Agroecology in Action
isidoro
revised 07-30-00

 


Applying agroecological concepts to the development of Ecologically Pest Management strategies

Miguel A. Altieri
Clara Ines Nicholls

1. Most of the scientist toda would agree that conventional modern agriculture faces an environmental crisis. Land degradation, salinization, pesticide pollution of soil, water and food chains, depletion of ground water, genetic homogeneity and associated vulnerability, all rise serious questions regarding the sustainability of modern agriculture.

2. The causes of environmental crisis rooted in a prevalent socioeconomic system which promotes monocultures and the use of high input technologies and agricultural practices that lead to natural resource degradation. Such degradation is not only an ecological process, but also a social and political-economic process. This is why the problem of agricultural production cannot be regarded only as technological one, but while agrreing that productivity issues represent part of the problem, attention to social, cultural and economic issues that account for the crisis is crucial.

3. The loss of yields due to pests in many crops, despite the substantial increase in the use of pesticides is a symptom of the environmental crisis affecting agriculture. It is well known that cultivated plants grown in genetically homogeneous monocultures do not possess the necessary ecological defense mechanisms to tolerate the of outbreaking pest populations. Modern agriculturists heve selected crops for high yields and high palatability, making them more susceptible to pests by sacrificing natural resistance for productivity. On the other hand, modern agricultural practices negatively affect pests' natural enemies, which in turn do not find the necessary environmental resources and opportunities in monocultures to effectively and biologically suppress pests. Thus while the structure of the monocultures is maintained as the structural base of agricultural systems, pest problems will continue to be the result of a negative treadmill that reinforces itself. Thus the major challenge for those advocating EBPM is to find strategies to overcome the ecological limits imped by monoculture.

4. IPM approaches have not addresed the ecological causes of the environmental problems in modern agriculture which are deeply rooted in the monoculture structure prevalent in large scale production systems. There still prevails a narrow view that specific causes affect productivity, and overcoming the limiting factor (i.e. insect pest) via new technologies, continues to be the main goal. In many IPM projects the main focus has been to substitute less noxious inputs for the agrochemicals that are blamed for so many of the problems associated with conventional agriculture. Emphasis is now placed on purchased biological inputs such as Bacillus thuringiensis, a microbial pesticide that is now widely applied in place of chemical insecticide. This type of technology pertains to a dominant technical approach called input substitution. The thrust is highly technological, with the limiting factor mentality that has driven conventional agricultural research in the past. Agronomists and other agricultural scientists have for generations been taught the "law of the minimum" as a central dogma. According to this dogma, at any given moment there is a single factor limiting yield increases, and that factor can be overcome with an appropiate external input. Once the hurdle of the first limiting factor has been surpassed-nitrogen deficiency, for example, with urea as the correct input-then yields may rise until another factor-pests, say-becomes limiting in turn due to increase levels of free nitrogen in the foliage. That factor then requires another input-pesticide in this case-and so on, perpetuating a process of treating symtoms rather than the real causes that evoked the ecological unbalance.

5. Emerging biotechnological approaches do not differ as they are being pursued to patch up the problems (e.g. pesticide resistance, pollution, soil degradation, etc.) caused by previous agrochemical technologies promoted by the same companies now leading the bio-revolution. Transgenic crops developed for pest control closely follow the paradigm of using single control mechanism (a pesticide) that has proven to fail over and over again with insects, pathogens and weeds (National Research Council, 1996). Transgenic crops are likely to increase the use of pesticides and to accelerate the evolution of 'super weeds' and resistant insect pests (Rissler and Mellon, 1996).

The 'one gene-one pest' approach has proven to be easily overcome by pests that are continuously adapting to new situations and evolving detoxification mechanisms (Robinson, 1996). There are many unanswered ecological questions regarding the impact of the release of transgenic plants and microorganisms into the environment. Among the major environmental risks associated with genetically engineered plants are the unintended transfer to plant relatives of the 'transgenes' and the unpredictable ecological effects (Rissler and Mellon, 1996).

Given the above considerations, agro-ecological theory predicts that biotechnology will exacerbate the problems of conventional agriculture. By promoting monocultures it will also under-mine ecological methods of farming, such as rotations and polycultures (Hindmarsh, 1991). As presently conceived, biotechnology does not fit into the broad ideals of sustainable agriculture (Kloppenburg and Burrows, 1996).

6. This view has diverted agriculturists from realizing that limiting factors only represent symptoms of a more systematic disease inherent to unbalances within the agroecosystem and from an appreciation of the context and complexity of agroecological processes thus understimating the root causes of agricultural limitations. A useful framework to accomplish this is to use agroecological principles.

Agroecology goes beyond a one-dimensional view of agroecosystems-their genetics, agronomy, edaphology- to embrace and understanding of ecologiacl and social levels of coevolution, structure, and function. For agroecologists, sustainable yield in the agroecosystem derives from the proper balance of crops, soils, nutrients, sunlight, moisture, and other coexisting organisms. The agroecosystem is productive and healthy when these balanced and rich growing conditions prevail and when crop plants remain resillient to tolerate stress and adversity. Occasional disturbances can be oovercome by a vigorous agroecosystem which is adaptable and diverse enough to recover once the stress has passed. Occasionally strong measures (i.e. botanical insecticides, alternative fertilizers, ect. ) may need to be applied by farmers employing alternative methods to control specific pests or soil problems. Agroecology provides the guidelines to carefully manage agroecosystems without unnecessary or irreparable damage. Simultaneous with the struggle to fight pests, diseases, or soil deficiency, the agroecologist strives to restore the resiliency and strength of the agroecosystem. If the cause of disease, pests soil degradation, and so forth, is understoodas imbalance, then the goal of the agroecological treatment is to recover balance. In agroecology, biodiversification is the primary technique to evoke self regulation and sustainability.

