Subcommittee E: Section 2
Development of Resistance in Pest Populations
by Cassandra Stuart
Steady exposure to pesticides and herbicides can lead to the evolution of resistant pests. This term refers to all plant pestsinsects, pathogens, and weeds. Most scientists and their research focus on the evolution of pesticide-resistant insects. It is believed that the cultivation of transgenic, bioengineered plants and crops will accelerate this process.
Insects breed with incredible speed. For example, the boll weevil, a pest found in southern U.S. cotton fields, produces a new generation every 21 days and as many as 6 generations in a growing season.1 Because of their rapid breeding patterns, insects evolve quickly. They therefore develop immunity to the chemicals we douse them in through natural selection and come back more strong than ever.
Commercial cultivation of pesticide-producing plants will create strong selective pressures and resistant biotypes will quickly evolve (within 3-5 years of constant exposure).2 Take for historical example the following: the first DDT-resistant mosquitoes in Pakistan were noticed in 1965, just 5 short years after DDT pesticides were introduced into the region. This pattern of swift resistance repeated with each new pesticide. In the last nearly 50 years, over 520 species of insects and mites, 273 weed species, 150 plant diseases, and 10 rodent species have developed genetic resistance to at least one pesticide.1 (note: only very recently have transgenic crops been widely used; such figures are likely based on primarily sprayed pesticides) Many species are now becoming resistant to most major classes of insecticides. Selection for resistance to one pesticide often confers cross-resistance to other related, chemically similar pesticides. Crops that are virus resistant and/or contain insecticides may lead to new viruses and stronger pests.
In terms of weed plant pests, gene flow can cause genetically engineered plants to pass on their herbicide resistance to weed relatives. Gene flow can occur when a plant pollinates a relative plant or weed and confers its genes to another plant. This could lead to the evolution of "super weeds" capable of invading and overcoming ecosystems.
Ideally, insect populations would be suppressed to economically lucrative levels but levels still allowing susceptible insects to survive and reproduce. Mixed cultivations of protected and unprotected host plants could achieve this goal. Growers would have to be informed and aware of local pest species movement. Even so, resistant genotypes could reproduce and create pests homozygous for pesticide resistance. Another possible way of achieving our ideal is to produce pesticides only in specific plant tissues (i.e. fruits, seeds, young leaves). Yet another option is creating plants with more than one type of resistance. Also, each toxin should be produced and distributed in higher concentrations than are necessary to kill the target pest. The results of using this more potent pesticide are that partially resistant insects are more likely to be killed than they would be at lower toxin levels and that evolution of resistant pests is effectively slowed.
A specific example of this pesticide resistance problem is regulating the use of Bacillus thuringensis (Bt) toxin, a registered and commercially sold pesticide. Cotton plants have been genetically engineered to contain a Bt gene that produces a lethal protein in pests. Unfortunately, the cotton plants do not produce enough Bt to effectively kill pests and genetic resistance is developing rapidly.1 Currently the United States Environmental Protection Agency (EPA) is requiring growers to maintain refuges of non-Bt plants in hopes of prolonging the efficacy of Bt.
Clearly the evolution of resistant pest biotypes creates a need for concern. If there is no way to effectively control insect populations with current pesticides, then the use of more environmentally detrimental methods of pest control may be promoted. There are not many feasible alternatives to pesticides like Bt, so they must be managed with care. Also, resistance and cross-resistance studies show that unwanted selection can occur, resulting in pest problems even more persistent than original problems. Pesticide-resistant insects may then feed on plants that were once undesirable and would be more difficult to control than previous biotypes. Decreased efficacy of pesticides poses a serious threat to many global ecosystems and represents one of the greatest risks of growing transgenic plants.
1 Miller, Jr., G. Tyler. Living in the Environment. Belmont, CA: Wadsworth Publishing Company, 1998.
2 "Evolution of Resistant Pests". BioScience. February 1997. Vol. 47 No. 2, pg.92-93.
3 Pennybacker, Mindy. "Fooling Mother Nature. Genetically Altered Food?" Sierra. Jan-Feb 1998. Vol.83 No. 1 pg. 14.
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