Submodule 8: How Invasive Plants Are Controlled

Biological Control

It is said that biological control originated with the Chinese in ancient times when they realized that adding ants to their citrus groves increased yields. These ants would forage on insects that were destructive to the citrus trees. Classical biological control involves using the natural enemies of weeds, including insects, pathogens such as bacteria and fungi, or nematodes, to control infestations. These biological agents typically eat various parts of the weed or cause a disease that slows its growth and spread. When biological control “agents” are purposefully introduced into a new environment, the idea is to restore an ecosystem or improve the ability of native plants to compete with the targeted weed. In many cases, invasive plants thrive because they are free of their natural enemies that evolved with them in their country of origin. Introducing biological control agents helps to decrease this competitive advantage that invasive plants have over native plants. Scientists travel to the native range of their target invasive plants and collect the natural enemies that control its population size in its native habitat. Then, they test these natural enemies in the new environment of the invasive plant to ensure that they will damage only the target plant, not native organisms. This is the first test for selecting a potential biological control for a weed.

Biological control can also involve other strategies. For example, grazing animals such as goats or sheep can damage or consume invasive plants. In other cases, native plants can be grown to shade out invasive plants. Biological control strategies attempt to reduce the severity of weeds infestations, but do not eradicate infestations.

Biological control methods typically work by affecting at least one of the following plant parts of targeted weeds:

• Roots anchor the plant and extract moisture and nutrients from the soil. Some biological control agents attach to roots and prevent their growth by releasing toxins (e.g., bacteria) or disrupt water movement into the plant (e.g., fungi). Nematodes and insects may also be used to injure roots, which facilitates entry, by fungi and bacteria.
• Leaves are the site of food production in plants; they capture energy from the sun and store sugars. Biological control agents that reduce leaf surface area can hamper this process.
• Seeds are commonly produced each growing season. Several pathogens and insects attack weed seeds, thus reducing the plant’s reproductive ability.

St John's Wort before (left) and after (right) biological control.

Successful biological control methods have many potential benefits.

• They are generally environmentally friendly.
• They are usually very host-specific, meaning that other non-target species are safe from harm.
• Many biological-control organisms (especially insects and pathogens) are meticulously tested for years before they are released into the environment. This testing starts in greenhouses and laboratories and, if initial results are promising, continues with field-testing.
• One key benefit is that biological control organisms are generally self- perpetuating; that is, they are reproducing organisms that can maintain their population naturally and continue to work on limiting the growth of weeds.
• Sometimes, because of infestation location, governmental regulations, or other reasons, biological control is the only option. Native plants can be threatened by chemical or mechanical control methods, while the specificity of biological control protects them from harm.

Hundreds of biological control agents have been studied and released for weed control. About one-third of these agents have been very successful, while the rest have shown more limited effectiveness. There are good agents for some common western weeds, and others currently in the research pipeline.

To use biological control methods, the first step is to contact local weed experts and ask them what kinds of biological control agents might be appropriate for your particular situation. Questions to ask include:

• What is the best biological control agent to control a particular invasive weed in my area and what is the best source for the agent?
• You may need a permit depending on where you buy the organism and what state you live in.
• Be sure to ask for certification.
• How do I know I am getting the correct organism? Are there particular factors that can impede the effectiveness of my biological control agent?
• Unsuitable temperature and moisture.
• Presence of predators of your biological control organism.
• Weed resistance to the biological control agent.
• How many control agents will I need?
• For example, releases of insects usually range from 50 for small releases up to 100,000 for large areas.
• What do I do with the organism once I receive it?
• Timing and location of a release can be very important to the success of the project. Each organism has special requirements to keep it viable.
• What do I do after I release the organism?
• Monitor the activity and numbers of the organism after a period of time, as well as progress made in weed control. If the control agent is an insect, you can use nets to collect and count.
• Photographic records can be helpful to document “before vs. after” scenarios