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Managing Rice Pests with Less Chemicals Heong, K.L.; Teng, P. S.; Moody, K., International Rice Research Institute, POB 933, 1099 Manila, Philippines ABSTRACT: Losses caused by pests remain an important constraint to achieving high rice yields. Potentials of protecting these losses have stimulated innovations in pesticide development. Today the rice pesticide market is valued at US $ 3.0 billion per year. With reducing land available for rice production and increasing demand for food production, attention is turning towards intensification through higher fertilizer inputs and cropping. Such intensifications may in turn increase pest intensities and demand for more pesticides. A large proportion of insecticide sprays administered by rice farmers in Asia is influenced by misperceptions and overestimations of damages. Most farmers apply their first sprays in the first 40 days after crop establishment to control leaf feeding insects. However, these pests do not occur in sufficiently high densities to cause yield loss. Instead, such early season sprays may contribute towards development of secondary pests, such as the brown planthopper. Strategies to reduce insecticide use need to focus on enhancing naturally occurring biological control and understanding farmers' decision making behavior. Most fungicides used in rice are in the sub-tropical countries, like Japan, Korea, Taiwan and Vietnam. An important strategy towards reduction in fungicide use is through host plant resistance and gene deployment strategies. With biotechnology, tools may be used to characterize population structures in order to enhance these strategies. Cultural practices, such as rotations, cultivar mixtures, crop mosaics and planting times are being investigated. As cost of labor increases, farmers are likely to resort to using herbicides. The best way to accomplish weed control is the simultanous application of a variety of practices. These will include cultural, mechanical and chemical methods. The potentials of using naturally occurring enemies, such as plant pathogens, and the use of allelopathy are also being explored.

Rice is grown in Asia to m e e t family and local m a r k e t needs. Less than half the crop goes to m a r k e t and m o s t o f that is sold locally, with only 4% traded internationally ( I R R I 1993). Rice occupies one-tenth o f the world's arable land. In m o s t Asian rice countries it occupies one-third or m o r e o f total planted area (Huke and H u k e 1990). Since World War II, world rice area has increased by 67%, yields increased by 95%, and total p r o d u c t i o n tripled, making rice the m o s t i m p o r t a n t h u m a n food crop. This e n o r m o u s growth has b e e n brought about by the diffusion o f m o d e r n cultivars, b o t h seeds and the associated farming technology. There is still potential for growth, as these highyielding cultivars have not b e e n universally a d o p t e d nor has their full potential b e e n realized by farmers. W i t h appropriate inputs such as water, fertilizer, and pesticides, and g o o d m a n a g e m e n t skills, the yield o f these cultivars can often be doubled.

A n i m p o r t a n t constraint to achieving higher rice yields is losses caused by pests. Pests are organisms that attack the rice crop or c o m p e t e with it for nutrients, causing yield reductions. The c o m m o n organisms that are pests o f rice are insects, weeds, molluscs, m a m m a l s , birds, fungal and bacterial pathogens, and viruses. M a n y o f these are herbivores that feed on the rice plant, others are parasitic disease organisms and weed species that c o m p e t e with rice for nutrients, light and water. However, only a few o f these organisms can potentially cause losses (Tab 1). U n d e r the worst situations, these pests can cause total crop losses. For instance, the brown p l a n t h o p p e r caused losses a m o u n t i n g to US $ 300 Million in Asia (Dyck and T h o m a s 1979). Blast disease is capable o f causing severe yield losses. In severely affected areas, tungro can cause losses o f 40-60°/0. Stem borers have caused large-scale destruction in West Java, where 8,000 ha were destroyed in 1989 (Rubia 1994).

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Tab 1

Examples of organisms that may harm or compete with the rice crop

specialists, researchers and farmers. Few estimates of actual field losses caused by pests in farmers' fields are available, and the most frequently quoted figures are from Cramer (1967). His analysis showed losses of 34.4% due to insects, 9.9% due to diseases and 10.8% due to weeds, with an overall potential production loss before harvest of about 55%. The potential of preventing such high losses has stimulated innovations in research, both in breeding resistance to pests and in developing pesticides for field applications. It has also encouraged input subsidies in some countries (eg a 40% subsidy for pesticides was introduced in Indonesia (Conway and Barbier 1990). Such a policy has inevitably encouraged pesticide use (Conway and Pretty 1991). The rice pesticides market is currently valued at US $ 3.0 billion (Woodburn 1993), and there is more pesticide being used on rice than on any other food crop. Rapid population growth in Asia will increase demand for food production. With less land available for rice production, such a demand can only be met through increase in yield per unit area. Attention will then be turned to intensification through higher fertilizer inputs and cropping. Such intensification may in turn increase pest intensities and a demand for more pesticides. Pesticides are by design biocides, and their value lies in their ability to kill noxious or undesirable organisms. Many pesticides are available and, depending on their intended target organisms, they are classified as insecticides, fungicides, herbicides, rodenticides and molluscicides. Insecticides, fungicides and herbicides constitute more than 98% of all pesticides used in rice. These three groups have different modes of action, application methods, and toxicity to the environment and human health. In addition, these chemicals are often perceived in different ways by both researchers and farmers. Their use in ricefields is likely to be influenced by ecological, economic, environmental, health and social factors. To address issues related to managing pests with less chemicals, it is necessary to focus on each of these groups separately.

