Describe the benefits and drawbacks of different methods of pest control.
The Problem with Pests
It’s not the size of the pest that is the problem; it is the damage caused by the pest that causes problems.
A pest is any species that is harmful to humans or that interferes with human endeavors such as growing crops, raising livestock, etc. Another way to view a pest is to think of any species that occurs where it is not wanted. Pest species can be as large as a deer and as small as a mite.
Deer can invade gardens or crops, causing economic damage when crops are destroyed. Mites can make us uncomfortable or even spread diseases. In either case, both species are considered pests when doing something or being somewhere where they are not wanted.
Worldwide, pests destroy about one-third of all food crops. Years ago, we thought the best pest was a dead pest, but ecologists now recognize that even pest species play important roles in their ecosystems. Now the goal is not to eradicate, or wipe out, the pest species, but to control the pest when necessary.
So, when is it necessary to control pests?
A rule of thumb for farmers is that it is necessary to control pests when the damage they are doing to a crop approaches 10% of the value of a crop, but once they surpass that percentage, the pests need to be controlled so their economic losses aren't too high.
Pest Control
Pesticides, when used carefully and properly, are one of the greatest developments in the chemistry realm. They have drastically increased food production, decreased disease transmission, and generally made our lives more comfortable by helping to reduce pests in and around our households.
When misused, however, pesticides can lead to long-lasting damage to species and ecosystems.
They can also make their way into our food chain and end up causing health issues in the very species that invented them.
There are several different types of pest control measures: chemical, biological, cultural, mechanical, and physical control. We will discuss each in this lesson.
Chemical Pest Control
Explore the tabs below to learn more about chemical pest control. Several very important concepts are discussed in the tabs: biomagnification, bioaccumulation, and Rachel Carson. Do you understand the difference between biomagnification and bioaccumulation?
Chemical treatment seeks a “magic bullet” that will eradicate or greatly lessen the numbers of the pest organism.
When we talk about chemical control of pests, we are talking about pesticides. Pesticides can be broken down into subcategories.
herbicides are chemicals that kill plants
insecticides are chemicals that kill insects
fungicides are chemicals that kill fungi
rodenticides are chemicals that kill rodents
Pesticides are useful in controlling pests but can be misused. There are a few pest control philosophies.
Chemical treatment is the philosophy we usually think of when we think of pesticides. It provides only short-term protection, and these chemicals often have highly damaging side effects in humans.
Early Pesticides
A soldier demonstrating DDT spraying equipment.
Early pesticides, known as first generation pesticides, were very strong, persistent (lasted a long time), and killed many other species in addition to the target pest. These included toxic heavy metals, such as lead, arsenic, and mercury. The philosophy at the time was to use pesticides to eradicate, or wipe out, all members of a pest species population without regard to its place in the ecosystem. Bees and butterflies suffered massive die-offs due to pesticide exposure that was meant for a different target species.
DDT (dichloro-diphenyl-trichloroethane) was an early pesticide that appeared to be a magic bullet.
It was extremely toxic to insects, seemed nontoxic to humans and mammals, and was extremely cheap, costing around $0.20/pound.
DDT is a broad-spectrum pesticide, which means that is effective against a multitude of insects, so farmers only had to spray one chemical to kill the majority of their pests.
DDT being sprayed on Texas children in the 1940s.
Additionally, because DDT is persistent, so farmers could spray once and the pesticide would last for weeks, months, or even years. The military even used DDT to control body lice and sprayed DDT over the island of Saipan, enabling the Marines to defeat both dengue fever and the Japanese army.
The characteristics that made DDT an excellent, cure-all pesticide also happened to be its downfall.
DDT controls mosquitos, which has saved millions of lives. Some countries where malaria is prevalent still use DDT because they have decided the risks of malaria outweigh the risks of DDT.
