HERBICIDES in Asian rice - IRRI books - International Rice ...

Community education also is important in creating a realistic perception of what
can be achieved by a biological control agent. Charudattan (1990) believes that the
number of pathogens that might be suitable for bioherbicide development will be
severely limited if the demand for efficacy is similar to that achieved with chemical
control. High efficacy, a demand which may originate in the private sector, is unnecessarily
rigid when biological weed management is considered within the broader
framework of IPM.
Concerns that weeds may act as reservoirs of disease and insect pests have not
been substantiated. The question should be, is weed suppression instead of elimination
an environmental luxury or an economic benefit
The presence of a diversity of weed vegetation near or within a crop that will
sustain populations of beneficial predators of harmful insects is an advantage of IPM
that has been recognized for some time (Altieri and Whitcomb 1979). Moody (1990)
listed numerous records of weed/insect and weed/disease associations, while acknowledging
that the listing was uncritical. Whether weed populations pose a risk as reservoirs
of crop diseases should be quantified.
In determining risk, the following questions arise: How many weed-crop disease
associations have been studied in the field How many of the associations reported
were based on greenhouse studies where no alternative food supply was available to
the potential insect vector or pest Was pathogenicity proved by realistic methods
Did the pathogen (particularly fungi) sporulate on the infected plant Was the virus or
mycoplasma able to be acquired from infected weeds by an appropriate vector
Merely recording the occurrence of a disease (especially of a virus) or of known
disease vectors on a weed species in proximity to rice provides only circumstantial
evidence that the weed may be acting as an inoculum or pest source. Speculations
(Karim and Saxena 1989) must be viewed with caution: each association should be
examined in detail. For example, Borromeo et al (1993) have shown that populations
of Pyricularia oryzae -infected weeds did not provide inoculum to infect rice crops in
the Philippines.
Similarly, the presence of an insect pest on both rice and an adjacent weed does
not automatically mean that the weed population is the source of the pest. Heinrichs
and Medrano (1984) found that different populations of the brown leafhopper
Nilaparvata lugens on the weed Leersia hexandra and on rice did not trade hosts. The
insect complex on weeds, however, is important in managing leafhoppers on rice; the
leafhoppers on the weeds are attacked by the same predators, parasites, and pathogens
as the leafhoppers on the rice. The number of leafhopper predators on weeds can
be several times greater than the leafhopper population, and can migrate to attack
leafhoppers on a rice crop (Xiaonan and Wenxi 1987).
RESEARCH ON BlOHERBlClDES FOR RICE
Most research on studying inundative bioherbicide weed control has been directed at
controlling weeds important in capital-intensive agriculture (Charudattan 1991). The
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