Environmental or cultural pest controls seek to modify the biotic or abiotic environment to make it less hospitable for pests. Thus, these controls rely on knowledge of pest and crop ecology. Many practices can be included in environmental (cultural) pest controls, including:
Increasing crop diversity. For example, intercropping peanuts with corn can reduce corn borer populations by 80%, because the peanuts harbor predatory insects that prey on the borers. Intercropping must be done carefully, however, as in some cases the intercrop may actually favor the crop pest in some way -- as is true of all cultural control practices, application requires thorough understanding of the pest's ecology.
Mixed cropping of various kinds (e.g., intercropping [planting one other species with the crop species] and polyculture [planting several different species together]) -- can be used for for insect control. Two ideas behind mixed cropping are that it:
In 56% of reported cases, polycultures did have lower herbivore densities than monocultures.
Dr. Chris Mundt in OSU's Department of Botany & Plant Pathology has done great work demonstrating that planting mixtures of varieties of rice in a given field -- or mixtures of wheat varieties -- rather than planting fields entirely to one variety -- can reduce greatly the rate of disease spread and of losses to disease. Some important pathogens, such as wheat rust fungi, are able to successfully infect only certain wheat varieties, so "mix them up!"
Another example involves planting sunflowers in amidst vegetables to decrease insect problems in the vegetables. How does this work? The sunflowers attract birds, some of which also eat insects, including pest insects! [Ag, Ecosyst & Environ 117 (2006)]
But what about crop yield? -- Competitive effects from the polyculture species may suppress crop yields more than the decrease in pest abundance increases them, in which case the netresult is a yield loss (or no gain, anyway). Research is active in this area.
Crop rotation is a form of cultural control that relies on increasing crop diversity, and, in some cases, is effective at controlling pests. For example, corn yields are generally 5-20% higher when grown with a 2 yr rotation with soybeans than under continuous corn cultivation. Researchers are not sure what causes this "rotation effect" (that portion of the enhanced yields that can't be compensated for by chemicals such as fertilizers -- largely nitrogen in this case, which is added to the soil by the soybeans) but it may be because of effects on soil borne pathogens.
In many cases, rotation seems to have its effect largely through "starving" the pest in years when a nonhost species is planted. For example, wireworms feed on potatoes but not on alfalfa, hence a rotation of potatoes with alfalfa can be beneficial in reducing wireworm populations -- they basically starve when the field is planted to alfalfa.
Pests are sometimes mighty adaptable however, there is some evidence that insect pests can adapt their life cycle to a regular rotation!
Altering the time of planting can also be an important cultural control. If the pest is one that emerges early, plant a variety that goes in late. Here in western OR, for example, growers tend to plant early varieties of potato to avoid problems with potato late blight, which tends to become a problem later in the season. This allows growers to simply avoid the pathogen.
The time of planting is, however, often constrained by the length of the growing season or the crop variety, so in some cases, this avoidance isn't practical. In addition, there is always the risk that some new pest problem will emerge under the changed planting regime.
Controlling adjacent crops and weeds can also be a useful strategy. For example, sometimes adjacent relatively low value crops can be planted to lure pests away from the more valuable crops; alfalfa lures lygus bug to it away from cotton for example (and the alfalfa can then be used for forage, or as part of a rotation sequence).
In some cases, growers plant strips of non-crop plants between crop rows, and then spray only the crop rows themselves with insecticide. The unsprayed strips provide refuges for beneficial insects -- and also for pest insects, thereby delaying the development of insecticide resistance in them.
Sanitation in the field or orchard can also be important. This involves removing residue from the previous season, on which many pests overwinter. For example, the apple scab fungus overwinters on fallen leaves and apple "mummies" from the previous year, so removing these from the orchard greatly reduces the inoculum for the following spring. I have observed the efficacy of this for my own apples (and the problems when I'm not good about sanitation under my trees...)!
Careful timing of irrigation can also be important, as in avoidance of irrigation at critical times when fungal spores are being shed. This can be important both for decreasing spore dispersal and because many fungal spores require humid conditions for germination.
Mulch, cover, or smother crops can be useful in weed control (and in soil retention!). People often think of using inorganic mulch -- such as plastic -- to accomplish this. For example, 40% of Florida's vegetable cropland is covered with plastic to decrease weeds, increase water retention, and warm the soil in spring. Growers are learning, however, that they can substitute organic mulch for the plastic -- living or dead. Some Florida tomato growers plant vetch (a legume) in the fall, mow it in the spring, plant tomatoes in the residue, and find that their yields are increased over those under plastic; the mulch decreases erosion, diminishes weeds, keeps the soil moist, and adds nitrogen.
An ideal mulch is one that is sown in the fall and that dies naturally at about planting time, so that extra efforts to kill or mow it (to diminish its competetion with the crop) are not needed.
Some mulches, such as rye and barley, are alleged to contain allelochemicals (chamicals that are toxic to other plants) that decrease weed establishment and growth in addition to their physical suppression by the mulch. (I'm not sure why the allelochemicals don't bother the crop too, but I think that the mulch crop is killed and its allelochemicals broken down before the crop is planted.)
In Iowa, some growers are experimenting with use of "smother crops" that are planted at or near the same time as the crop. These are low, dense growing species that suppress weeds yet competes minimally with the crop. Experiments are being conducted with vetches, clovers, and "sava" medic.
The province of Ontario, Canada made a committment a few years ago to decrease pesticide use 50% by 2002. An important part of their strategy involves use of integrated weed management (IWM), as about 70% of the pesticides used there are herbicides (as in the US).
IWM includes in ontario includes:
Overall, IWM (and IPM in general) takes a systems approach to understanding pest dynamics and controls. Researchers to model what we understand of the various interactions, identify gaps in understanding by so doing, and then attempt to fill in the gaps by research. The hope is that models that are somewhat generalizable can be constructed, so that control systems don't need to be reinvented for every unique case.
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This page is maintained by Patricia Muir at Oregon State University. Page last updated Dec. 27, 2007.