Perhaps the most common way that growers control pests in annual crops is through the use of crop rotation. For instance, growers typically rotate corn with soybeans throughout much of the Midwestern U.S. This change of crops sets back the population of certain pests. For potatoes, it is often necessary to have several seasons of other crops planted between each potato crop, otherwise pests build up to damaging levels.
Some pests spend different parts of their lifecycle on different hosts. A damaging and even deadly bacterial disease of grapes is spread by an insect called the glassy-winged sharpshooter. A state-wide program in California has been in place for the last 15 years to minimize the damage caused by this pest to the highly valued wine grape industry. One element of this program is based on the fact that the insect spends the winter feeding in citrus orchards while the grapes are dormant. By disrupting the pest’s life cycle stage on citrus, the state greatly diminishes the risk to grapes.
California uses a similar off-season program to control the sugar-beet leafhopper which is a vector of the curly top virus disease of peppers and tomatoes. In the winter, the hoppers survive on weeds in rangeland and unused cropland and then return to the tomato and pepper crops in the summer. Through a state program, California monitors for off-season populations and treats non-crop areas as needed to protect the next season’s vegetables.
Another way that growers can control insect pests through life-cycle disruption is an approach called mating disruption.
Growers place synthetic versions of the insect’s mating hormones throughout a field or orchard so that the males can’t detect the gradient of the hormone that guides them to females. This sort of program works best with relatively low populations of the insects, so it is best paired with a targeted insecticide program.
In certain cases, it is possible to release large numbers of male insects that have been raised to be sterile. These males out-compete the wild ones to mate with the females, which then generate few offspring within the population. This strategy has often been applied to control mosquitoes that vector (or spread) human diseases or to control introduced exotic pests like the Mexican fruit fly.
Historically, sterile male release programs have been based on the use of gamma irradiation to sterilize the insects. These males may be compromised in other ways, so programs must use large numbers of sterile males to effectively compete with the wild males. Scientists have developed a technology that genetically modifies males to need a specific, supplied nutrient in their diet to survive. Growers or others involved in insect control programs can release such males to mate, but the males will not live long and their offspring will not survive because they inherit the requirement for the nutrient.
Field-testing of this technology has demonstrated that it can be effective to lower the populations of the mosquitoes that spread human diseases like dengue fever, Zika virus or chikungunya virus. This approach could also be helpful for controlling some of the newly introduced exotic pests that now threaten various crops around the world.
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