Managing Insect Pests with Less Insecticides Weeds are a constantly present, nonsporadic pest problem. Weed control, in some form, is continuously required and costs farmers throughout the world millions of dollars annually. Worldwide, losses due to weeds alone amount to a quantity of food sufficient to feed more than 250 million people (Stoskopf 1985). Favored by new cultivation techniques, weeds are becoming more of a problem in tropical agriculture. An increase in the number of weeds increases the workload, and reduces both the yield and the quality of the crop. Production losses can reach 30-40% in fields that are poorly weeded (Anon 1988). While incidences of pest outbreaks may be considered rare given the large hectarage of rice grown each year, they often become major concerns to policymakers, extension

The largest proportion of the world rice pesticide market is in insecticides. In 1993 an estimated US $1,114 million or 37% of the total was spent on insecticides for rice (Woodburn 1993). Of this, a large portion was in Japan (34%), China (11%), and Korea (10%), where insect pests are often cited as serious constraints to rice production (eg Kiritani 1979). Large yield increases attributed to insecticide use have been documented under various conditions (Lim and Heong 1984). These figures often represent the rare and abnormal cases, but play significant roles in influencing policies that encourage insecticide use. Farmers also believe that insects are the main constraint to higher yields (Litsinger et al. 1980; Heong et al. 1985; Heong 1984; Escalada and Heong 1993), and often

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overestimate losses due to insects (Lazaro et al. 1994; Rubia 1994). In addition, researchers and policymakers also perceive insects to be most important (Rola and Pingali 1993). Farm surveys in the Philippines and Vietnam showed that about 90% of the sprays administered by rice farmers were insecticides (Heong et al. 1994a). About half of these were organophosphates and the main chemicals were methyl parathion, monocrotophos, methamidophos, and chlorpyrifos. When classified by their potential hazards to human health, about 37% of the sprays were "highly or extremely" hazardous chemicals belonging to W H O Category I.

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35 30 25 20 15 10 5 0

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Early

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Insecticide Misuse

Misuse is defined as the "improper or incorrect use". Thus when an insecticide is used for the wrong target pests or at the wrong time, or both, it can be considered to be misused. In Leyte, Philippines, when farmers' sprays were analyzed by matching timing of sprays and their intended targets, more than 80% of the sprays applied did not match (Heong et al. 1994b). In this case study, about 43% of the insecticides were used against rice bugs (Leptoeorisa spp.), which commonly colonize ricefields in the tillering and ripening stages. However, they are destructive only during the milky stage of the crop, which lasts 7-10 days. The insecticides applied against rice bugs at any other time will not prevent crop loss. Only 17% of the sprays were targeted for this pest at the milky stage (Fig 1). The remaining 83% of the sprays represent misuse of the product. Ladybirds (Micraspis sp.) are common predators, yet about 4% of the sprays were targeted at them.

Early Insecticide Use

Most rice farmers apply their first insecticide sprays in the first 40 days after crop establishment (Heong et al. 1994a). Their main insect targets are lepidopterous larvae, usually referred to as "worms". Farmers in both the Philippines and Vietnam strongly believe that these larvae, particularly the rice leaffolder Cnaphalocrocis medinalis, will cause significant yield loss. However, field infestations with as much as 67% damaged leaves did not cause any yield loss (Miyashita 1985). Bautista et al. (1984) estimated the economic thresholds for the rice leaffolder at booting stage as 1.5 and heading stage at 1.3 larvae per plant. However, they grossly overestimated leaf consumption per larva by at least four times (Heong 1990). Using a computer simulation approach, Graf et al. (1992) suggested a threshold of 3 larvae per hill at the heading stage. Fabellar et al. (1994), on the other hand, found that leaf consumption of 20 larvae per hill did not seem to be sufficient to significantly reduce crop yield.

Fig 1

Percent insecticide sprays used at various crop stages for rice bug control by farmers in Leyte, Philippines

The average densities observed in the rice crop are far below these thresholds. In Japan, the average is less than 2 per hill (Wada and Shimazu 1978) whereas in the Philippines it is less than 1 per hill (Guo 1990). Populations rarely reach more than 5 larvae per hill. There is, therefore, practically no evidence to show that early sprays against rice leaffolders are economically justifiable. However, high proportions of insecticide sprays are used early in the season to control them. Insecticide use among Asian rice farmers seems to be based on perceived needs and perhaps fear, rather than real needs. Indigenous attitudes, such as a belief that all insects, particularly worms, are harmful has tended to make farmers become victims of insecticide abuse (Bentley 1989).

