Rodents including rats, mice and squirrels are gnawing animals having chisel like teeth. They have highly developed sense of smell, taste and hearing. They are omnivorous and feed on grains, vegetables, fruits, meat and other products in the houses or in the field. Rodents are responsible for causing enormous losses to crops and stored grains.
Besides feeding on these products, they destroy a substantial quantity by spillage and contamination with their droppings, urine, body hair, etc. Rats daily consume food equal to about 10 per cent of their body weight and the figure may go up to 30 per cent in case of mice.
They damage about 20 times the amount they actually consume, by their gnawing activity and by polluting the food grains. Rodents represent about 35 per cent of the total mammalian fauna but their population outnumbers the total population of living mammals put together. They inflict, on an average, about 5-20 per cent loss in food production alone.
Of the 84 species of order Rodentia in India, 50 belong to the family Muridae. The important ones belong to two sub-families, viz., Murinae and Gerbillinae. Murinae has 18 species of genus Rattus, 2 of Bandicota, 7 of Mus, 2 of Apodemus and a few each under eight other genera. The subfamily Gerbillinae has 2 species of Gerbillus, one each of Tatera and Meriones. Family Sciuridae has 10 species of flying squirrels and 13 species of tree squirrels in India.
On the basis of the reports on the damage inflicted by rodents to crops and stored food materials, about 10 rodent species are of major importance. The commonest species found in houses throughout India are Rattus rattus, Mus musculus and Rattus norvegicus.
The species of field rats most widely distributed throughout the country and causing damage to crops are Bandicota bengalensis, Rattus meltada, Tatera indica and Mus spp. However, in certain regions they may be outnumbered by some other species, for example, Meriones burrianae in the Indian desert and T. indica in peninsular India. The population structure and intensity of rodents differ with time and agro- climatic conditions of the region.
In crop fields of Punjab, T. indica dominated in early seventies and was replaced by R. meltada in mid and late seventies. With intensification of paddy cultivation during the last two decades, B.bengalensis has become the most predominant rodent in summer. According to the estimates made in wheat and groundnut fields, the population of Tatera indica is 50-75 individuals per hectare in the peak season.
1. Indian Mole Rat, Bandicota Bengalensis (Gray) (Rodentia: Murinae):
This is a fierce animal and grunts when angry. A god swimmer, it is covered w ith thick harsh fur, greyish-brown or black on top and greyish-white on the belly. It is heavily built and has a pig-like face and a short stumpy head. It measures 16-24 cm in length (without the tail) and the average male weighs 326 g and the female 287 g. The tail, which is scaly and has 160-170 rings, is often shorter but is sometimes as long as the body.
It breeds throughout the year and the numbers of the young per litter vary from 6 to 15. In southern India, the mole-rat breeds during September-October and January-March, coinciding with the maturity of paddy. The non-breeding adult individuals live separately in their own burrows.
The mole-rat hoards large amount of food in its burrow. Up to 7.3 kg of wheat ears has been found hoarded in a single burrow. The burrows are underground and shallower than those made by Tetera indica. There are 2-12 openings in a burrow and they are generally closed with loose excavated soil during the day- time.
The tunnel has 2-5 lanes of which some are blind and have food-chambers at the end. The food materials, such as groundnut, paddy, wheat, etc. are placed in these chambers in an orderly manner and they are sealed with balls of soil. The brood chamber is at the end of a zig-zag alley, having a lining of grass and straw.
The presence of the mole-rat in its burrow is detected from a freshly dug earth piled on the closed opening. The burrows are generally found in the fields of groundnut, wheat, gram, sugarcane, maize, sorghum, cotton, paddy and in fruit orchards. They are also concentrated in uncultivated areas where there may be sarkanda (Saccharum munja) growth. The mole-rat cuts the entire paddy ear but eats only part of it. Like other species, it migrates from the harvested fields to the newly sown crops.
2. Soft-Furred Field Rat, Rattus Meltada (Gray) (Rodentia: Murinae):
It is smaller than the mole-rat and its fur is soft and dark brownish-grey except the belly which is pale grey. This species is found in wetlands as well as in drylands having many habits similar to those of B. bengalensis. These rats make simple burrows in the field as well as in the bunds. It is 10.0-15.6 cm long and weighs about 64 g.
The tail is a little shorter in length than the rest of the body. It has two subspecies- R.m. pallidor occurs in northern and eastern India, and R.m. meltada in southern India. In northern India, this nocturnal and gregarious rat produces young ones in two breeding seasons, viz. March and August. A female produces 1-4 litters during the breeding season and the interval between two litters is 20-44 days.
In captivity, the number of young ones per litter varies from 1 to 8. In the spring, they attain sexual maturity in 3-4 months, whereas in the autumn in 7 months. This rat makes burrows which are smaller and narrower than those of B. bengalensis and T.indica. There are 1-4 openings to a burrow and they are always kept open, but a few bits of grasses are kept in front of the openings.
