Biological Control of Pests: Overview, Examples & Pros/Cons

When controlling pests in your garden you have a few options: Chemical treatment (pesticides), or biological control. Pesticides are effective but can have an adverse effect on beneficial insects as well as the pests you’re targeting. They can also contaminate soil and water.

Biological control is a method of controlling pest populations naturally, without the use of toxic chemicals. Instead, biocontrol makes use of natural predators to control pest populations and protect garden plants.

Rather than killing off the pest population entirely, you’re creating an ecological dynamic that maintains the pest population at a minimum. Here’s everything you need to know about the biological control of pests.

What is Biological Control?

Biological control is a method of pest control that manages the population of pests without chemical pesticides. This method can be used to control insects, small animals, and parasitic plants. This method typically involves introducing natural predators into the ecosystem.

Biological control has the advantage of being effective, environmentally friendly, and self-sustaining. The plants, predators, and natural deterrents used in biological control are called biological control agents.

When gardeners introduce lady bugs into their gardens to control aphid colonies, they’re practicing biological control — in this instance, the lady bugs are the biological control agents. Likewise, planting mint to keep spiders away from entrances, uses mint as the biological control agent and is actually a form of conservation biocontrol, or habitat manipulation.

Examples of Biocontrol

Farmers have used biocontrol to control pests for centuries. Here are a few examples:

• In the 1880s, California citrus farmers introduced the Australian vedalia beetle to their plantations in an effort to decrease and control the population of cottony cushion scales in the area. The success of this example of classical biological control allowed the citrus industry in California to boom.
• Nematodes that target insects have been used by farmers to control the populations of vine weevils. Specifically, Heterorhabditis bacteriophora nematodes, which release bacteria in the soil that infect and kill vine weevils, are used.
• A spray of the entomopathogenic virus CpgV has been used successfully to control the population of codling moths, which infest apples and pears making them unsuitable for eating.

Three Main Biological Control Strategies

While it seems simple, getting biocontrol right can be a challenge, especially if you’ve never done it before. There are three main strategies used in the biological control of pests, each having different steps and using different agents. These three strategies are classical biocontrol (importation), augmentative biocontrol (supplemental), and conservation biocontrol.

Classical Biocontrol (Importation) 

The first strategy used to control pest populations naturally is importation, also known as classical biocontrol. This method is used on exotic pests and involves using a natural predator.

The goal of importation is to identify useful natural enemies of a specific pest, introduce them to the area, and permanently establish them so that further human intervention isn’t necessary.

When it’s done on a large scale at the state level, for instance to save the California citrus farming industry, it involves a process of collecting potential enemies, a rigorous testing and quarantine phase, mass production and release, and then finally, follow-up studies.

But gardeners often don’t have the time, budget or desire to take on a project of that size to protect their prized roses. Instead, gardeners might order a batch of lady bugs from their local nursery to help control aphid colonies when lady bugs don’t already exist in their area.

Augmentative Biocontrol (Supplemental)

Another strategy used in biological control is supplemental, or augmentative, biocontrol. This strategy relies on boosting the population of a natural enemy when not enough of a predator exists naturally to effectively control pest populations. This can be done two ways.

The first method is by adding a small number of new members to an already established population of natural enemies at certain times of the year. This is called innoculative release. It’s expected that these enemies will reproduce and provide more long-term pest control.

The second method is called inundative release, and involves releasing large numbers of enemies to achieve immediate pest reduction. This is a corrective measure and is used to fix an out of control pest problem quickly.

Sticking with the lady bug example, releasing a large amount of lady bugs to augment the lady bugs already present in your roses would be inundative supplemental biocontrol. Releasing smaller numbers to augment your lady bug populations throughout the year would be innoculative.

Conservation Biocontrol

The third and final strategy used in biological control is conservation. Conservation seeks to modify the environment to provide ideal habitats for control agents so they can thrive and control pest populations. Habitat control usually means providing adequate resources, like plant diversity and nectar to support the control agent populations.

The problem with conservation is that while you can do everything in your power to provide your control agents with ideal habitats, you may not always see an increase in control agent populations, a decrease in pest populations, or a better yield of rose blossoms. It just doesn’t always work.

Biocontrol Agents

Biocontrol agents can be predators, pathogens, parasites and parasitoids, and even other plants. The only requirement is that they work to eliminate a pest or augment a predator population. Here is a breakdown of different biocontrol agents and how they work.

Predators

Predators feed on the pest you’re trying to control. Most predators that are used as agents in biocontrol are used because they have the ability to eat large quantities of the pest. And the most optimal predator agents are those that can lower pest populations without causing an infestation of their own.

Common predators used as biocontrol agents are lady bugs, lacewings, and preying mantids. One great example of a predator agent are ladybugs. These insects all prey on plant damaging insects, but don’t generally damage plants themselves.

Some other great predator agents include dragonflies (which help lower the mosquito population), centipedes, house lizards, and even toads.

Pathogens

Pathogens include bacteria, viruses, protozoa, fungi and nematodes that infect and kill the targeted pest.

Often these are available commercially as biological or microbial pesticides. The advantage of microbial pest control is that they’re generally harmless to humans and nontarget insect populations.

