INDIA'S Green Revolution in the'60s had a baneful side to it too- the use of high-yielding seed varieties led to a spurt in fertiliser and pesticide application. This was especially true of rice, the staple food of a majority of Indians and other Asian people. The continent accounts for over 90 per cent of the worlds rice production. But Asia's warm and humid climate, the intensive use of fertilisers, and the practice of multiple cropping created conditions favourable for pest growth. Also, the high-yielding rice varieties being promoted were susceptible to pests. The pests became a menace and the scientific community's only answer to the problem was to encourage farmers to use more pesticides.
But far from being an answer to the pest menace facing rice farmers, pesticides are today becoming a part of the problem. Scientist, who in the '60s had promoted pesticides, are now asking farmers to reduce pesticide use. Besides adversely affecting the health of the farmers, pesticides can kill the natural enemies of the pests, instead of the pests, and thereby cause, rather than prevent, a pest outbreak. Having finally recognised the drawbacks of pesticides, several Asian nations are implementing integrated pest management (IPM) programmes to reduce pesticide use in rice, the region's largest crop.
More than 100 species of insects attack the rice crop, but most of the damage is caused by 3 pests - brown planthoppers, stemborers and leafhoppers. Even though plant breeders have developed several varieties that are resistant to the major rice pests, farmers have not decreased pesticide use. There are 2 main reasons for continued high pesticide use: varietal resistance has not lasted - for example, in 1977, in Indonesia the resistance Of IR26 to brown planthopper broke down in 3 months - and often a given variety is resistant to few but not all rice pests.
Says Peter Kenmore, the regional programme coordinator for Food and Agricultural Organisation's (FAO) integrated pest control programme in rice in South and Southeast Asia, "Nearly every recorded outbreak of the planthopper in the tropics has been associated with use of insecticides."
A study conducted by the International Rice Research Institute (IRRI) at Los Banos in the Philippines has shown that of the 39 commonly used insecticides by the Filipino farmers can cause brown planthopper resurgence.
In order to understand how the excessive use of pesticides can cause pest outbreaks, one has to understand the ecology of the rice field.
Every rice field has an abundance of insects but only some of these are pests. A majority of the other insects are predators and parasites that target rice pests, thereby keeping the pest population under control. Pesticides can potentially disrupt the natural control process by killing the natural enemies, not the pests, and thereby permitting the pests to multiply unchallenged. This argument presumes that the natural enemies of rice pest are more vulnerable to pesticides than the pests, a fact that is borne out in the case of the brown planthopper.
The brown planthopper has 3 predators, each of which attacks the pest at a different stage of its development. The plant bug Cyrtorlhinus lividipennis) feeds on the planthopper eggs, devouring up to 200 eggs a day; the water strider bugs (Microvelia douglasi atrolineata) kill newly hatched brown planthoppers and wolf spiders (Lycosa pseudoannulata) concentrate on older planthoppers, consuming upto 45 adults every day.
So what goes wrong? Explains K L Heong, entomologist and coordinator of the IPM programme at IRRI, "The brown planthopper eggs are embedded in the stem of the rice plant and, therefore, are protected against pesticides, unlike their predators', the plant bug." Similarly, adult planthoppers are protected by a hard abdomen unlike their predators - spiders, who have soft abdomens and are, therefore, easily wiped out by pesticides. Besides, when the plant canopy becomes thick, which is after about 45 days, penetration of pesticides is difficult and they become less effective. Heong draws an analogy between the use of pesticides against the brown planthopper larvae and the use of bombs by the Americans against the Vietnamese soldiers. Like the Vietnamese soldiers safe in underground tunnels, the pests hide inside the protective cover and are unharmed by the pesticides.
The disruption of the natural balance between the pests and its enemies can also accelerate the process by which the resistance of rice varieties to major pests is broken. Biological variations among the pests permit some strains of the pests to feed on a pest-resistant variety. Without pesticides, natural enemies are able to control the population of pests that can feed on the resistant variety. But with the natural enemies killed, as happens when pesticides are used, the pesticide-resistant strains can multiply unchecked and ravage the crop.
Besides playing havoc with the paddy ecosystem, pesticides can adversely affect the health of rice farmers and those who spray pesticides. The use and unsafe handling of hazardous pesticides can lead to pesticide poisoning while prolonged exposure to pesticides can produce ' chronic ailments such as cardiopulmonary, neurological and skin disorders .
In the Philippines, of the 4,031 acute pesticide poisoning reported by the Department of Health hospitals between .1980 and 1987, 603 proved fatal. Says IRRI agricultural economist P L Pingali, "The number of poisoning is likely to be underestimated, since most Ases do not reach the hospital, and rural health officers may not always correctly diagnose pesticide poisoning."
According to Heong, farmers tend to overuse pesticide because they are unable to differentiate between pests and friendly insects. A study of pest management practice of 45 farmers from the province of Laguna, conducted by IRRR and the University of the Philippines at Los Banos (UPLB) found that 31 per cent farmers thought that all insects were enemies of rice. Only a few farmers were able to identift spiders, dragonflies, and grasshoppers as natural enemies of rice pests. Most farmers (80 per cent) resorted to pesticides when they saw "any" type of insect because they believed that could damage the rice plant.
Sometimes even when farmers are able to differentiate between pests and predators they overuse pesticides because they overestimate the damage caused by a particular pest. The rice leaffolder (Cnaphalocrocis medinalis) is one such pest. Farmers believe that the rice leaffolder causes substantial yield reductions by feeding on the leaves of young rice plants. However, research at IRW has shown that it takes as many as 15 rice leaffolders per rice plant to affect the yield and the pest population rarely grows to that level. In fact, the rice plant can compensate for any early leaf damage by these insects by later growth.
|Standing up to a pests|
|Resistance of a few International Rice Research Institute (IRRI) varieties to inspect pests|
|Variety||Year of release||Brown planthopper||Yellow September||leafhopper|
|IR64||1985||Resistant||Susceptible||No data available|
|IR74||1988||Resistant||Mildly resistant||No data available|
|Sick on pesticide|
|Hypothesised health effects of chronic exposour to pesticides|
|Eye||Pterygium||Vascular membrane on eye|
|Skin||Eczema||Lichenification and fissuring|
|Respiratory||Bronchial asthma||Wheezing cough|
|Cardiovascular||High blood pressure||
|Gastrointenstinal tract||Chronic gastrities||Nausea and vomiting|
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