Brown Planthoppers - A Secondary Pest Caused by Early Insecticide Use

It is now well established that broad spectrum insecticides when used in ricefields induce brown planthopper populations (Kenmore et al. 1984; Heinrichs and Mochida 1984; Joshi et al. 1992; Schoenly et al. 1994). Farmers who spray early for rice leaffolder control generally apply such broad spectrum insecticides, as the organophosphates and pyrethroids (Heong et al. 1994a). In a recent study, Cohen et al. (1994) showed that foliar sprays of deltamethrin disorganized the rice arthropod community structure favoring herbivores. It was estimated that the deltamethrin sprays would significantly reduce the mean food chain length (a measure of the food web structure) and cause an increase of about 4 million herbivores per ha per sampling date (Schoenly et al. 1994).

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Resistant Cultivars and Insecticide Use

% farmers using pesticides 100

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Proportion of farmers using pesticides in Nueva Ecija, Philippines from 1960 to 1990. Arrows indicate when various IR varieties were introduced. Except for IR8, the varieties introduced were resistant to insect pests.

The herbivores were mainly two species of planthoppers, the brown planthopper and the whitebacked planthopper. The planthoppers are species characterized by their relatively high reproductive capacities, short life spans, and high mobility. MacArthur and Wilson (1967) described such species as "opportunists" or "r-strategists", which have been selected for their ability to harvest food efficiently. Through a synoptic model, Southwood and Comins (1976) showed that r-strategist's " b o o m and bust" dynamics are related to the ephemeral nature of its habitat, and its population growth is strongly influenced by its finite net rate of increase. Insecticide sprays or any environmental disruptions in the rice ecosystem thus tend to cause habitat instability that will favor r-strategists. In addition, the brown planthopper and whitebacked planthopper typically embed their eggs in the leaf sheaths of rice that would escape insecticide sprays. Egg parasitoids and predators that normally cause high egg mortality, on the other hand, are killed instead. While the extra insects in the sprayed field may not be sufficient to cause serious damage to the crop, it does represent extra pests that can colonize another field. Since these pests are primarily monophagous, the extra pests produced from the insecticide use become potential threats to other farmers with crops at the susceptible growth stages. Thus, early sprays for rice leaffolder control may well be the primary cause of secondary brown planthopper problems (Heong 1993). In Vietnam, where there was a higher proportion of sprays directed at leaf-feeding insecticides, there was proportionally higher numbers of sprays used for brown planthopper control (Heong et al. 1994a). In Leyte, Philippines, only one spray was used for leaf feeder control and practically none was used for hopper management.

Rice varieties that can provide an inherent resistance to insects would seem ideal for reducing pest problems. Since the 1960s, entomologists and plant breeders have concentrated on developing rice cultivars that have resistance to several important pests (Khush 1989). Rice cultivars with multiple resistance to insect pests are now grown on more than 20 million ha in Asia and Central and South America (Pathak and Khan 1994). However, because of inherent genetic variability in pest populations, newly incorporated genes had not been able to remain effective for long periods in most cases. Despite the widespread adoption of resistant cultivars (Ostaka and David 1994), insecticide use in many developing countries has increased or remained high. For instance in the Philippines, where resistant cultivars were sequentially introduced since 1970, insecticide use had correspondingly increased (Fig 2). A survey in 1985 showed that farmers either use the same amount or more insecticides on resistant cultivars (Escalada 1985). Although farmers have adopted resistant cultivars, they do not seem to possess the corresponding knowledge-base necessary for their use. It has also been demonstrated that early insecticide use on a brown planthopper-resistant cultivar resulted in significantly more brown planthoppers (Joshi et al. 1992). Insecticides thus seem to mask any beneficial effects the resistant cultivars might have. For instance, in Indonesia where insecticide use was dramatically reduced, farmers reverted to growing Cisadane, a cultivar thought to have its resistance broken down earlier (Gallagher et al. 1994).

Insecticide Reduction Strategies While elimination or eradication of pests has a clear objective, management of pests does not. How much pest damage can be tolerated? Pimentel et al. (1993) argued that we should be able to reduce "cosmetic" values in crop management. Thus increased presence of insects and insect damage in the rice crop may be tolerated by farmers if they do not cause sufficient loss to warrant action. A widely acceptable concept of economic decisionmaking to avoid crop loss is that of the economic threshold (Stern et al. 1959). They defined economic damage as the amount of injury that will justify the cost of control. In most cases, the threshold represents time for control (Norgaard 1976), assuming that the pest population will increase substantially. Since population developments are uncertain, decisions using economic thresholds are often made under a great deal of uncertainty (Pedigo et al. 1986). In the case of rice leaffolders, where populations typically decline with crop age, insecticides applied when the threshold is reached need not necessarily imply an economically rational action (Heong 1993). Planthopper population characteristics in the tropics differ markedly

