Biological Control of Invasive Weeds

Managing Invasive Weeds with Biological Control

Many ranchers and rangeland managers are continually trying to find ways to halt prolific spread of invasive weeds. The first step in any control program is to identify the weed that is taking over your pastures, fields or rangelands. Depending on the weed, you might be able to use biological controls such as targeted grazing, and/or insects that feed on those particular weeds, to reduce the invasive weed numbers.

Biological weed control involves the use of natural enemies—such as insects, pathogens, or grazers—to reduce non-native, invasive plant populations to manageable levels. This is a sustainable, cost-effective method often used within Integrated Pest Management (IPM) strategies to control weeds but generally will not eradicate them completely. Total elimination is often impossible.

Biological control is not a new idea. In ancient times, the Chinese discovered that increasing the ant populations in citrus groves helped decrease destructive populations of large boring beetles and caterpillars. This use of a natural enemy to control a pest marked the beginning of biological control.

Common techniques today include introducing host-specific insects or utilizing livestock for grazing. These methods do not completely eliminate the target weeds but may exert enough pressure on them to reduce their populations to more acceptable and manageable levels. Biologic controls tend to be long-term actions and will only work with certain weed species. This strategy is most often in use on perennial and biennial species in natural areas, rangelands, and other perennial ecosystems. It is less common in conventional annual cropping systems where crop rotation, harvest, and soil disturbance can more easily disrupt biocontrol organisms.

An invasive weed generally came from another country such as Europe or Asia where it was part of the natural ecosystem. It had natural enemies there, to help keep it in balance with the other native vegetation. When brought to a new environment like North America it didn’t have the same ecosystem checks and balances. With no natural enemies, these weeds can spread and may crowd out the native vegetation.

Today, foreign and native organisms that attack certain weeds can be evaluated for use as biological control agents. As a weed management method, biological control offers an environmentally-friendly approach that complements conventional methods. This can help meet the need for new weed management strategies since some weeds have become resistant to certain herbicides. Biological control agents target specific weeds, and this technology is safer for applicators and consumers than use of chemicals.

If the right “enemy” for a certain invasive species can be found, this can have a dramatic effect for control of species that have been introduced into a new environment. For instance, prickly pear cactus is not native to Australia, but was introduced at some point by settlers. By 1925, Australia was struggling with 60 million acres of grazing land in Queensland and New South Wales that had become heavily infested with prickly pear cactus. Hundreds of square miles were impenetrable to humans or animals.

Then the discovery of a small moth from Argentina that utilizes the cactus for its life cycle. Some of these moths were imported into Australia and released. The moth larvae burrow into the cactus, to grow and multiply. Within 10 years these moths decimated the prickly pear cactus population. Now the cactus covers only 1% of the area it occupied in 1925.

Invasive, noxious weeds in the U.S. include leafy spurge and yellow starthistle that infest millions of acres of rangeland and wilderness areas, and estimated to cost tens of millions of dollars annually in lost grazing and associated economic effects. Leafy spurge and yellow starthistle can both spread and form dense stands, competing with native plants, reducing plant diversity, and degrading wildlife habitats.

Leafy spurge (Euphorbia esula) is native to Europe and Asia, introduced to North America in the early 1800s, likely as a contaminant in imported seed shipments. It was first documented in Massachusetts in 1827 and later spread widely via contaminated grain and soil, becoming a major invasive species across northern US states and Canada.

Biological control for leafy spurge involves using specialized, non-native flea beetles that feed on the roots and leaves. These beetles can accomplish up to 95% reduction in weed density within 3 to 5 years. The beetles are highly effective, cost-effective, and can thrive long-term, particularly in sunny, well-drained habitats. Goats and sheep can successfully graze leafy spurge as an effective biological control to reduce its spread. Goats are especially useful because they generally seek out the weed to eat it, whereas sheep learn to eat it after becoming accustomed to the taste.

Yellow star thistle (Centaurea solstitialis) is native to the Mediterranean region of Europe, Western Asia, and Northern Africa. It’s introduction to California was in the 1850s—likely as a contaminant in Chilean alfalfa seed during the Gold Rush—and has since spread throughout the western U.S. as a major invasive species.

Biological control for yellow starthistle involves using introduced insects from its native European range to feed on flower heads and reduce seed production. The most effective agents are the hairy weevil (Eustenopus villosus) and the false peacock fly (Chaetorellia succinea), which can significantly reduce seed output. They rarely eliminate established infestations by themselves, however, and work best as part of an integrated approach

Biocontrol is a slow process, however, and efficacy is highly variable. Since it can take several years for biological control agent populations to build up to levels capable of weed suppression, many of them have not been present or studied long enough to determine their ability to control their host weed. Only after monitoring the target weed and biologic control populations over time will the impact of newly introduced control agents be determined.

These agents impact their target weeds directly and indirectly–directly by destroying plant tissues and interfering with plant functions. They cause indirect impact by increasing stress on the weed, which may reduce its ability to compete with the more desirable plants. This is why integrated management practices can more effectively reduce target weed populations. After the weeds receive the biologic control agents that weakens them, they are less able to compete with desirable plants. Seeding or planting competitive species can reduce the available light, water, and nutrients to already-stressed weed populations. In addition, small weed populations in the surrounding area can be controlled by herbicides or physical means to reduce weed spread while the biologic control agents attack the primary infestation.

