Tiny terrors

  • 29/11/1995

Tiny terrors FLEET-FOOTED and wary, the rodent is an adversary humans can undoubtedly do well without. But so far, no device has yet proven to be combative enough for this nuisance that has per~Bandicoota bengalensis , SIS e smce ancien Imes. Apparently, even the Egyptians and Mesopotamians (if archaelogical findings are any indication) had waged a relentless battle against rodents more than 7000 years ago. Can the rodent menace never be eradicated? Evidently not, according to Ishwar Prakash, earlier with the Central Arid Zone Research Institute ( CAZRI) Jodhpur and now a professor of eminence at the Desert Regional Station of the Zoological Survey of India (ZSI}. With more than 30 years of expertise in this field to his credit, Prakash is probably one of the most experienced rodentologists in the country. According to him, eradication of the rodent population is impossible. Rodent population control is a more practical option.

The rodent population is distributed equally throughout the country and can be classified three broad heads -Sylvan (wild species like the Bandicoota indica), Peri-commensal (which includes .. the common Indian Gerbil, Tattera indica and ~ Bandicoota bengalensis) and Commensal (like the " common house rat, Rattus rattus and the common Know house mouse, Mus musculus).

Rodents pose a twofold menace -they harbour a variety of fleas which can cause diseases from the deadly plague to the less fatal Salmonella fever, and secondly they are a serious threat to agricultural crops and livestock.

In order to prevent the outbreak of plague, it is important to mipimise the scope of rodents of different ecological habitat coming in close proximity of . one another. But with a drastic change in land- Erad,cat, use pattern and increasing irrigated agriculture, rodent the lush green crop fields and continuous presence o I SOl moisture In a Itlon to t e supply of green food, several rodent species multi- impossit plied drastically. Under favourable conditions, it Rodent is common to find "a rodent mother begetting a litter of 15 annually instead of five," observes populati Prakash. A recent Zoological Survey of India trol remc report endorses his claim. A study carried out by a British Zoologist Ryley in 1913, on different more prc rodent species (B bengalensis, R rattus, T indica) option m at Mt Abu had classified each one as 'scarce'. The advent of irrigated agriculture in the region, however, has made the ecology so conducive, that all these species are :bundant' now (ZSI report, 1993-94).

Rodent population management has to be a continuous process, the key factor being the surveillance of the P commensal rodents and the fleas they carry. A rodent (specifically a rat) can carry a maximum of six fleas under healthy conditions. The fleas (which carry y pestis) like all other living organisms need a source of food and conducive habit for sustenance. If a rodent exceeds its carrying capacity, the fleas in this over-pop- ulated surrounding become particularly dangerous. Incessant attempts to feed on the host result in regurgitation of blood and a large number of bacilli back into the bite-wound. Fleas nesting in a densely populated rodent tend to fly off in search of another host, most typically a rodent which carries a lesser number of fleas. The P commensal rodents, the most important link in the plague infection cycle, must be checked periodically for the number of fleas they carry. Rodents harbouring a large number of fleas and whose serum test positive for plague bacillii are particularly dangerous , thy enemy: examining rodents and their fleas in Seed vectors. Prakash insists that such studies should be carried out at least once every four months to warn us in time (See box: Pugmarks of death).

Among the various methods of rodent population man- agement on a large scale level, spraying chemicals like DDT/BHC ( whose efficacy is ensured only when repeated .f every 6-12 weeks), is preferred. Fumigation is by t,on o far the most effective method. It involves spraying the rat burrows with cyanogas, ensuring an ,. .almost 100% morality rates for the rodents, the Ieas an t e1r arvae. ut cyanogas lS extreme y 'ble. poisonous and has to be handled by skilled staff under suitable supervision. Generation of such skilled manpower, however, has ~ion con- remained a problem in the country. Some other Jains a modern methods in vogue involve the use of .anticoagulants.

'"actlcal For the purpose of rodent population management on the medium scale, that is, in agricul1ural crop fields, baiting and trapping are the most effective methods. The ideal time to plant fike pfa baits is when the ground is ploughed before sowing the rabbi and the kharif crops. The burrows are loosened and the rodents come out more frequently in search of food and search for safer places to hide.

Rodents are initially suspicious of the availability of an entirely new source of food in the locality. The bait must also be as attractive as other natural sources of food. Normally cereals like rice, wheat, millets prove irresistible lures. On the first and the third day of a week-long 'baiting programme' the pre-baits (usually a mixture of the cereals and a little groundnut oil) are planted. These are harmless, just to induce the rodents to shake off their 'bait-shyness' and accept the new food. On the fifth day, poisoned bait is introduced. An ideal poison bait is 2% acute rodenticide (usually Zinc phosphide), 2% groundnut oil (which enables the powdery poison to stick to the grains) and 96% food grains. Zinc phosphide when ingested, releases the deadly Phosgenegas and claims a very high mortality rate. The sixth and the seventh days are scheduled for collection and disposal of the dead rodents and closing the burrows. Trapping also remains a useful device; the common snap-trap or the breakout trap is often the most effective. Sixty to seventy traps should be planted per hectare.

Certain chemicals like Cadmium chloride are proven anti-fertility agents in rodents. But these are extremely species -specific and not feasible for use to effectively keep the rodent population in check.

Though all these methods look quite effective, the difficulties arise during t1k execution. Wrong timing in planting the baits is a significant factor. Also, till recently, the government's reluctance to hand over the rodenticide directly to the agriculturists, for fear that these lethal chemicals would be mishandled, proved to be a major stumbling block.

A number of traditional local practices for keeping away rodents have been known to yield good results. Tribals in the hilly areas of Arunachal Pradesh, Mizoram and Nagaland periodically plant ginger around the main crops. The pungent ginger quite successfully deters the rodents from nearby bamboo forests. Rodents are not too fond of crops like sorghum and clusterbeans also. Some local com munities in the Deccan use these to keep the 1 rodents away.

Tribals like Irulas and Kurwas in Tamil Nadu and Nats in Bihar are traditional rat catch ets. They need to be encouraged more, as all vis- tas should be explored for effectively countering the rodent menace. "Both urban dwelling and rural houses can be very easily made rat-proof. It just requires imagination (like tin-plating the edges of the floor and walls) and a little will" says Dr Prakash. "Keeping them out is the most effective strategy for combating rodent-borne diseases."

Rodent population management essentially, remains a national problem. It calls for a basic change in the attitude of both the urban and rural people as well as the government which should set up properly manned units to monitor rodent population periodically, specially in the plague prone belts of Andhra-Tamil Nadu, Karnataka and Gujarat- Maharashtra.

Procrastination with plague prevention programmes can prove to be costly. The appalling callousness shown by a majority of the urban population in letting garbage pile up everywhere, must stop while the culture of tolerance inspired by religious taboos towards these harmful rodents, must be done away with. "The rodent population management programme has a gloomy and dark future," sighs Dr Prakash, unless, of course, we change our attitude towards it."

Given the backdrop of mounting large-scale rodent infestation, a bleak prediction indeed, considering that attitudes in our country are not easy to change!

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