SCOPE 49 - Methods to Assess Adverse Effects of Pesticides on Non-target Organisms

Integrated management strategies have as their goal providing maximum economic yields to the farmer, while improving or maintaining the production site and protecting the environment. Examples of integrated pest management (IPM) procedures to improve economic yields while reducing inputs include: the integration of pesticides with cultural techniques for disease control; and biological, behavioural, and environmental controls of pests such as weeds. Biotechnology promises to have a major impact on IPM through the use of recombinant DNA techniques to genetically engineer microbes for the control of diseases, insects, and weeds, and to generate plants that resist insects, pathogens, and herbicides. Examples of each management procedure follow:

1. Cultural controls: crop variety selection or host-plant resistance, double cropping and/or crop rotation patterns, minimum or reduced tillage, use of organic fertilizers, and prescription pesticide application.

2. Biological controls: natural pathogens or predators of insects and weeds. Several cultural techniques mentioned above (such as variety selection) relate directly to biological control.

3. Behavioural controls: release of sterile insects, pheromone traps, and development of lethal natural products that attract insects. These techniques have only recently received attention in IPM; they relate to pest behaviour modification based on insect communication.

4. Biotechnology: microorganisms with novel features that are currently impacting IPM strategies include viruses and bacteria that were developed for control of insects, weeds, and/or fungal pathogens. These practices are likely to impact substantially on IPM procedures regarding the development of plants having novel characteristics such as herbicide tolerance and insect resistance.

5. Environmental controls: alterations of soil pH, moisture status, use of topography, temperature, and light intensity are among the environmental approaches. However, such manipulations are difficult to perform. While closely related to cultural practices, environmental controls relate to alterations in the physical environment to maintain yields and reduce stress.

Finally, an excellent opportunity exists to introduce expert systems and computer-aided decision-making processes into IPM strategies. Many different IPM options should be assembled into a stepwise approach that evaluates each practice, and recommends the most appropriate choice. The best options for a specific cropping system will always depend on the probability that desired results will occur; thus, computerized applications are an obvious choice for future IPM planning and implementation.

5.1 CULTURAL CONTROLS

Cultural methods can be modified by using knowledge of the ecology of cultivated plants for control of diseases, insect pests, and weeds. The five major ones are as follows.

Early planting

Sowing crops over the winter provides a plant that is more physiologically active at an earlier period than plants sown in the spring. Accelerated crop-growth in the spring may break the plant-damaging cycle of key pests at cenoses and thus avoid the disease-susceptible stages of the crop. As an example, short season cotton has been incorporated effectively into IPM programs for cotton farming in Texas (USA) to control the boll weevil and pink bollworm. Similarly, fall sowing of sugar beets decreased damage caused by pests and diseases, including beet yellow disease and mildew of beet.

Late planting

This technique may uncouple plantpest synchronization. For example, if wheat is sown late, it is less likely to be attacked by Hessian flies that emerge in the spring.

Crop rotation

Sequential crop rotation may be used to reduce insect pests, plant pathogens, and weeds. Maintaining crop rotation in alternate years or at other intervals can effectively control pests. For example, planting sugar beets decreases the effects of weeds, diseases, and pests that survive in the soil and on crop debris. Crop rotation provides an inexpensive control for important weed species such as Avena fauna and Elytrigia repens, as well as for pests such as eelworms and for diseases like black leg.

Soil cultivation

Cultivation can achieve a plentiful harvest by acting as an effective control for late infestations of weed and diseases that would lower the sowing density of emergent plants. However, if the sowing density of seeds per unit area is too low, the risk of damage from other pests increases. For example, if a sugar beet crop is planted at too low a density, then the crop can be attacked more frequently by aphids.

Reproductive control of pests

Technology is available to reduce the reproductive success of pests. The normal reproductive process of pests is affected more by simple, biologically effective agrotechnical operations, and less by chemical means. For example, such techniques have been used experimentally for control of C. pomonella and C. molesta.

For example, the technique has been employed against the Hessian fly and cabbage yellows.

5.2 BIOLOGICAL CONTROLS

Biological control is agricultural management using a biological agent to reduce the impact of a specific pest or pathogen. These methods include numerous procedures such as production and introduction of insect or weed parasites, predators, and pathogens into the agreocosystems. Alternative methods include the use of pesticides that do not reduce a natural population of indigenous biological control agents and the development of more vigorous, pesticide-resistant plants, and microbes that foster plant productivity. Much recent research has focused on the use of microbes that either enhance plant development or protect plants from chemicals, pathogens, or pests. Plants and organisms used for biological control may also have the ability to cause environmental injury either through careless development or by self-replication in a susceptible environment.