Effects of Early Pesticides
Because of the persistence of DDT, it often entered the food chain at the bottom as it became concentrated in the lower members of the food chain.Bioaccumulation is problem where small, seemingly harmless amounts of something received over long periods of time eventually reach toxic levels because the body has no way to metabolize these compounds. This is what happened with DDT, especially in the aquatic food chain as DDT was flushed into ponds, lakes, rivers, streams, and the ocean.
DDT was present in insects and those organisms that ate insects were exposed to DDT in doses that were actually higher than in the individual insects because they ate multiple insects. The organisms that ate the insect-eaters were also exposed to DDT in even higher concentrations because they ate multiple insect-eaters. This problem, where bioaccumulation becomes compounded through a food chain is called biomagnification.
Bioaccumulation occurs in individual organisms over periods of time because they continue to consume contaminants over their lifetime. Biomagnification is where contaminant concentrations increase in each level of the food chain in an ecosystem.
How does biomagnification occur? DDT was present in primary consumers and those organisms that ate the primary consumers were exposed to DDT in doses that were actually higher than in the individual consumers because they ate multiple consumers. The organisms that ate the secondary consumers were also exposed to DDT in even higher concentrations because they ate multiple secondary consumers.
Rachel Carson wrote the book Silent Spring but died before she could see the effects of her work.
Rachel Carson wrote the book Silent Spring, which detailed the effects of biomagnification and bioaccumulation of DDT in birds, causing the decline of important species such as bald eagles, brown pelicans, and peregrine falcons. This caused eggs to have very thin shells that collapsed before the baby birds could hatch. Her book led to the banning of DDT in the United States and other industrialized countries in the early 1970s and she is credited with stimulating the start of the modern environmental unit and the creation of the EPA.
In the early days when pesticides were first developed, we didn’t understand their potential impact on the ecosystem and ourselves. We didn’t know about bioaccumulation and biomagnification. Now, because of courses like this one, we fully recognize the damage associated with the misuse of all chemicals, not just pesticides. One cautionary note: just because we understand the risks associated with chemicals in the United States doesn’t mean that all countries of the world follow our lead in being careful with them. Much of our food is imported from other countries so the obvious question is, “Are the same precautions with pesticides taken there as they are in the United States?”
DDT has long been banned from use in the United States and many other industrialized countries. In addition, many other pesticides have been banned in the United States by the EPA: chlordane, heptachlor, aldrin, dieldrin, copper arsenate, and mercuric chloride are some of the others. Additionally, all pesticides today must be carefully labeled as to their ingredients, their use, and final disposal.
Explore the interactive below to learn more about the legal requirements of a pesticide label:
The EPA requires this label and also requires training in the use of pesticides for any commercial operators that use them.
Nonpersistent Pesticides
Nonpersistent pesticides are those that break down into simple nontoxic products within a few weeks of their application. This avoids the issues of bioaccumulation and biomagnification. They are, however, far more toxic than DDT and account for 70% of all pesticide poisonings and are also more pest-specific in nature than persistent pesticides. Think of these types of pesticides as a “rifle shot” at a pest rather than a “shotgun blast” that kills many more species than the target pest.
Even though these types of pesticides are better, they can still result in the decline of species such as butterflies and bees if not used correctly.
For example, carbofuran is a nonpersistent pesticide that is used to treat alfalfa. As many as 2 million birds have been killed by this pesticide and as little as ÂĽ teaspoon can be fatal to humans.
Pesticide Treadmill
When the same pesticide is continually used, the pest can develop resistance to the pesticide. No pesticide, at least those that are approved for use in the United States, kills all members of the pest species. When the pesticide is applied, it will kill most of the pests, but not all. Those pests that aren’t killed survive because they have some sort of resistance to that pesticide. Since those are the individuals that are left after the pesticide is applied, those are the individuals that reproduce. Now, future generations of these pests have resistance to that pesticide.