Biocontrol of certain weeds may not work in your area, even if it does work somewhere else. The four main methods of biological weed control are classical, inundative, conservation and grazing. The classical method involves releasing an organism such as a specific insect into areas infested with the targeted weed. The biocontrol organism sustains itself by feeding on or infecting the weed and reducing the weed population over time. These biocontrol organisms are very host-species specific. They involve natural enemies from the places where the weed originated. The practice of weed biological control in the U.S. began in the 1940s and has resulted in some spectacular successes in suppression of the targeted weed.

Since about 1960, a group of experts has been advising APHIS on decisions to release a new non-indigenous weed biological control agent into the environment. The Technical Advisory Group for Biological Control Agents of Weeds (TAG), an independent voluntary committee, was first formed in 1957 to provide advice to researchers. Today, TAG members review petitions for biological control of weeds and provide an exchange of views, information and advice to researchers.

Inundative: An organism is released or applied to control the target pest. Mass release of insects to overwhelm the pest or applying a bioherbicide both fit this category of “inundating” the pest. These techniques are intended for relatively quick and shorter-term control, and the release or application can occur multiple times.

Conservation: A cropping system is manipulated to increase the populations of natural weed-suppressing organisms.

Grazing: Cattle, sheep or goats are utilized to reduce weed populations.

If you choose to use insects for biological control you need to use the right species of insect. Shelby Filley (Oregon State University, OSU Extension Service, Douglas County Regional Livestock & Forages Specialist, now retired) says some of the “bugs” that feed on a certain weed may already be there but you might not notice them. “People often call our Extension Service asking where they can buy the bugs, and we try to help educate them about what these biological control agents look like,” she says.

The Oregon Department of Agriculture is responsible for the biological control program in Oregon. “They tell us that these bugs are everywhere and that we can collect them ourselves if we think we don’t have enough. Citizens can collect their own bugs from a patch that has them, and take them to their own place if they need more of them,” she explains. These beneficial insects are widespread, but in some locations, for some reason, they are not present, and a person can transfer some into that area.

“Oregon State University has an integrated pest management program (Oregon IPM Center). I’m not sure if all the Departments of Agriculture in the different states have a similar program, but use of biocontrol agents is now quite popular in the U.S. There are some commercial businesses that sell these insects,” says Shelby.

She thinks it would be helpful for counties to have field days to catch some of these bugs and show people what they look like, what plants they feed on and which bugs might be useful to help reduce the viability of certain invasive plants. “Interested people could learn about timing—what time of year they can find them, etc. because they are all different,” she says. Proper timing is also important for placing them in weed patches.

“Many types of larvae feed on certain plants. Usually a person would be catching and transferring the adults because they would lay eggs that would hatch into many larvae, unless they’ve already laid their eggs when you catch and transfer them. The Oregon Department of Agriculture entomologist could advise you on this.”

People need to realize that after the introduction of some of these biological control agents, weed populations in many regions have already diminished. “For instance, in years past, tansy ragwort was responsible for many cases of poisoning in livestock, and millions of dollars in death losses. Then the ODA and OSU released the cinnabar moth and the population of tansy ragwort dropped, and so did the death losses. There are actually two other biocontrol agents that are more important for tansy ragwort control than the beautiful moth that everyone loves, but since these releases years ago, we haven’t seen major livestock losses,” Shelby says. The bugs helped, but some people are still afraid of this invasive weed because they remember the bad years from earlier times.

Many of the invasive weeds in North America originated in Europe and Asia in a similar climate, but some of their natural enemies do not exist on our continent. Invasive weeds came as seeds in grain or in immigrants’ belongings or clinging to livestock hair–imported from other continents. We go to the home country to find the biocontrol agents that are host specific for the target weed. These agents receive rigorous testing to ensure safety to native plants, agricultural crops or endangered species before we release them.

Over the years there have been many biological agents brought to the U.S. after researchers found them in their native countries and did studies to make sure they are safe to bring to this continent. “The testing is very intense,” says Shelby.

“A few years ago a fungus was found that was really good at controlling non-native blackberries. These berries are hugely invasive here on the coast. The fungus is more viable on the coast where there’s more moisture, and it wasn’t able to survive very far inland in drier weather. The researchers found that the fungus was specific for certain types of blackberries and didn’t work on subspecies. Luckily it was not a problem for the commercial blackberry industry. But this illustrates the difficulty in matching a biological control agent with the weeds you want to target,” she says.

We also need some set-aside areas to preserve the biological controls, just as we now have some set-aside fields for pollinator insects (where no pesticides are in use). The biological control insects need some areas where they have their desired plants to feed on. “If there is a tansy ragwort plant in your pasture and there is plenty of forage for the livestock, leave that plant alone and don’t remove it. It may help keep the bug population alive to help control any new plants coming in,” says Shelby.

“One experiment on controlling tansy ragwort with herbicides versus biological control showed that the biocontrol was still helping years later. The spraying only lasted one season and the tansy ragwort came back and was even worse. We need to be careful to leave some of those plants. This includes some of the native plants like milkweed, that the monarch butterfly depends on,” she says. “We need to think about the insects we are dealing with and how to keep them for when we need them.”

Along with biological controls there is also a need for better pasture management as part of the strategy to reduce unwanted weeds. “A healthy pasture provides more competition for weeds and keeps them at lower levels. It really is an integrated pest management strategy,” she explains.

Targeted grazing of weed patches with livestock can also help. Sheep or goats can eat some weeds that are toxic to cattle. In some situations cattle can control weeds if they group together (using temporary electric fencing, herding or virtual fences) and concentrate in small areas to graze and knock down/eat the targeted weed.

June 2026

By Heather Smith Thomas

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