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DNA "fingerprints" of the blast pathogen to guide selection of rice genes conferring resistance

from those in temperate regions (Kuno and Dyck 1985; Heong et al. 1992). Since natural mortalities and natural enemy abundance in the early crop period are high (Kenmore et al. 1984; Cook and Perfect 1989; Fowler et al. 1991; Heong et al. 1992), early season spraying in anticipation of pest population buildup can have negative effects. Computer simulation studies have also shown that early season spraying has poor efficiency (Heong 1988; Cheng et al. 1990). To use economic thresholds, farmers would need to recognize the insect pests and count them before acting. For most rice farmers in the tropics, this might not be readily acceptable. The relevance of the economic threshold concept in tropical third world environments, though useful in some cases, may need to be reviewed. There is widespread unnecessary insecticide use in tropical rice (Heong et al. 1994a). Early season insecticide use not only does not benefit rice production, but it can be detrimental to the ecological balance, causing secondary brown planthopper problems. Thus, an immediate strategy

may be to reduce early spraying of insecticides. As farmers' beliefs and risk aversion are deeply entrenched, research efforts to better understand why farmers use insecticides in the first place would be useful. There are perhaps several explanations. In the 1960s and 1970s when there was an urgent need to increase food supply, agricultural research was strongly oriented to chemicals (Rossiter 1975). Documented successes in the use of fertilizers and insecticides from experimental stations (eg Pathak and Khan 1994) have influenced recommended solutions to production problems. Nonchemical sciences, like ecology in general, and insect ecology and agroecology in particular, were underdeveloped and generally not represented in agricultural faculties (Butte11993). This formative period of agricultural research also occurred during the agrochemical era, which has strongly influenced pest management research and rice "green revolution" programs. Prophylactic insecticide applications were "packaged" into these programs and farmers were encouraged to use them at regular intervals just like fertilizers.

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Tab 2

Cultural methods known to work against rice diseases (Teng 1994)

Farmers, who were encouraged by chemical company advertising, extension programs, and the simplicity of chemical applications, widely adopted insecticides. However, the corresponding knowledge-base to use them judiciously was lacking. As pointed out by Bentley (1989), farmers' technical knowledge about pests is related to the pests' high visibility rather than the loss it can cause. These and other factors (Kenmore et al. 1984; Escalada and Heong 1993) may have contributed to the widespread unnecessary insecticide use in tropical rice. To improve the situation, national policies that can contribute to overuse of insecticides should be reviewed, and hopefully replaced with policies that will strengthen the implementation of integrated pest management (IPM) principles, training and promotion of noninsecticide approaches among farmers will have a significant impact on insecticide reduction.

Managing Diseases with Less Fungicides Most of the fungicides used in rice are in the subhumid subtropics, in countries like Japan, Korea, Taiwan, and Vietnam. Resistance to fungicides in the blast or sheath blight pathogens has not been reported (Sato 1988), and given the lack of available control methods when an epidemic results, use of chemicals will probably continue in these countries. Rational fungicide use, assisted by predictive systems, has been adopted in temperate rice. In the tropics, however, such systems are not used, except in Thailand where preliminary work has begun (Disthaporn 1994). Environmental conduciveness for blast epidemics has been determined using computer simulation models that are able to predict the likely effects of climate at a specific locality. This information was used by extension workers, in addition to spore traps and field monitoring, to better inform farmers of when not to spray fungicides

(Disthaporn 1994). Several disease predictive systems in temperate rice countries (Korea, Japan) appear to be successful in rationalizing fungicide use for control of blast and sheath blight (Kim 1987; Ishiguro and Hashimoto 1991). Host plant resistance has been the basis for plant protection in rice for centuries (Bray 1986), and was greatly enhanced by IRRI since 1960. IRRI genotypes with single or multiple gene resistance to diseases are now widely grown (Bonman et al. 1992), but this success has also led to increased selection pressure on pathogens to overcome the resistance. Much of the resistance in rice is of the "vertical", single gene type, and while this has been useful, the many disease epidemics that have occurred attest to its weakness when used as the sole method of plant protection. For example, the blast epidemics in South Korea (1976-79), which resulted from pathogen evolution to overcome resistance in the Tongil-type rice varieties (Dalrymple 1986; Kim 1979), had serious political and social consequences in that country. On the other hand, the managed deployment of major genes conferring resistance to the tungro vector, in the form of mandated variety rotation, has reportedly been successful to control this damaging disease in South Sulawesi, Indonesia (Sama et al. 1991). Biotechnology tools may be used to characterize the population structure of the blast and bacterial blight pathogens. At the same time, molecular marker technology can be applied to determine which genes are present in rice varieties that can match the virulence of the prevailing pathogen population (Fig 3). Knowledge of diseases is thus "captured" in the seed before release to farmers. Andow and Hidaka (1989) found that traditional varieties grown under organic culture in Japan had less disease and lower arthropod populations than conventional farms growing high-yielding cultivars. Methods to mix host genes by deploying these genes either in pyramids in the same cultivar, as multilines, or as multivars (mixtures of cultivars, each with a different gene for resistance) may be useful in disease management. Another way of reducing fungicide use is through cultural control. This includes all those techniques that will make the environment less favorable for pests to develop or multiply, but which still favors rice production. Cultural practices, such as rotations, cultivar mixtures, crop mosaics, and planting times have been part of the local practice in many rice-growing areas (Oka 1988). Flooding, stubble burning, fallow, intercropping, terracing and plant spacing are other practices that have been used by rice farmers for centuries to help in producing a good crop (Bray 1986; Thurston 1990; Oka 1988). The reasons for their effectiveness are often not known, and their impact has not been scientifically measured, but a variety of such methods are known to be effective against blast, sheath blight, and rice tungro (Tab 2). Infection rates of the sheath blight fungus on trap plants has been found to be reduced by the type of organic fertilizer used due to microbial activity in the soil (Fig 4).

