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Evidence Mounts Questioning Insecticide Use in Tropical Rice
March 7, 1995
A new analysis of insecticide use in rice in the Philippines
and Vietnam has found that a large proportion of the
insecticides currently used are unnecessary. Researchers at
the International Rice Research Institute and the Visayas
State College of Agriculture in the Philippines and the
Ministry of Agriculture in Vietnam interviewed over 950
farmers in selected areas of these countries to compare
insecticide use patterns. During the study, researchers found
that high proportions of the sprays were targeted at leaf-
feeding insects and accounted for 42% and 28% of insecticide
sprays in Vietnam and the Philippines, respectively. However,
insecticide use patterns in areas studied did not appear to
reflect pest situations. In both sites, there were no
serious pest attacks during the periods when the surveys were
done.
In Vietnam, farmers use insecticides to protect their crops
from leaf damage in the early stages because they perceive
that this damage can lead to yield loss. Farmers often
believe that insects are the main constraints to high yields.
Studies have shown, however, that such leaf damage does not
reduce yields. Using insecticides in this manner can lead to
a disruption of natural control mechanisms, which then
require additional sprays against the secondary pest, the
brown planthopper.
The study points out that large yield increases attributed to
insecticide use have been documented often under conditions
of "maximum protection," and although these figures
frequently represent rare and abnormal cases, they continue
to be used by researchers and policy makers. Researchers'
perceptions of pest losses are often based on generalizations
from single-period and short-term experiments. The authors'
evidence indicates that the results of such experiments may
have influenced policy makers to assume that intensification
of rice cultivation necessarily leads to increased pest
losses and that high production is not possible without
higher pesticide inputs. The authors conclude that these
perceptions, although not necessarily true, have strongly
influenced the rapid increase in insecticide use on tropical
rice.
A study in Indonesia has shown that when farmers reduced
insecticide sprays on rice crops from more than four to about
one per season, average yields rose from 6.1 to 7.4 tonnes
per hectare. Other recent studies show that when pesticide-
related health impairments are explicitly accounted for, the
natural control (i.e. using no pesticide) option becomes most
profitable.
Almost all of the farmers interviewed (96% in Vietnam and 89%
in the Philippines) sprayed at least one pesticide during a
growing season. Application rates were higher in Vietnam,
with a mean of seven sprays per farmer, compared to only
three in the Philippines. Over 90% of the pesticides sprayed
were insecticides. Approximately half of the insecticides
used were organophosphates, including methyl parathion,
monocrotophos, methamidophos and chlorphyrifos.
Of the insecticides sprayed, 22% in the Philippines and 17%
in Vietnam were WHO Class Ia (extremely hazardous), primarily
methyl parathion. Another substantial proportion were WHO
Class Ib (highly hazardous) -- 17% in the Philippines and 20%
in Vietnam. These included monocrotophos, methamidophos,
azinphos ethyl, carbofuran and triazophos.
The study concludes that farmers protect their crops from
leaf damage in the early stages because they perceive that
this damage can lead to yield loss. However, in most cases,
natural populations of leaf-feeders are unlikely to cause
sufficient yield loss to justify any use of insecticides. As
a result, a large proportion of insecticides used in rice
production is unnecessary. The authors recommend focusing
research and training on ways to change farmers' perceptions
of pests, and thereby reduce the amount of pesticides they
use.
Source: "An analysis of insecticide use in rice: Case studies
in the Philippines and Vietnam," International Journal of
Pest Management, 1994, 40(2).
Contact: K.L. Heong, Entomology and Plant Pathology Division,
International Rice Research Institute, P.O. Box 933, 1099
Manila, Philippines.
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