7. From a management perspective, the agroecological objective is to provide a balanced environment, sustained yields, biologically mediated soil fertility and natural pest regulation through the design of diversified agroecosystems and the use of low-input technologies. The strategy is based on ecological principles that lead management to optimal recycling nutrients and organic matter turnover, closed energy flows, water and soil conservation and balanced pest- natural enemy populations. The strategy exploits the complementarities and synergisms that result from the various combinations of crops, trees and animals in spatial and temporal arrangements. These combinations determine the establishment of a planned and associated functional biodiversity which performs key ecological services in the agroecosystem.

8. The optimal behavior of agroecosystems depends on the level of interactions between the various biotic and abiotic components. By assembling a functional biodiversity, it is possible to initiate synergisms which subsidize agroecosystem processes by providing ecological services such as the activation of soil biology, the recycling of nutrients, the enhancement of beneficial anthropods and antagonists, and so on.

In other words, ecological concepts are utilized to favor natural processes and biological interactions that optimize synergies so that diversified farms are able to sponsor their own soil fertility, crop protection and productivity. By assembling crops, animals, trees, soils and other factors in spatial/temporal diversified schemes, several processes are optimized. Such processes (i.e. organic matter accumulation, nutrient cycling, natural control mechanisms, etc.) are crucial in determining the sustainability of agricultural systems.

9. Agroecology takes greater advantage of natural processes and beneficial on farm interactions in order to reduce off-farm input use and to improve the efficiency of farming systems. Technologies emphasized tend to enhance the functional biodiversity of agroecosystems as well as the conservation of existing on-farm resources. Promoted technologies are multi-functional as their adoption usually means favorable changes in various components of the farming systems at the same time.

10. For example, legume based crop rotations, one of the simplest forms of biodiversification can simultaneously optimize soil fertility and pest regulation. It is well known that rotations improve yields by the known action of interrupting weed, disease and insect lifecycles. However, they can also have subtle effects such as enhancing the growth and activity of soil biology, including vesicular arbuscular mycorrhizae (VAM), which allow crops to more efficiently use soil water nutrients.

Another practice is cover cropping or the growing of pure or mixed stands of legumes and cereals protect the soil against erosion; ameliorate soil structure; enhance soil fertility, and suppers pests including weeds, insects, and pathogens. cover crops can improve soil structure and water penetration, prevent soil erosion, modify the microclimate and reduce weed competition. Besides these effects, cover crops can impact the dynamics of orchards and vineyards by enhancing soil biology and fertility and by increasing the biological control of insect pest populations.

11. Perhaps the most dramatic example of the integrative effects of a multi-purpose technology in simultaneously enhancing IPM and soil fertility management is organic soil fertilization.

Some studies suggest that the physiological susceptibility of crops to insects may be affected by the form of fertilizer used (organic vs. chemical fertilizer). Studies documenting lower density of several insect herbivores in low-input systems, have partly attributed such reduction to a low nitrogen content in the organically farmed crops.

12. The ultimate goal of agroecological design is to integrate components so that overall biological efficiency is improved, biodiversity is preserved, and the agroecosystem productivity and its self-sustaining capacity is maintained. The goal is to design an agroecosystem that mimics the structure and function of natural ecosystem, that is systems that include:

(a) Vegetative cover as an effective soil-and water-conserving measure, met through the use of no-till practices, mulch farming, and use of cover crops and other appropiate methods.
(b) A regular supply of organic matter through the regular addition of organic matter (manure, compost and promotion of soil biotic activity).
(c) Nutrient recycling mechanisms through the use of crop rotations, crop/livestock systems based on legumes, etc.
(d) Pest regulation assured through enhanced activity of biological control agents achieved by introducing and/or conserving natural enemies.

13. The process of converting a conventional crop production system that relies heavily on systemic, petroleum-based inputs to a diversified agroecosystem with low-inputs is not merely a process of withdrawing external inputs without compensatory replacement or alternative management. Considerable ecological Knowledge is required to direct the array of natural flows necessary to sustain yields in a low-input system.

The process of conversion from a high-input conventional management to a low-externalinput management is a transitional process with four marked phases:

(a) Progressive chemical withdrawal.
(b) Rationalization and efficiency of agrochemical use through integrated pest management (IPM) and integrated nutrient management.
(c) Input substitution, using alternative, low-energy input technologies.
(d) Redising of diversified farming systems with an optimal crop/animal integration which encourages synergisms so that the system can sponsor its own soil fertility, natural pest regulation, and crop productivity.

During the four phases, management is guided in order to ensure the following processes:

(a) Increasing biodiversity both in the soil and above ground
(b) Increasing biomass production and soil organic matter content
(c) Decreasing levels of pesticide residues and losses of nutrients and water components
(d) Establishment of functional relationships between the various plant and animal farm components
(e) Optimal planning of crop sequences and combinations and efficient use of locally available resources.

14. The challenge for EBPM scientists is to identify the correct management techniques and crop assemblages that will provide through their biological synergisms key ecological services suchh as nutrient cycling, biological pest control, and water and soil conservation.

The exploitation of these synergisms in real situations involves agroecosystem design and management and requires an understanding of the numerous relationships among soils, plants, herbivores, and natural enemies. Clearly, the emphsis of this approach is to help to restore natural control mechanisms through the addition of selective biodiversity within and outside the crop field, through a whole array of possible crop arragement in time and space.