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Disease management in general has relied on diversity present at the genetic and environmental levels. Host genetic diversity, when used with knowledge of environmental diversity, can greatly enhance the durability of disease resistance and reduce the need for chemical use.

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Managing Weeds with Less Herbicides

Weed control does not change the inherent production capacity of the plant or its biological potential for using nutrients, water and light. It helps to change the production environment, thereby allowing more of the inherent capacity of the plant to express itself in higher yields than otherwise would occur. Thus the ability of weed control methods to increase output or to contribute to an increase is constrained by the upper level of the plant's potential output (Anon 1975). Many factors cause loss of agricultural production, and there is little doubt that weeds are of major significance, Because weeds are so common and so widespread, people do not fully appreciate their importance in terms of the losses they cause and the cost of their control. Weed management for most of the small-scale farmers is one of the most time-consuming activities: land preparation and subsequent hand weeding commonly take 50% or more of the total time required to produce annual crops. Work requires time (his or her own, the family's or hired), energy (the availability of which is determined by physical strength, endurance, health and diet) and perhaps the financial or other sources to hire labor. Hence, opportunity costs must be added to those caused by crop loss and the costs of control. Weeds are more of a problem in upland rice than in lowland rice. Although fields in lowland rice culture can be submerged after weeding and thus checking the germination of weed seeds, in upland rice weed seeds are encouraged to germinate during the weeding process because new weed seeds are brought near the soil surface (Vega 1957). It is only when the rice plants have effectively shaded the ground that weed seeds stop germinating. The cost of weed control is greater, the chances of weed control are less, the adoption of new weed control technology is slower and the attainable yield is lower (the risk is greater) in rainfed lowland and upland rice - rice cultures in which weed control is needed the most. In Laos, upland rice has a huge labor requirement for weeding. Families spend up to 200 days for weeding 1 ha of rice or 1020 days to produce 100 kg of rice (W. Roeder pers. comm.). The use of herbicides is often a more economical means of weed control than hand weeding. As a general rule, the higher the wage rate or the more severe the weed infestation, the more likely that herbicides will be economical. When the prices of rice and herbicides are considered, weed control with herbicides is economically attractive to farmers. Herbicides are a highly productive input, and the marginal return for every dollar invested in herbicides is strongly positive. Current production trends,

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together with likely economic developments, suggest continued increase in the use of herbicides for weed control (Moody 1994). According to Woodburn (1993), it is realistic to expect that over the next 6-8 years, the average global expenditure on rice herbicides could exceed US $10 ha -1 compared to US $ 7.50 at present, resulting in an increase in the rice herbicide market in China and India from US $ 67 million to over US $ 550 million. Herbicides will represent the major growth area in the pesticide industry in the developing countries in the next decade. Herbicide use is increasing not only in the most favorable production areas but also in the less favorable

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Percent of control 100,

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areas. In India, for example, rice herbicides that are widely used in Punjab, Haryana and western Uttar Pradesh have now started making inroads into the southern and eastern parts of the country (Mehta 1992); the Indian rice agrochemicals market is expected to be one of the fastest growing for the foreseeable future (Woodburn 1993). While herbicide use in rice has increased dramatically, crop losses due to weeds have not declined and, in fact, may have increased. Weed problems persist because of their great reproductive capacity, their massive recycling potential, inadequate prevention, ecological shifts and changing weed populations. New pest problems continually arise as a result of evolutionary developments in many pest organisms, introduction of new pests to an area, changing cultural practices or crop intensification (Stoskopf 1985). Profitable and efficient chemical weed control discourages the development of other practices and this makes chemical methods seem more necessary (Anon 1975). The lack of profitable alternatives to chemical weed control will seriously hinder any reduction of dependence on herbicides. In wet-seeded rice, there is great (sometimes complete) reliance on herbicides for weed control. No single weed control technique is perfect, and because the weed population constantly adapts to its physical environment, a multilateral approach is required to ensure sustainability. There is a need to find ways to reduce dependency on herbicides (Moody 1994). Differential responses of ecotypes to herbicides has been observed in weed species. Variation among Echinochloa species and among strains in their susceptibility to herbicides has been observed in the Philippines (Fig 5). Echinochloa crus-galli spp. hispidula