1. Mechanisms of biocontrol, such as parasitism, antibiosis, competition, stimulation of plant defences, or a combination of these factors;

3. Formulation of biocontrol agents, such as delivery systems, carrier development, and application technology.

5.3 BEHAVIOURAL CONTROLS

Behavioural controls utilize some chemicals to modify insect pest behaviour, and control pests without the use of toxins, thereby playing an important role in area-wide control systems. At present, behavioural modification methods (e.g., pheromones) have been used to confuse or trap the male population. This technique has been applied successfully for control of Pestinophora gossypiella in cotton, Cydia pomonella in apple orchards, and Eupoecillia ambiquella in vineyards. These methods are not effective with extremely large insect pest populations.

Recently, chemists and entomologists have focused on relationships between plants and insects, with the aim of altering insectplant communications (i.e., antifeedants, plant attractants, and plant repellents) with chemical behaviour mediators. A series of possibilities includes reducing the success of female insects in finding plant hosts, altering reproductive physiology, or both. Attractants have also been developed to capture female insect pests; for example, in vineyards some weed plants are more attractive to female European grape vine moths. This approach may form the basis for removing female populations that cause damage by generating offspring that directly damage commercial crops.

The visual orientation and colour preference of insects may also be used for effective control. Whitefly populations in greenhouses, cherry fruit flies in cherry orchards, and sarfly in plum orchardseach has been controlled effectively employing colour attractants.

5.4 IMPACT OF BIOTECHNOLOGY ON IPM

Microbial biocontrol agents are equipped to survive and to perform the role that nature has intended. However, providing high levels of disease control in genetically uniform crops exposed to massive quantities of pathogen inoculum is not equivalent to natural evolution. Characteristics of some possible benefit to an organism in the natural state may be of little use or even detrimental when employed in agriculture. The tailoring of biocontrol agents to meet the requirements of current agricultural practices will most likely entail genetic alteration of those microbes. Although lagging somewhat behind the advances made in the genetic manipulation of bacteria, this process has already begun in fungi.

Several researchers have used mutants of biocontrol agents, either to understand mechanisms in the biocontrol process or to enhance the efficacy of a biocontrol agent. The difficulty in making mutant fungi is that mutagenesis is a relatively non-specific process: all too often, silent mutations are made in addition to the desired ones. A mutant strain may perform well when assayed for a given trait in vitro (such as antibiotic resistance or antibiotic production), but it may be unable to sustain itself in nature, because other undetected and later expressed mutations may make it less competitive.

5.5 FUTURE PROSPECTS FOR BIOCONTROL

Research on biocontrol has provided a wealth of information, not the least of which is how little we know about this phenomenon. If successful strategies for biological control of plant diseases are to be developed, much research needs to be aimed at the following questions:

1. What mechanisms are responsible for the observed effects of biocontrol agents on plant pathogens?
2. How do biotic and abiotic environments alter the viability and metabolism of biocontrol agents, and the mechanisms involved in the biocontrol process?
3. What genes are responsible for the production of biocontrol mechanisms? Can these genes be isolated, transferred, and expressed in other genomes? 
4. Will genetic manipulation of biocontrol agents result in superior biocontrol strains?
5. How can biocontrol agents be mass-produced most efficiently, economically, and safely?
6. What are the most effective formulations for storage, application, and activity of biocontrol agents?
7. What are the optimum timings, frequencies of application, and treatment concentrations for biocontrol agents?
8. How readily can biocontrol agents be integrated in agriculture with other means of disease control and with crop production practices?

With definitive responses to these questions, agriculture will probably be able to make practical and economic the use of fungal and other agents to control plant diseases. The biocontrol agents alone or in combination with other means of disease control may achieve this end.

Research on population ecology and genetics of parasitehost and predatorprey systems has provided new and exciting information for the control of insect pests and weeds. Authoritative responses to the following questions would probably improve substantially the value of biological pest control.

1. How can `new association' parasites (predators) from related hosts of the pest species be selected to obtain successful biocontrol agents that will provide effective, permanent control of insect pests and weeds without further manipulations?
2. Relative to `new associations', how important are the ecology and genetics of searching, transmission, reproductive capacity, and virulence?
3. Are there different regions (continents versus islands) where `new association' biological control agents should be sought?
4. Since viruses, bacteria, fungi, and protozoa can be genetically engineered to improve their pathogenicity for insect and weed control, what ecological and genetic characteristics will make these microbes effective biocontrol pathogens?
5. How is it possible to genetically engineer microbes as biocontrol agents that provide effective, permanent control of a pest?

IPM clearly represents a highly promising approach that requires patient and careful application to reap high crop yields while minimizing injury to ecosystems.