Available biological and microbial pesticides usually contain Bacillus thuringiensis or B.t, which is a microbe that lives in the soil. It creates proteins that are toxic to some insects. Others contain entomopathogenic nematodes, which also live in the soil and parasatize insects. You might also find products containing granulosis viruses, which infect and kill insect larvae.

Parasites and Parasitoids

Another common control agent used in biological control are parasites and parasitoids. Parasites, like roundworms, target kill or sterilize pests. Parasitoids are parasites that attack other parasites, usually by laying their eggs inside the body of the pest, using them to incubate their eggs.

Once the eggs hatch into larvae, they feed on the host’s body, which usually ends with the host insect dying. This type of agent is typically slower to kill off pests than the use of predators is.

Wasps and flies are two of the most common parasitoids that use other insects as hosts. Here are four commonly used wasps:

• Ichneumonid wasps, which mainly hunt for caterpillars.
• Braconid wasps, which are smaller than Ichneumonid wasps but also attack caterpillars. They also target greenflies and other insects.
• Chalcid wasps, which are even smaller than the Braconid. They target both the eggs and the larvae of scale insects, strawberry tortrix moths, caterpillars, and even green and white flies.
• Tachinid flies, which target caterpillars and beetles, among many others.

Adult parasitoids also feed on nectar, pollen and sap, so growing flowering plants that they can feed on can help attract them.

Plants

Using plants as biocontrol agents falls under the realm of habitat manipulation. And different plants accomplish different things. Some plants may act as decoys, attracting pests to them and sparing your crops. Others may provide resources for beneficial insects that prey on pests.

Here are the types of plants you might use in your biocontrol efforts:

Companion plants. Companion plants are secondary crops that you grow alongside primary crops. In general, they enhance the overall health of your crops as well as promote an ideal habitat for predators.

Repellent plants. As its name suggests, repellent plants are used to keep pests and harmful microorganisms away from your main crop through chemicals that these plants may release in the air and the odor they produce. Some examples of repellent plants that you can use include basil, citrus plants, and citronella.

Barrier plants. Used mainly to keep plant diseases from infecting main crops, barrier plants are also used by farmers to prevent pests from getting to the crops. They do so by physically surrounding the crops with barrier plants. The barrier plants protect crops by intercepting pathogens and pests. Some commonly used barrier plants include sunflowers, sesame, and pearl-millet.

Indicator plants. Indicator plants help in the early detection of potential pest infestations in gardens and farms. By using such plants, you can easily monitor if your lot is prone to an infestation so that you can act on it quickly. Some examples of indicator plants include tomato plants and bean plants.

Trap plants. Trap plants are also used to keep pests away from the main crop. These plants are grown to divert a pest’s attention away from the main crops, minimizing damage to the main crop.

Insectary plants. Insectary plants are used to boost and promote a pest’s natural enemies. They’re plants with nectar, sap, and pollen that these natural enemies, such as parasitoids, feed on. Some examples of insectary plants include flowering plants, buckwheat, and licorice mint.

The Pros and Cons of Biocontrol

The goal of biocontrol is to decrease a pest population in a sustainable way that requires little human intervention in the long run, making it a cost-efficient method over time. Biocontrol is also environmentally friendly, since it doesn’t make use of harmful chemical pesticides.

Because biocontrol can be complex, it may be daunting to gardeners. Many farm or garden owners tend to want to stick to what they know and do what is familiar. But while pesticides can create pesticide-resistant pests in the long-term, biocontrol remains effective as long as your control agent is well-managed.

Side Effects

Successful biocontrol usually manages to regulate itself effectively, but ecosystems are complex and even unpredictable. If you introduce a control agent that hasn’t been studied or used before, it may lead to some unintended consequences.

One possible negative biocontrol outcome is that the control agent you’re promoting in your environment targets a species other than the intended pests. Rather than targeting your pest populations, biocontrol can sometimes result in decreased populations of beneficial insects, too.

Alternatively, the introduced biological control agent can become pests themselves. To avoid this, avoid using biocontrol agents that haven’t been thoroughly studied and proven effective against the target pest.

How to get Started

 

Learning how to use biological control involves a lot of research and is a continuous learning journey. If you want to try this method of controlling a pest population yourself, there are a few crucial steps you need to follow.

Do your homework. Before you even plant your crops or purchase your control agents, you should do your research. Know which control agents work best with your crops and with your environment. You might try to find a local gardening group that practices biocontrol — they’ll have a lot of useful information to pull from. 

Don’t try to reinvent the wheel. Before using biocontrol, you need to have a plan and you need to stick with it. Take advantage of the methods that others have already successfully used to control your pest.

Do a bit of research and ask questions at your local nursery. Often, local garden experts will be knowledgeable about local pests and will be able to point you in the right direction. You want to have a well thought-out plan so you don’t end up switching back to chemical pesticides half-way through, ruining any progress you’ve made.

Know where to get your supplies. Before you start, you need to make sure you have everything you need. Ask around and find a reliable supplier who has decent product knowledge so you can ask questions when they come up.

Timing is key. Make sure to time the release of your agents correctly — different control agents and methods require different timing. You want to give your control agents the best chance of accomplishing your goal as possible.

Keep learning. Continue researching your methods. You may notice that you hit a few snags along the way or things didn’t work the way you expected them to. Keep track of your progress so you can adjust as needed. You might also keep an eye out for educational opportunities like seminars, since biocontrol is an evolving science.