IPM Strategies
To address an actual or perceived pest problem, one must first develop a strategy. An IPM strategy is a plan for management that uses a particular type of approach to address the pest problem. Tactics are the methods used to achieve the goals of your strategy. The strategy should try to embrace all three principles of IPM if possible. Upon discovering a pest in your cropping system, the IPM strategies that you may employ can be part of three possible responses:
- No action,
- Reduction of the pest population, or
- Reduction of the host crop’s susceptibility.
No action is obviously the plan of action when there are no potentially damaging pests in fields. It can also be used as a strategy when pests are present, but their injury is not resulting in damage or damage is below the EIL. This management option requires diligent scouting and considerable amounts of sampling. The “no action” management option is a sound practice provided your scouts are attentive to changes in pest populations and the crop environment, and are prepared to respond to pest populations approaching the EIL.
Reduction of the pest population is employed when pest numbers exceed the ET or show potential to rapidly reach this action threshold. This can be accomplished by changing the pest’s potential reproductive rate and/or its potential survival rate. The general equilibrium position (GEP) of a pest species is relative to the long-term average density of the pest population. If the GEP of the pest is typically considerably lower than the EIL, then management tactics need only be applied to dampen peaks in the pest population that may exceed the EIL. Traditionally, these peaks have been dampened through the use of pesticides. The pesticides reduce the population spikes through outright mortality or reduced fitness and reduce the potential survival rate of the pest. This tactic will lower the pest’s GEP in a location where this tactic is consistently used, but the repeated applications can be detrimental to the environment and greatly increase the potential for resistant biotypes.
IPM tactics include the prudent use of pesticides, but they also include a battery of other tactics that can decrease existing pest populations and/or reduce their reproductive potential. If a pest population’s GEP is consistently near or above the EIL for a given variety, then strategies that lower the pest’s GEP should be employed. By lowering the overall GEP of the pest, peaks in the population are less likely to exceed the EIL. Non-pesticidal approaches to lowering GEP include three main strategies:
- Reduction of the field’s carrying capacity,
- Reduction of the pest’s reproductive potential, and
- Reduction of the pest’s survival rate.
The above Graph represents a management tactic that lowers the carrying capacity of a host crop and effectively lowers the pest’s GEP. EIL, economic injury level; ET, economic threshold; and GEP, general equilibrium positions are shown.
The general goal in reducing the carrying capacity of a field is to make the environment less favorable for the pest to complete its life cycle. This can be accomplished in a variety of ways. Planting of resistant varieties, rotation with non-host crops, crop sanitation, tillage practices, and other manipulations of the field environment can affect the field’s carrying capacity for a pest. The reduction of carrying capacity automatically lowers the position of the pest’s GEP. The amount the GEP is lowered is relative to the efficacy of the strategies used to lower carrying capacity.
Using strategies that reduce the pest’s reproductive rate or efficiency can lower the GEP without lowering the carrying capacity of the field. These strategies include sterile insect release and chemicals that interfere with normal reproduction in the pest species. The result is suppression of new generations of the pest without affecting the favorability of the environment for the pest.
Reduction of the crop’s susceptibility is a highly effective strategy, but is limited by the development time of breeding efforts and the speed at which pest populations overcome host resistance. Here, the crop’s properties are modified so that the crop plant is not as favorable a host to the pest. These alterations or modifications in the plant’s phenotype or growing environment usually inhibit at least one stage in the pest’s life cycle. Selecting crop varieties that possess resistance or tolerance to a pest is an effective strategy, provided it is alternated with other tactics. Consistently using one resistance mechanism against a pest population leads to heavy selection pressure and the evolution of resistant populations of the pest. If a crop variety possesses tolerance, the plant is usually able to withstand moderate to high pest populations without the resultant injury resulting in economic damage. Tolerance does not typically lower the GEP of the pest. Resistance is an inheritable factor that when expressed, interferes with the feeding preferences or life cycle of a pest. A resistance factor that is uniform in the crop plant population will usually lower the GEP of the target pest. Combining more than one of these strategies can greatly enhance the effectiveness of your pest management system. Using multiple tactics is an important part of IPM and typically provides more consistent pest management. Other tactics in a multi-tactic strategy may compensate for failure of an individual tactic and prevent severe losses. Alternating or rotating tactics within these management strategies will further improve your program by preventing the evolution of resistance to a single pest management tactic. Multiple tactics may also impart a synergistic effect that provides added suppression of pest numbers and injury.
Most crop pests fall into four major categories:
Sub-economic, Occasional, Perennial and Severe pests.
