5.6 - Pest Control Methods
Learning Objectives
The main objective is to describe the benefits and drawbacks of pest control. Essential knowledge includes recognizing that pests can develop pesticide resistance over time if pesticides are over-applied. Also, GMOs can be a solution for reducing pesticide use but may result in biodiversity loss. The associated skill is proposing a solution.
I. Pesticides
Pesticides are chemicals that are toxic to pests. Pests are organisms, like rodents or fungi, that may harm crops.
A. Types and Short-Term Effects
Pesticides are categorized by the target pest:
- Rodenticides: Kill rodents.
- Insecticides: Kill insects.
- Herbicides: Kill plants (weeds).
- Fungicides: Kill fungi.
In the short term, pesticide use may decrease the pests attacking crops and help maximize profits.
B. Drawback: Unintended Target Species
A major unintended consequence of pesticide use is that the chemicals can kill species they were never meant to target.
II. Pesticide Resistance and the Treadmill
Over time, overusing pesticides can cause pests to actually evolve resistance.
- Genetic Diversity: Within pest populations, there’s genetic diversity. Some pests naturally have genetic mutations that give them adaptations letting them survive the pesticide.
- Artificial Selection: Pesticide use acts as a form of artificial selection. It kills all the organisms that don’t have genetic resistance and leaves behind only the ones that do.
- Result: Over time, the entire pest population evolves genetic resistance to that pesticide.
- Pesticide Treadmill: This creates a never-ending cycle called the pesticide treadmill. As pests become resistant, farmers have to spray more often, use higher doses, or switch to newer pesticides, and the pests evolve resistance to those too.
- Example: Data shows that the more often BT corn (a crop with built-in insecticide) gets used, the more insects evolve resistance to it.
- Real examples: Palmer amaranth (pigweed), a major weed in cotton fields, was first found resistant to the herbicide glyphosate in one Georgia county (Macon County) in 2004. By 2010 resistant pigweed had spread to more than 50 Georgia counties. Western corn rootworms in Iowa had evolved resistance to a Bt toxin in corn by the late 2000s.
III. Genetic Modification (GMOs)
Genetic modification is another method of pest management, where the genes of the crop get altered so a resistant trait is added to their genome.
A. Examples of Modified Crops
- BT Corn: This corn has been given a gene from the bacterium Bacillus thuringiensis (Bt) that lets it produce a Bt crystal protein. The protein acts as a built-in insecticide: caterpillars and other insect pests that eat the corn are killed, but it doesn't harm people or most other animals.
- Roundup Ready Crops: These crops, like soybeans and cotton, have a genetic modification that lets them resist the potent herbicide Roundup (glyphosate). That means farmers can spray large amounts of the chemical, killing weeds without damaging their crops.
B. Impact of GMOs on Pesticide Use
- BT Corn: Using BT corn has decreased the amount of insecticide used. Since the corn manufactures its own insecticide inside its cells, there’s an inverse relationship between the percentage of BT crops planted and overall insecticide use.
- Roundup Ready Crops: The opposite happened here. The introduction of Roundup Ready crops in 1996 led to a steady rise in herbicide use. Because the crops are resistant, farmers can basically douse their fields with large amounts of herbicide.
IV. Environmental Drawback: Loss of Genetic Diversity
Fields of GM crops are planted with one uniform variety, so the plants are nearly genetically identical, almost like clones. When farmers across a whole region plant the same few varieties, the crop has very little genetic diversity.
- Catastrophic Risk: If a new disease or pest can harm one of these plants, it can harm nearly all of them, so the losses can be catastrophic.
- Real example: In 1970, about 85% of the corn planted in the US shared the same 'Texas' cytoplasm, a trait seed companies had bred into their hybrids. A new strain of southern corn leaf blight attacked plants with that trait and destroyed about 15% of the US corn crop. These weren't GMOs, but it shows the same risk of a genetically uniform crop.
- Contrast with Diversity: In a diverse population, some organisms are resistant and can survive a blight. In a cloned crop, they’re all susceptible, and effectively none survive.
- Metaphor: This risk is basically putting all of our eggs in one basket.
Big picture
Pesticide use is a race against evolution. Farmers want to maximize profit with pesticides, but the inevitable genetic diversity in pest populations means every application acts as a brutal selection pressure, weeding out the susceptible pests and leaving behind a stronger, resistant generation. That creates the pesticide treadmill, forcing farmers to keep spraying more or switching to new chemical weapons. GMOs offer a genetic shield, but they risk betting the crop’s entire survival on a single genetic strategy, which leaves the whole population open to catastrophic failure if that shield ever breaks.