubs are yellow or yellowish white with fine black spots on its body. Grubs bear only three pairs of legs. They continuously and voraciously eat the greens of the plants as a result only yellow spots are seen on the leaves. If these yellow spots are examined the larvae or the grubs are easily seen. After removing the grubs only white venetion exists in the leaves. The leaves attacked by them become dry and finally roll into a pipe. Within this leaf-pipe the larva is transformed into pupa. Pupa emerges as an imago and within 7 days becomes an adult. Like the grubs the adults also feed on the green of the leaves. Time taken to complete the life history of Hispa armigera is 15 days.
Nature of damage
Hispa attack the paddy plants from the date of its germination on the Seed plot. Before the appearance of flowers in paddy both the adult and the grubs eat of the greens of the plants. As a result the plant dies before fruiting. As the greens are eaten up only white parallel markings of the viens on the leaves becomes prominent and this is the identifying symptoms of Hispa attack. They attack the plants in swarms field after fied and do great damage to plants and yields. So they are also known as major pests.
Period of attack
Aus and Aman-From May to October.
Distribution or where prevalent: From May to October its intensity reaches its peak in West Bengal, Assam, Orissa and Madras. They are less intensive in Punjab, Kerala and Karnataka.
Rice bug – Leptocorisa varicomis (B-Gandhipoka)
Rice bug belongs to the order-Hemiptera, class-Insecta. Their mouth parts are sucking type. They possess repulsive smell and hence they are called gandhipoka.
The Adult is an ugly looking insect, about 15 mm. in length. The body is light green in colour and possess one pair of fine antenna and two pairs of wings.
Life history of Leptocorisa is completed through eggs, nymphs and adults.
Eggs: The female lays granular eggs on the upper surface of the leaves in 10-12 rows. The eggs hatch into nymphs within 6-8 days time.
Nymphs are very small but look like miniature adult but they do not possess any wing. The nymphs develop into adults within 17-25 days time. The life history is completed within 4-5 weeks time and the adult may live upto 5 months.
Nature of damage
Both the adults and the nymphs do great damage to paddy plant. They suck the milk from the paddy grain by their sucking mouth as a result there is no seed formation. Even they suck the juice from the stem and leaves.
Period of activity
They are very much active during July to October during fruiting times of the crops. They are regarded as major pest.
Distribution or where Prevalent: They are abundantly found in West Bengal, Bihar, Orissa, Assam, Karnataka, Kerala, UP etc. In North Bengal they are very much active perhaps due to weather condition.
As these pests live inside the stem of paddy plants and as they are capable of quick migration only controlling measures applied in a particular field will not help to control the damage done by these parasites. However, the following controlling measures may yield successful results. They are:
- Rotation of crop: If one paddy crop is followed by another crop other than paddy then the pest being monophagous, will not find host plants. A generation gap will be created and they will be destroyed due to want of food.
- Removal of weed grass: During adverse condition the pest take shelter in weed-grasses. So the farmer should take care to remove all the weed-grasses. Thus the pests will be destroyed.
- Burning of stubbles: As the larvae and pupae may remain within the stubbles they should be burnt before ploughing.
- Light traps: From the beginning of cultivation light trap should be used to attract the pest and kill them.
Use of Insecticides: Insecticides are poisonous chemical substances that kill the pests. Before transplanting the baby plants should be rinsed with 0.1% DDT* solution. Spraying of 0.025% parathion or 0.08% endrin at the rate of 60-80 gallons per acre, protects the crop from the damage of Tryporyza.
No effective biological controlling measures have yet been discovered to control these pests.