Ecological Basis for Low-Toxicity Integrated Pest Management 185dynamics and are not usually a major yield-reducing pest. Rice water weevil (Lis-
sorhoptrus oryzophilus) introduced from the Caribbean area in North America and
North-East Asia is a problem pest requiring intensive sampling (Way et al, 1991)
that deserves greater research on its natural enemies. In upland ecosystems, white
grub species and population dynamics are not well studied and are difficult to
manage. Way et al (1991) provide an overview of insect pest damage dynamics,
while Dale (1994) gives an overview of rice insect pest biology.
Diseases
The need to grow more rice under increasingly intensive situations leads to condi-
tions that favour diseases. High planting density, heavy inputs of nitrogen and soil
fertility imbalances result in luxuriant crop growth conducive to pathogen inva-
sion and reproduction. This is made worse by genetic uniformity of crop stand
that allows unrestricted spread of the disease from one plant to another, together
with continuous year-round cropping that allows carry over of the pathogen to
succeeding seasons. Reverting to the less intense, low yield agriculture of the past
may be out of the question, but a thorough understanding of the ecological condi-
tions associated with the outbreak of specific diseases may lead to sustainable forms
of intensification. We briefly describe the specifics for three major diseases of rice,
namely, rice blast, sheath blight and rice tungro disease.
Blast (Pyricularia grisea, Magnaporthe grisea) occurs throughout the rice world
but is usually a problem in areas with a cool, wet climate. It is a recognized prob-
lem in upland ecosystems with low-input use and low yield potential, as well as in
irrigated ecosystem with high input use and high yield potential (Teng, 1994).
Fertilizers and high planting density are known to exacerbate the severity of infec-
tion. Plant resistance is widely used to control the disease, but varieties often need
to be replaced after a few seasons because pathogens quickly adapt and overcome
the varietal resistance. Recent work by IRRI and the Yunnan Agricultural Univer-
sity demonstrated that the disease can be managed effectively through varietal mix-
tures (Zhu et al, 2000; see Box 9.3).
Sheath blight (Rhizoctonia solani) is a problem during warm and humid peri-
ods and is also aggravated by dense planting and nitrogen inputs above 100kg/ha–1.
No crop plant resistance is known for sheath blight. A number of bacteria (Pseudo-
monas and Bacillus) isolated from the rice ecosystem are known to be antagonistic
to the pathogen. Foliar application of antagonistic bacteria at maximum tillering
stage appeared to effect a progressive reduction of disease in the field over several
seasons (Du et al, 2001). Incorporation of straw and other organic matter, with its
effect on soil fertility, pH, and possibly on beneficial microorganisms may reduce
sheath blight incidence in the long term.
Rice tungro disease, caused by a complex of two viruses transmitted by the
green leafhopper (Nepthettix virescen), is a destructive disease in some intensively
cultivated areas in Asia where planting dates are asynchronous (Chancellor et al,
1999). Overlapping crop seasons provide a continuous availability of host that