If the farmer applies this same pesticide to his or her crops after the pest has developed resistance to the pesticide, very few of the target pests will be killed. The farmer will need to increase the amount of pesticide to achieve an effective kill rate. This is called the pesticide treadmill because it is an unending cycle of spraying, seeing poor results, increasing the dosage, spraying again, and so on.
The pesticide treadmill is a cycle in which farmers use more and more pesticides to control pests, but the pests become resistant to the chemicals and require even more pesticides.
Over 1000 pest species have shown resistance to pesticides and breaking the cycle of the pesticide treadmill is an ongoing goal for the U.S. Department of Agriculture (USDA) and the Environmental Protection Agency (EPA), both of whom have a hand in regulating the production and use of pesticides in the United States.
Pesticide resistance is often responsible for a phenomenon known as resurgence. The pest population not only recovers but explodes to levels that are higher and more severe than they were initially. Or, when the initial pest population has been controlled, another species starts to explode because it can now occupy the niche left open by the first pest. This is known as a secondary-pest outbreak.
Over 1000 pest species have now shown resistance to pesticides.
Modern agriculture grows crops in a monoculture, or one crop at a time. Genetically modified crops have allowed widespread use of identical crops. These crops have boosted yields, but also increased pests. Because the crops are genetically identical, a pest for one plant will be a pest for ALL the plants. These pests can quickly destroy an entire crop. The use of monocultures, however, has led to improved pest control measures without increasing the amount of chemicals needed to control the pest.
The goal for pests today is control, rather than eradication. Through careful management of pests, humans can co-exist with them which helps keep ecosystems intact.
One consequence of using common pest-control methods such as pesticides, herbicides, fungicides, rodenticides, and insecticides is that organisms can become resistant to them through artificial selection. Pest control decreases crop damage by pests and increases crop yields.
Ecological Control
Another method of pest control is ecological control. This method seeks long-lasting protection by developing control agents on the basis of the pest’s life cycle and ecology. These strategies are highly specific and emphasize the protection of people and domestic plants and animals rather than completely eradicating the pest.
Bacillus thuringiensis (Bt) isn’t a predator of pests, but it is a type of bacteria found naturally in soil. Bt can be concentrated and used as a spray on insects. When the insect ingests the bacteria, the proteins in the bacteria kill the pest but these proteins cannot be activated by mammals so species higher up the food chain are not impacted.
Genetic Control
A pheromone trap that has a species-specific pheromone in it, attracting the pest species into the trap.
Genetic control of pests is another type of biological pest control. We can develop genetic traits in the host species that provide resistance to the pest, or the pest can be genetically altered. For example, scientists have effectively used sterile males to control pest populations. The natural population is flooded with sterile males – ideally 100 sterile males for every male. Then, statistically, the females are more likely to mate with the sterile males than the regular males, preventing them from having offspring.
Hormones are chemicals produced by organisms that control developmental processes and metabolic functions and pheromones are a specific type of hormone secreted by one individual that influences the behavior of another individual of the same species. Usually, insects produce pheromones to attract a mate. Using these natural chemicals to control pests is advantageous because they are nontoxic and highly specific. Farmers can also spray a pheromone over their fields, confusing the pest and making it difficult for the males to find the females and mate.
Integrated Pest Management
With IPM, chemicals are used as a last resort and are used very carefully to avoid the mistakes of the past.
Integrated pest management (IPM) is a method of managing pests by using different control methods including physical-mechanical control, cultural control, biological control, and other control methods. This approach tries to control pest populations by using all suitable methods – chemical and ecological – that brings about long-term management of pest populations and also has minimal environmental impact.
You should know that Rachel Carson wrote Silent Spring. There's usually at least one question on the AP exam about her.
You should understand the pesticide treadmill.
[CC BY-NC-SA 4.0Links to an external site.] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN LESSON.
Requirements Changed
LWU: Lesson - Impacts of Irrigation (Topic 5.5) đź“– LWU: Lesson - Integrated Pest Management (Topic 5.14) đź“–