was more susceptible to a low concentration of butachlor than E. glabrescens and E. colona. The herbicide reduced shoot length by more than 50% in 4 of 20 E. glabrescens strains and in 2 of 6 E. colona strains. Farmers have many options for the control of weeds. Cultural practices (together with appropriate cultivars), some of which have evolved specifically as weed management practices, will form the basis for any weed control program. Lower herbicide rates or better herbicide performance is achieved when optimum cultural practices are used. Day (1972) noted that, however, powerful and effective herbicides may be, they are virtually worthless in the absence of other sound management practices chemicals supplement traditional methods but do not serve as substitutes for them. Weeds are most effectively controlled by the simultaneous application of a variety of practices, the total effect of which is usually greater than the effects of individual measures employed separately. Recognizing that weeds are active participants in the dynamic agricultural environment has led to the current development and use of ecosystem-oriented methods of weed control or integrated weed management systems. These systems involve the judicious integration of cultural, mechanical and chemical control procedures. Pimentel (1993) stated that by optimum use of alternative or nonchemical weed control methods, herbicide use in rice could be reduced by as much as one-third while maintaining crop yields. Since 1948, average annual growth in rice production has been 3% owing to factors such as increase in area planted, plant breeding, fertilizer application, irrigation, pest management and other inputs. In the future, there will be a need to greatly improve productivity on currently cropped lands while maintaining the sustainability of that

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increased productivity (Barbier 1989). Maximum benefits will accrue to rice producers and consumers if further increases in yield can be achieved at lower production costs. Panganiban and Sumangil (1985) noted that farmers are usually not satisfied with general herbicide recommendations and experiment by themselves with material, timing, rate and complementary water control. Fajardo and Moody (1990) also commented on the innovativeness of wet-seeded rice farmers in their approach to chemical weed control. Herbicide rates are set so that the product will work under a wide range of conditions. However, farmers in most countries in South and Southeast Asia apply herbicides at less than the recommended rate (Navarez and Moody 1979; Abeyratne et al. 1984; van de Fliert and Matteson 1990). This may be an economic measure but most claim that effective weed control is achieved (Navarez and Moody 1979). On the other hand, the manufacturer may have calculated in a safety margin for farmers who underdose and for farmers whose land preparation and water control are less than desirable (Mabbayad and Moody 1985). It is often possible to reduce the recommended rate because it is often based on worst case situations for the most difficult-to-control weed species. The worst case approach is no longer acceptable. Instead, the rate should be adjusted to give exactly the required effect and no more under the prevailing conditions. In a number of experiments conducted at the International Rice Research Institute and in farmers' fields in the Philippines, rates of application of pre-emergence herbicides, such as butachlor and pretilachlor + fenclorim, could be reduced by up to 50% of that recommended without loss in efficacy or reduction in crop yield (Mabbayad and Moody 1985; Castin et al. 1992; Pablico and Moody 1992). As herbicide rates are reduced, levels of weed control become more variable and more sensitive to environmental factors. The lower the weed density, the more economical it is to cut rates. This is not because it takes less herbicide to kill the weeds. The rate required to kill a certain proportion of the weeds is fairly constant, regardless of the weed density. However, 10% survival at a low weed density (10 weeds out of 100) is not nearly as damaging as 10% survival in a high density infestation (100 out of 1,000). In the densely infested field, a higher rate of herbicide aimed at killing a larger proportion of the weeds will be economically justified. However, in the lower weed density, the cost of extra chemical is greater than the yield loss it is intended to prevent. Water, land preparation, seeding and weed control are interrelated. Good land preparation is an effective and economical way of keeping down weed infestations in annual crops (Pothecary 1970). Cost reduction per ton can be achieved by good land preparation which leads to more efficient water use. This also permits better weed control at less cost per hectare since less herbicide is required. Diop

