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Integration of Biological Control Into Ipm Systems for Aquatic Weeds

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Integration of Biological Control Into Ipm Systems for Aquatic Weeds INTEGRATION OF BIOLOGICAL CONTROL INTO IPM SYSTEMS FOR AQUATIC WEEDS James J. Marois, Department of Plant Pathology, University of California, Davis, CA 95616 USA ABSTRACT The development of effective long-term control programs for aquatic weeds is dependent upon the ability to integrate biological, chemical, and cultural control strategies. Successful IPM programs are dependent upon a sound knowledge of the cropping system (especially the inputs and outputs of the system), the biology of the aquatic weed pest, and the biology of the control agent(s). There are several ways to analyze these complex systems, from the molecular to the community level. This presentation will emphasize the ecological interactions that should be con- sidered. INTRODUCTION approach may be to augment or inundate the system with the control organisms at specific times in the Integration of biological control of paddy and cropping history. This is especially true when the aquatic weeds into integrated pest management control agent is a fungus or bacterium, since these programs is a necessary goal for the implementation are relatively easy to produce in large quantities and of sustainable rice production systems. To reach are dependent upon specific environmental this goal, however, a number of obstacles must be conditions for greatest efficacy. When a fungus is overcome. Most important is the current status of used to control a weed in this manner, it is referred biological control of aquatic weeds which, in this to as a bioherbicide (Emge and Templeton 1981). discussion, will be defined narrowly as the use of Successful integration of control agents, either beneficial microorganisms or their gene products pathogens or insects, is dependent upon the for pest control. Although considerable research compatibility of the biological control system with has been done and several successful systems have the entire crop production system. been implemented for the control of weeds, these Most of the difficulties in developing are usually restricted to low-input systems such as functional biological controls are related to the public lands and range lands. This is because most complexities of the biological systems being of the work has been based on the classical approach managed. In spite of this difficulty, there are to biological control, in which the strategy is to increasing economic and social pressures to develop establish pathogens or herbivores in an area where practical biological control strategies. This paper the weed is a problem. Classical biological control addresses the potential for integration of biological is dependent upon long-term population dynamics; control with other weed control strategies. Also however, in intesively managed systems such as discussed are perspectives of research at population Asian rice production, where three crops are grown and community levels which should be considered each year, it is not possible for the long-term if viable biological control strategies are to be interactions necessary for suppression of the weed developed. population to take place. Thus, a more appropriate Keywords: Ecology, biological control, IPM 210 BIOLOGICAL CONTROL IN INTEGRATED PEST weed) (Supkoff et al. 1988). Because this weed is MANAGEMENT important in low input systems, such as range land, it is not economically feasible to control it with Biological control for aquatic weeds will be chemical herbicides. implemented most readily when no other means of Biological methods can also be used to increase control are available, when public awareness of the level of disease control attainable with chemicals. pesticide use is high, and when pesticide applications It is possible for resident microorganisms to increase are particularly hazardous or disruptive to the directly the efficacy of a chemical treatment. For environment. Such opportunities for implementation example, herbicides may stress a plant so that it is are becoming more common. In the past, pesticides more susceptible to infection by plant pathogens, appeared to control aquatic weeds relatively cheaply competition from other plants (including the crop because the negative impacts of pesticides on the plant), or to being eaten by insects. A combination environment were costs which were not transferred of herbicides with biological control agents may to the users or manufacturers of pesticides. Recently, result in the control of weeds which cannot be a volatile political climate has developed concerning controlled by chemicals alone, or a reduction in the exposure of workers and consumers to pesticides, amount of chemical applied, by reducing either the and there is an increasing body of information number of necessary applications or the rate needed describing the negative effects that pesticides can for each application. have on cropping systems and surrounding In each of these instances, the beneficial ecosystems. Cosequently, many older pesticides organisms must have a tolerance to the herbicide are no longer available for use, and registration of applied to control the weed. This is often the case new materials is becoming more difficult and with herbicides, since they usually affect only plants. expensive. It appears that despite the complexity of In some production systems, however, the beneficial biological control programs for aquatic weeds, such organisms may be affected adversely by an herbicide. methods will have to be utilized in future In such cases, it would be necessary to apply the management strategies. Fortunately, there are many organisms at suitable intervals after pesticide possibilities for this type of approach. application, or develop resistant strains of the Integrated pest management has been defined beneficial organisms. many different ways for different purposes. I will Biological control measures can also be used use the definition as proposed in a USDA report to minimize the potential of some herbicides to (1982) “... (the combining) of two or more pest increase other, nontarget, weeds. In plant pathology, suppression methods into practical systems of IPM this concept is termed “disease trading” (Kreutzer to reduce pest problems.” Although other definitions 1960). For example, applications of herbicides that may be more appropriate in other situations, the are specific for broadleaf weeds can lead to rapid goal here is to identify opportunities for effectively infestations of weedy grasses. Herbicide treatment combining biological control methods with other can reduce population densities of these broadleaf disease control strategies which are based on weeds which may be important competitors, not chemical, cultural, and genetic methods. only with the crop plants, but also other weed species. A similar situation occurs whenever land is Integration of Biological Control with severely disturbed. Many organisms, including Chemical Control weedy plant species, have developed specific life histories to exploit severely disturbed areas. Often There are many weeds which cannot be these species are considered r-selected species controlled effectively with present chemical (MacArthur and Wilson 1967), which have methods, often because of the toxicity of the characteristic traits such as rapid reproduction, chemicals to nontarget species or high costs. effective dissemination, and tolerance to conditions Biological control measures might be adopted most associated with disturbed systems. These species rapidly by growers in these instances because no often do not do well in undisturbed systems, usually chemical alternative would be available. An example because they are poor competitors in communities is the use of the rust fungus Puccinia chondrilina for of high diversity. the control of Chondrilla juncea (rush skeleton 211 Integration of Biological Control with should be much less specific, and selection for Cultural Control resistance should be less likely to occur. Cultural controls are commonly used to create CONSTRAINTS TO DEVELOPMENT OF an environment which is not conducive to the BIOLOGICAL CONTROL development of weeds. Establishment of beneficial organisms within this environment might increase In most of the scenarios described, biological levels of weed control which cannot be attained by control can best be used to augment other weed the cultural practices alone. For example, water- control strategies rather than to replace them. hyacinth (Eichhornia crassipes) can be controlled However, the development of such augmentative by draining waterways for a period of time. However, biological control methods is difficult because of this method is very labor-intensive and is often not the complexity of biological systems and the impact practical, especially when considering the negative of economic and sociological factors. impact that such a practice has on the rest of the aquatic environment. However, if the repopulation Biological Limitations of the waterway by water-hyacinth could be reduced by beneficial organisms, from fish to fungi, it may Biological limitations on the biological control be possible to delay the need to repeat such a drastic of weeds are directly associated with the very practice cultural practice. of agriculture. As recognized by Feeny (1976), fast- Although many situations can be envisioned growing plants often rely on their “unapparency” as in which biological control could be integrated with a defense against natural predators. Unapparency is cultural control, the potential for integration may be defined as the period
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