In southern India, the breeding season coincides with the maturity of paddy in September- October and again in January-April. They may live alone, in pairs or in groups of l-9.The number of young ones per litter is 2-12 and there may be more than one litter in a season. In the summer when food is not available, they feed on grass or on crabs and snails. The length of the burrow ranges from 88 to 240 cm and the width, 3 to 7 cm. There are 1-4 bed chambers, each of the size 13 × 3 cm.
In southern India, these rats outnumber B.bengalensis in July-August but their population declines steadily and they are hardly seen in the spring. They are fairly good swimmers and have the characteristic habit of cutting a paddy ear and swimming back to the bund with the booty.
3. Indian Gerbil, Tatera Indica (Hardwicke) (Rodentia: Gerbillinae):
It is also known as the antelope-rat and is found in sandy soils. A poor swimmer, it is light brown or reddish from above and white on the belly. It is 11.6-20.3 cm long and weighs 115 g. The tail is 20-30 per cent longer than the body and it is hairy, being of dark shade on the upper and lower surfaces and a pale shade on the sides. The tail ends in a tassel. It is a very sensitive and fast-moving rat and lives in colonies.
In captivity, it has two breeding seasons:
(i) From last week of March to mid-May
(ii) From the last week of August to mid-October.
A female produces 1 -3 litters, there being 3-8 young ones in each. In the field, there are 5-10 young per litter.
Unlike the mole-rat and the meltad, the Indian gerbil has its burrows in sandy areas, dryland patches near the crop fields or river beds. The burrow opening is often located under the cover of a thorny bush or some vegetation, but the scattered freshly excavated soil helps in its detection. The burrows are deeper and shorter in length than those of the mole-rat. Like other gerbils, T.indica feeds on seeds in the winter; stems, rhizomes and insects in the summer, and on leaves and flowers of plants in the monsoon.
4. Indian Desert Gerbil, Meriones Hurrianae (Jerdon) (Rodentia: Gerbillinae):
It inhabits the arid and semi-arid regions of Punjab, Haryana, Rajasthan and spreads up to Pakistan, Afghanistan and Iran. It is a diurnal rat and digs extensive burrows with 10-15 openings. Its head and body is 100 mm, tail is equal to or slightly shorter than head and body is covered with black tassel of hairs at its tip. Merion gerbil breeds all the year round with peaks during February-March and July-September. Litter size varies from 1 to 9 (average 4.4). The gestation period averages 30 days. It causes damage to crops and debarks the trees.
5. Indian Field Mouse, Mus Booduga (Gray) (Rodentia: Murinae):
It is found in dry as well as in wetlands and is a fairly good swimmer. It is brown from above and whitish or dull grey on the underside. The tail is equal to or shorter than the body and is dark above and pale below. The mouse measures 5-8 cm and weights 16-19 g. In southern India, it breeds in September-October and February-June.
It makes small burrows without any branches, having only one bed chamber. The field-mice live in pairs and are common in the same habitat as of the mole- rat. The length of the burrows ranges from 50-116 cm and its diameter is 2-3 cm. The number of young ones per litter is 6-13. It feeds on paddy grains and at times steals from the food chambers of B.bengalensis.
Damage Caused by Rodents:
The nature and extent of damage vary from crop to crop. Rats prefer cereals to dicot seeds and grains. In the wheat crop, little damage is done to the seedlings and most of the damage is caused at the ripening stage. The estimated loss to cereal yield is 52 kg per hectare in the Punjab. The presence of alternative crops and the lack of shelter reduce the damage to some extent.
Damage to the groundnut crop in the seedling stage is high. Later on, only branches are cut but at maturity, the loss is again heavy because the rats dig out pods and eat the kernel, leaving behind empty shells. At places where no alternative food crops are available, the damage can be the maximum during the growing season. The estimated loss of groundnut yield is 47 kg per hectare in the Punjab.
The damage to sugarcane in the seedling stage is negligible and it starts from October onwards and continues up to harvest. The attacked canes are not a complete loss, but are gnawed usually from the 1st internode to the 5th. In the lodged crop, however, all internodes may be damaged, resulting in greater loss.
The rats uproot paddy nurseries to eat the seeds and later on they also cut down the plants. In the vegetative phase, the damage starts when the plants form 3-4 nodes and damage continues in the reproductive phase till harvest. In the advanced tillering stage, damage can be heavy but some of the early-attacked tillers recover and produce late earheads which may mature.
After maturity, the damaged patches of the crop are easily detected as the plants are still in the vegetative or in the flowering stage. It has been noticed that the rats restrict their activity within 5-10 metre radius around the burrow. Damage by rats to coconut fruits is characteristic in that they make one or two holes near the stalk and feed on the carpel.