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and Moody (1989) concluded that returns to land preparation and weed control increased as the amount of land preparation increased. Water, judiciously used, is a most effective method of weed control (Jayasekera and Velmurugu 1966). Poor water control leads to reduced weed control efficiency with all methods of weed control but particularly with herbicides; there is increased herbicidal efficacy with continuous flooding. Rajadurai and Panner Selvam (1988) reported that lower rates of thiobencarb (50-750/0 of the recommended rate) were effective in controlling weeds in submerged fields, whereas the recommended rate had to be used when the field was saturated. In Sri Lanka, when there was standing water in the field, lower rates of propanil could be used and it could be applied later in crop growth (ARS 1980). For example, to achieve the same degree of control as 1.5 kg ai/ha -1 at 14 days after seeding in the presence of standing water, 3.0 kg ai/ha -1 had to be applied at 7 days after seeding when there was no standing water. Ho (1994) reported that during 1990-93, herbicide use in the Muda area, Malaysia, declined as a result of the strategic extension campaign on integrated weed management. Farmers who used proper cultural practices needed to apply herbicides only once, whereas farmers with poor land preparation and improper water management had to apply herbicides three to four times per season to control weeds. Morales et al. (1993) conducted a study comparing the performance of unselected (farmers') seed, selected seed (farmers' seed which was thoroughly cleaned) and certified seed of IR64. A crop grown from the pure seed yielded 10.3% higher than that from the farmers' seed and there were fewer off-types (0.03 vs 10.32%). There was also a significant reduction in the number of weed species and weed biomass; the biomass ofL rugosum was reduced from 58.6 to 2.7 g/10 m -2. Use of certified seed resulted in an additional yield increase of 5.6% but had little further effect on weed growth. Crop competition is one of the most useful weed control methods available to farmers. In addition to reducing weed weight and weed competition, closer plant spacing results in a greater number of options from which a suitable weed control practice can be selected (Moody et al. 1983). Pablico et al. (1994) reported that cultivar IR41996-50- 21-3, which tolerated anaerobic conditions at seeding, competed better with weeds (154 gm -2 in the untreated check vs 348 gm-2), and suffered less yield loss (13 vs 380/o) from weed competition than standard cultivar PSBRC4. The traits that made IR41996-50-2-1-3 more competitive were superior stand establishment, faster seedling growth, higher tiller number, greater leaf area index and greater biomass. More intensive herbicide application was needed with the standard cultivar than the '°anaerobic" cultivar to achieve the same degree of weed control. When a weed population occurs in patches, herbicide use and production costs can be reduced by spraying just

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the patches rather than the entire field (Dent et al. 1989; Thornton et al. 1990; Audsley 1993) without taking account of the environmental value of reducing the amount of spray material used (Audsley 1993). The economic feasibility of patch spraying will depend on the size and distribution of the patches in a field (Wiles et al. 1992); the smaller the area occupied by the patches and the more expensive the herbicide, the greater the savings. Audsley (1993) noted that there was less value in patch spraying when patches were large. The use of naturally occurring enemies such as plant pathogens appears to offer an environmentally friendly solution to weed problems. Biological control methods are not expected to replace herbicides but to supplement their judicious use or possibly increase use of more selective ones. In practice, however, there are many problems associated with the development and effective use of biological control agents that will probably prevent them becoming a significant method of weed control in arable crop production, at least until the year 2000 (Williams 1992). Allelopathy, the direct or indirect effect of one plant on another through the production of chemical compounds that are released into the environment, is a natural phenomenon that may provide an alternative to, or lessen dependence on, the use of herbicides, which are the most widely used of all chemicals in agricultural production. Allelopathy produces a variety of impacts in agricultural and natural ecosystems, including influences on plant succession, patterning of plants, inhibition of nitrogen fixation and nitrification, and chemical inhibition of seed germination and decay. The major challenges of weed scientists are to minimize the negative impacts of weed allelopathy on crop growth and yield and to exploit allelochemicals as additional crop protection strategies (Putnam 1985). Rice that possesses high allelopathic activity may be able to control some weeds and reduce the amount of weeding required, or the need to apply herbicides for weed control. By the year 2050, the world's population is projected to increase to 11 billion, more than double that of today. This will increase the pressure on land for food production. As agriculture is intensified, so will man's battle with crop pests increase. Effective weed management technology will become more important than it is today. Cultural, chemical, genetic and biological control methods will all contribute in this battle (Mehta 1992). The benefits of utilizing rice herbicides in ricefields are not in dispute (Woodburn 1993). However, current concerns over the environmental impact of pesticides may lead to a reduction in the number of rice herbicides available to the farmer and to social and financial pressures to reduce herbicide use. Only herbicides that have been shown to be of no risk to farmers and the nonfarming community, and have no residual effects on surface and ground waters, should be used. More than for other groups of pests, farmers have many options for the control of weeds. They include manual