Sub-economic pests have a low GEP and they do not exceed the EIL. Their damage does not usually warrant action (no action tactic), but if other sub-economic pests are present at the same time, action may need to be taken against the pests.
Occasional pests have a GEP that is significantly below the EIL, but they can develop population peaks that exceed the EIL. These peaks are sporadic and are driven by favorable environmental factors and host susceptibility. Reduction of the pest population is the management strategy most often used. Management of occasional pests requires routine scouting of crops to detect problem populations that may only be present every few years or at various growth stages of the crop.
Perennial pests have a GEP that is relatively close to the EIL and pest populations often, with favorable conditions, exceed the EIL in most years.
Severe pests have a GEP that is constantly above the EIL. As soon as a population of a severe pest is noticed, economic damage is usually occurring. Reasons for the GEP being higher than the EIL include dense spatial populations of the pest, high market value of the host crop, high (often lethal) injury by the pest, and other host-pest interactions that are unacceptable in the cropping system.
Management strategies for the control of pest populations are governed by the biological principles of the pest. Understanding all aspects of a pest’s biology (biochemical, morphological, reproductive, ecological niche, etc.) will help you understand how existing management strategies work and how to troubleshoot a system that is not giving acceptable results. A key concept to learn for any pest you wish to control, is the organism’s life cycle.
Prevention Tactics
Curative Tactics
Curative tactics are applied when a pest has already established itself in a field and the damage or potential for damage indicates action should be taken. This action, or “curative tactic”, usually involves significantly reducing pest numbers in order to lower the GEP of the pest population.
The four groups of curative tactics are as follows:
Physical Intervention by Humans
This type of curative tactic may well have been the first used by pioneer agrarians. Physically removing or crushing pests as they were encountered in crops may or may not have had a dramatic effect on their populations, but early agronomists recognized that allowing them to feed and reproduce could mean even greater losses to their food crops. Today, this type of tactic is still used on specialty crops or where the pest is easily recognized upon introduction to the crop. For diseases, this usually encompasses rouging of diseased plants during the season or excising (pruning) diseased organs from affected plants. For insects, this includes hand removal of insects, larva, and pupae as well as mechanical sweeping, and trapping of insects. Weeds are represented in this category by hand weeding, and mechanical cultivation.
Biocontrol/Natural Enemies
Biocontrol is basically managing pests with the use of biotic factors. These biocontrol factors often consist of pathogens and parasites or other natural enemies of target pests. Many pests and pathogens that we consistently see in our fields would be much worse if it were not for natural enemies of the pests that provide a dampening effect. This dampening effect on pest populations is almost always present in some form, but with the use of biocontrol agents the dampening effect can be enhanced. This enhancement of mortality and/or loss of fitness can be implemented so that its effects occur throughout the season, or with some biocontrol agents the impact is sudden and can reduce population peaks with timed application. Biocontrol organisms can help manage pest populations by competing with the pest for resources, or by parasitism or predation of the target pest species. Specific biocontrol strategies for crop pests will be presented in subsequent materials.
Chemical Control
Once considered the “cure-all” for pest problems, chemical control has come under much scrutiny by environmental regulating agencies. Consistent with the goal of reducing chemical inputs and protecting the environment, IPM strategies try to minimize the amount of chemicals applied per application and also the frequency of applications. For many pest problems, chemical control is the most practical and cost effective approach to bring population peaks back down below the EIL. In these cases, pesticides (chemicals toxic to the pests) are applied in a prudent and timely manner.
Volatile chemicals with pesticidal action are used against a variety of pest types in a process known as fumigation. Fumigation is widely used in crop storage areas and greenhouses, but it is also used to rid soil of pests. Fumigation can be quite effective for pests within the soil, but the expense of treatment usually limits its use to high value horticultural crops. Many of the volatile pesticides used in fumigation are effective on more than one pest group.
Specific chemical types in each pesticide group will be discussed as we cover management strategies for crop pests. Regardless of the chemical used, there are several considerations that need to be addressed before application begins.
- Mode of action (MOA): The general MOA needs to be selected so that the pesticide can target an appropriate weak point in the pest’s life cycle and/or general physiology.
- Delivery: How the pesticide is delivered to the target pest and how it interacts with the immediate environment must be determined.
- Selectivity: Will the pesticide only affect the target pest, or will other organisms at the site be negatively affected?
- Resistance: How many times has the pest population been exposed to this pesticide before? How effective has the response been over time? Is there strong potential for the buildup of resistance in the pest population to the pesticide?
Part 2 of this article to be continued in the next issue The Writer is an IPM consultant Biocon Kenya limited