Successful ecological pest control needs to address the complex interactions between the pest and the abiotic and biotic environment of the ecosystem. For agroecosystems, interactions between pest species and the environment are further complicated by the diversity of cropping systems (i.e., monoculture vs. crop rotations) and surrounding habitats (e.g., pasture systems, forests, riparian areas, etc.) that influence effective pest management strategies. A further consideration in assessing problems and developing IPM programs is the scale of the area involved; for example, agroecosystems can be viewed at the level of an individual plant, a research plot, a field, multiple fields (whole farm), a watershed, or a basin. Local movement of pests may be less important at small (plot) scales but critically important in insect population dynamics at larger (watershed or basin) scales, especially when considering the surrounding habitat.
Agroecosystem bioenvironmental control seeks to achieve successful pest management through an understanding of the complexities of system interactions, followed by application of this understanding to efficiently achieve relative stability of pest populations below damaging levels without resorting to use of control techniques such as insecticide application. Current research suggests that pest outbreaks can be mitigated by utilizing the following agroe-cosystem cultural management techniques.
Crop rotations/practices. Using crop rotations, intercropping, or strip cropping in place of monoculture cropping systems to effectively control many soil-inhabiting insect pests is an important stratagem in mitigating pest infestation, as evidenced by higher numbers ofnatural insect enemies in the intercrop and/or reduced herbivore colonization and residence time in the intercrop. Further research is needed, however, to identify those crops and crop rotations that best minimize invasion by different insect pests.
Tillage systems and management timing. Tillage system type can distinctly influence the soil environment and thus affect insect survival. For example, reduced tillage systems have been shown to increase the population growth of various predatory insects and mites in the soil due to higher crop residue levels. Pest management studies on no-till fields appear to be more variable, that is, no-till fields may be at higher risk than conventionally or reduced tilled fields for some pest insects and at lower risk for others depending on the environmental conditions (particularly precipitation and temperature). Studies have also shown many pest insect attacks on crops can often be minimized by careful studies of their life cycles and changes in the timing of management operations (e.g., planting, tillage, harvesting), thus disrupting the pest-crop interaction and thereby reducing pest attacks and avoiding seasonal carryover of pests from crop to crop. Many experimental studies have produced site-specific conclusions about the mechanisms by which tillage practices and the timing of tillage practices affect insect pest infestations; however, there appears to be limited potential for extrapolating these experimental results to other locations or cropping systems until the actual processes involved undergo further evaluation.
Resistant crop varieties. Using crop varieties with greater resistance to attack by pests has been a major tool in minimizing the use of insecticides and in developing sustainable IPM strategies. Agribusiness and domestic/international agricultural research centers are heavily involved in this process which has been accelerated through genetic engineering of new strains and varieties (particularly by techniques such as the insertion of the Bacillus thuringiensis (Bt) gene into crops). A potential problem is that widespread and long-term planting of Bt crops may make pest resistance more probable through increased pest exposure to Bt toxins in time and space. Also, gene transfer to other species (particularly weeds) is a concern. Currently, a significant part of US cropland is planted with strains resistant to some form of pests and this trend likely will continue to increase.
Most of the above management and cultural practices to control insects have been researched and are commonly used as important crop protection components. The following biologically related approaches are less frequently used and accepted.
Insect attractants and traps. Many chemical and visual lures attract insects and can be used to monitor or directly reduce insect populations. The effective use of attractants and traps requires knowledge of the pest- or crop-specific details involved in individual applications. Using insect attractants can complement chemical control in several ways. For example, they can be used to assess when insect pests are most active, to optimize the timing of insecticide sprays, or the attractants can be combined with an insecticide to kill all the insects drawn to the attractant.
Biological organisms. Beneficial predators, parasites, and pathogens are often very effective at providing insect pest control. For example, ladybugs, lacewings, and certain mites are common predators of insects, and some tiny wasps and fly species are parasites of insects. Biological control organisms are usually very sensitive to pesticides, that is, broad-spectrum insecticides used on a regular long-term basis tend to eliminate populations of most parasites and predators (and increase the dependence on regular insecticide treatments). As a practical example, populations of predators and parasites have been maintained in cereal fields in Europe by leaving the margins of the crop untreated with insecticides.
Alternative hosts. Some insect pests feed on more than one crop species. To attract pests away from the crop, the provision of alternative hosts through activities such as the controlled growth of weeds, or by planting trap plants or crops to lure insect pests away, can supplement insecticide control. For instance, studies have shown that populations of pest predators were much greater in maize crops populated with broad-leaved weeds or grasses than when these weeds were controlled by herbicides.
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