The loss to rabi crops by the rodents has been worked out at 11.5 per cent in wheat, 5-8 per cent in barley and 9.9 percent in gram. The loss to the kharif crops is still greater. The percentage loss varies from 4.6-5.4 in paddy, 4.1-25.8 in groundnut, 14.0 in maize, 2.2 in sugarcane and 5.0 in coconut. Rodents cause more damage at -seedling and ripening stages of the crops.
Average damage to wheat is 2.9 per cent at seedling and 4.5 per cent at ripening stage, and to pea is 1.1 per cent at seedling and 5.9 per cent at ripening. In winter maize they cause 10.7 per cent damage. The rat damage in wheat crop near sugarcane fields, waste land, canals and roads may reach up to 25 per cent. The extent of damage varies with fluctuation of the rat population in an area, which changes from year to year.
Apart from eating the grains and other crops, rats are known to spread deadly diseases among man and livestock. Rattus rattus, Bandicota bengalensis and Tatera indica carry the bubonic plague. Moreover, they gnaw electrical wiring, disfigure furniture, burrow inside the houses and cause the sinking of the floors and walls. In the field, their burrows interfere with irrigation, causing reduction in crop yields.
Management of Rodents:
The various techniques to manage rodents include cultural, mechanical, biological and chemical control practices.
1. Cultural Control:
Deep ploughing upto 45 cm, reduction of size and trimming of field bundhs at the time of land preparation would certainly go a long way in controlling rodents. Weed management and removal of burrows reduce availability of alternative food and shelter. Simultaneous planting prevents rodent migration from harvested to unharvested fields.
2. Mechanical Control:
Guarding of rat attacks by means of rodent-proof containers and plastering storage structures help in checking rodent infestation. The rodent problem can be solved to a large extent by obstructing the entry of rats in houses and stores. Trapping is an economical and effective way of reducing rodent population. Trapped rats should be killed by drowning cages in ponds and dead rats should be buried deep in the soil. The trapped rats should not be released in the fields as they usually come back to their original place.
3. Biological Control:
A number of predators like snakes, owles, eagles, mangooses, etc. contribute to natural regulation of rodent population. Keeping cats in houses also checks the rat population.
4. Chemical Control:
The two most commonly employed chemical control measures include poison baits and fumigation.
i. Poison Baits:
The most effective method of controlling rats is the use of poison baits.
The poisons used in the baits are of two types:
(a) Acute poisons which are used in a single dose, e.g., strychnine hydrochloride, zinc phosphide, norbormide (Raticate), sodium fluoro-acetate, thallium sulphate, ANTU (Alphanaphthyl thiourea).
(b) Chronic poisons which act as blood anticoagulants and are used in multiple doses.
They include hydroxy coumarins (Warfarin, Fumarin, Tomarin, Recumin) and indandions (Pival, Radione, Valone). These poisons are lethal when consumed for several days, as they cause external and internal haemorrhage. The other anticoagulant rodenticides (Brodicacoum, Bromadiolane) are lethal in a single dose but the rats die after several days of poisoning.
Zinc phosphide (2% bait) is the most commonly used rodenticide in India. The bait is prepared by mixing 1 part of zinc phosphide with 40 parts of whole or cracked grains of wheat, gram, maize, bajra or sorghum smeared with vegetable oil. Racumin bait (0.0375% bait) is more effective against the bandicoot rat than other species.
The bait is prepared by mixing 50 g of 0.75 per cent racumin powder, 20 ml of groundnut or sunflower oil and 20 g of powdered sugar in 1 kg of cracked wheat or any other cereal. Another bait (0.005% biomadiolone bait) can be prepared by mixing 20 g of 0.25 per cent bromadiolone powder, 20 g of oil and 20 g of powdered sugar in 1 kg of any cereal flour.
The mixture is wrapped in paper packets which are distributed in the field at 10-15 m distance. For better results in the field, the poison operations with zinc phosphide should be preceded by careful prebaiting, i.e., false baiting with non-poisoned bait for 1-2 days. Field operations with zinc phosphide result in 70-80 per cent reduction of rodent population.
The rest of the population acquires bait shyness after a single exposure to the bait. Poison baiting with zinc phosphide should not be repeated within 2-3 months. The residual population should be controlled by the fumigation of burrows or by baiting with warfarin.
ii. Fumigation of Burrows:
Another effective method of rat control is killing them inside the burrows by fumigation. Aluminium phosphide tablets (2 tablets of 0.6 g or half tablet of 3 g per burrow) have been found to be very effective and safe. After introducing a tablet into a live burrow, the opening is closed tight with soil. The chemical reacts with soil moisture and deadly phosphine gas is generated.
These methods of killing rats are effective only when carried out on a large scale, covering large contiguous areas, and are repeated time and again. The aim should be to kill more than 90 per cent of the population, otherwise they breed so fast that population reaches the same level within a few months.