Maclean et al. - 2002 - Rice almanac source book for the most important e

Insects attack all parts of the rice plant.
Hundreds of species feed on rice, but only a few
cause yield loss. The most important and widely
distributed pest species are stem borers,
leaffolders, planthoppers, and gall midge. Stem
borers are chronic pests, found in every field in
every season, but generally at low levels.
Planthoppers and gall midge usually create
localized outbreaks, causing high yield losses in
relatively small areas. Biological control by natural
enemies plays a critical role in the
management of all insect pests. Resistant rice
varieties are of importance in the control of
planthoppers and gall midge. No strong sources
of resistance to stem borers have been found in
rice germplasm, although modern semidwarf rice
varieties generally have lower levels of stem
borer damage than the traditional varieties they
replaced. Insecticides are used extensively
against planthoppers in temperate areas of Asia,
where mass immigration of planthoppers from
tropical areas is a frequent problem.
Bacterial blight, blast, and sheath blight are
the most important diseases of rice and have a
worldwide distribution. Three insect-vectored
viral diseases are also of importance: tungro in
Asia, hoja blanca in South America, and rice
yellow mottle in Africa. Bacterial blight and
blast have been successfully controlled by
resistant varieties for many years. However, the
evolution of resistance-breaking strains of these
pathogens has necessitated the continuing release
of new resistant varieties. Strong sources of
resistance for sheath blight have not been
identified in rice germplasm. Sheath blight is a
particularly important disease in intensive ricegrowing
conditions where high levels of nitrogen
fertilizer are applied.
Integrated pest management
Modern approaches to crop protection rely on
management rather than control or eradication.
In this approach, a pest species is considered a
pest only when it reaches numbers that can cause
yield reduction. Natural factors—such as natural
enemies—that prevent pest populations from
increasing are emphasized. Indeed, most
organisms inhabiting rice fields are never
harmful. In addition, the use of rice cultivars that
are resistant to major pest species is encouraged.
These cultivars do not need prophylactic
treatment to control the insects or diseases to
which they are resistant. Using a combination of
control tactics instead of relying on just one
tactic, such as host-plant resistance or pesticides,
and basing the decisions for control on sound
economic grounds is called integrated pest
management or IPM.
Pesticides should be used only as a last
resort to bring abnormal pest densities down
when crop loss is expected to exceed the cost of
treatment. Pesticides are costly to farmers, can
disrupt natural biological control, and are
damaging to human health and the environment.
In Thailand, for example, it is estimated that
40,000 rice farmers suffer from varying degrees
of pesticide poisoning every year. In the
Philippines, there is a similar high level of
poisoning and the overall cost of crop loss from
pests is less than the farmers’ resulting health
costs. The total extra costs of pesticide use in
Thailand, including health, monitoring, research,
regulation, and extension, amount to $128
million per year.
Environmental effects are also serious in
Thailand, where pesticide residues are found in
nearly all samples of soils, river sediment, fish,
and shellfish. One survey found organophosphorus
insecticide residues in three-quarters
of tangerine samples and one-third of all
vegetable samples.
Farmer education is a central feature of
IPM, and various approaches have been
employed. Many countries have implemented
“farmer field schools,” which entail season-long
weekly meetings in which farmers learn about
the value of natural enemies and other aspects of
growing a healthy crop. Another approach is the
mass communication of “simple rules” to
farmers. A campaign to encourage farmers to
experiment in eliminating early season
insecticide applications has been very successful
in Vietnam and is now being attempted in
Thailand. The campaign used billboards, cartoon
characters, information handouts, and humorous
radio programs; it reduced rice farmers’
insecticide use by an estimated 70% and the
proportion of farmers believing that insecticides
bring higher yields fell from more than 80% to
just 13%.
Landscape manipulation is a new approach
to IPM. The concept of using alterations to the
rice field and its surrounding areas involves the
roles of bordering vegetation and levees, and
26 Rice almanac