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methods, mechanical methods, cultural techniques and herbicides. Weed control is best accomplished when it is considered in the overall management program of crop production. Thoroughness in the way the land is prepared, carefulness in the way the crop is planted and promptness in attacking the weed problem are vital. Although herbicides are of great importance in modern agriculture, the principal methods of weed control are the basic cultural and managerial practices of "good farming". These have been the cornerstone of rice weed control practices throughout the world. The integration of chemical methods of weed control into rice husbandry systems is vital in providing acceptable weed control at economic levels. In order to be successful, weed control in rice must include cultural, mechanical and chemical methods as well as crop management (Turner 1983). Other factors supporting the need to develop an integrated approach to addressing weed control problems are the development of herbicide-resistant weeds, the buildup of tolerant weed species, changes in the weed flora, reduction in species diversity, and growing public concern about the harmful effects of pesticides on human health and the environment. The goal will be to reduce use rates and frequencies of herbicide applications, in combination with other practices, to achieve the degree of weed control needed. An axiom that would apply might be "as little as possible, and as much as necessary" (Williams 1992). Pests are important constraints to rice production. They can potentially cause as much as a 55% reduction in production. However, the chance of obtaining such a loss is extremely low. Nevertheless, this potential has provided the incentive to search for synthetic pesticides. However, pesticides are not a perfect answer to managing pest problems. Besides being a danger to human health and the environment, pesticides are often inefficient in pest management. The reasons for this are (Conway and Pretty 1991): 1) Development of resistance to pesticides. 2) Causing pest resurgences by killing offnatural enemies. 3) Causing the development of secondary pest problems of killing off natural enemies that keep them under control. 4) Pesticides need to be applied repeatedly, and they cannot produce lasting or stable controls. Rice ecosystems, planted with monocultures of uniform varieties and provided with high inputs of fertilizers, seem to provide ideal conditions for pest development although there is now evidence to show that this need not be the case (Way and Heong 1994). Quite often, farmers resort to pesticides as the only means of avoiding serious crop losses. Various studies of farmers' pesticide use indicate that there are numerous opportunities for reducing pesticide use in rice production. A great deal of pesticide misuse stems from lack of knowledge and misperceptions. Research into the design of better communication mechanisms that will facilitate farmers' participation in research and training activities can

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help improve current systems. In addition, research to better u n d e r s t a n d farmers' perceptions and decision making in pest m a n a g e m e n t will contribute towards the design o f m o r e sustainable strategies and policies. F a r m e r s perceive and r e s p o n d to insects, diseases and weeds differently, and research to address these issues will be useful. The dynamics o f organisms i m p o r t a n t in a rice crop will usually d e p e n d on area-wide interaction, in particular nonrice crops, fallow areas and "wild" habitats (including bunds) associated with ricefields. Such an ecosystem, comprising a group o f plants, animals and abiotic c o m p o n e n t s , is conceived to be sufficient to d e t e r m i n e the a b u n d a n c e o f its c o m p o n e n t species in space and time. Diverse nonrice habitats in the rice ecosystem m a y serve as a source o f natural enemies. U n d o u b t e d l y nonrice habitats m a y occasionally serve as a source o f localized invasions o f polyphagous pests such as a r m y w o r m s and locusts. However, m o s t i m p o r t a n t pests are specific or narrowly oligophagous for rice, while n u m e r o u s predators are polyphagous feeding on a variety o f food. At the m o m e n t , field data on the potential role o f these nonrice habitats are conspicuously lacking. This ecosystem approach to pest m a n a g e m e n t opens up new research opportunities that can lead to d e v e l o p m e n t o f innovative pest m a n a g e m e n t strategies, especially since the limits o f m a n y pest species lie b e y o n d regional boundaries. N u m e r o u s examples where the impact o f natural e n e m i e s were e n h a n c e d

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t h r o u g h increasing habitat diversity have been described in cotton, maize, grapes and citrus (Conway and Pretty 1991). The use o f resistant cultivars offers an excellent alternative to pesticides in controlling pests. However, natural selection often works to c o u n t e r b r e e d e r s ' selection. New populations develop that are n o t resistant, and c o n t i n u o u s efforts in b r e e d i n g for resistance are required. Nevertheless, it is still one o f the m o s t effective alternatives to pesticides, and research towards better u n d e r s t a n d i n g pest adaptations to resistance genes and m e t h o d s to deploy cultivars w o u l d yield dividends. It is b e c o m i n g increasingly evident that the c o m p e n s a t o r y ability o f m a n y rice cultivars is i m p o r t a n t against pests (Way and H e o n g 1994). Besides c o m p e n s a t i o n from stem b o r e r attack (Rubia et al. 1989), yield is often unaffected by leaffolder damage ( G r a f et al. 1992; H e o n g 1990; H u et al. 1993) and by whorl m a g g o t damage (Litsinger 1991; Viajante and Heinrichs 1986). This opens up opportunities for breeding to enhance c o m p e n s a t o r y abilities against defoliation and tiller destruction and perhaps other pests as well. In her book, Silent Spring, Rachel Carlson (1962) referred to "The Road N o t Taken". This other road involves use o f ecological and n o n c h e m i c a l means. This, however, does not advocate total e l i m i n a t i o n o f chemicals, rather j u d i c i o u s and efficient use. The destination o f this new road is also n o t total e l i m i n a t i o n o f pests, b u t pests at tolerable levels.

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