Non-native species of plants and wildlife are in every ecosystem, both on land and in the water. You’ve probably got a few in your back yard in the form of an ornamental shrub or flower, not to mention an aphid or two. According to the U.S. Forest Service, the United States harbors at least 5,000 different non-native species of plants alone.

While some of these foreign flora and fauna fit in just fine, others take over. They become invasive, crowding out native species and wreaking havoc on not only your garden, but the health of our waterways, forests, fields, highlands and everywhere in between.

The reason why these non-natives go rogue is because they have no natural population controls like they did in Asia, Europe or wherever they came from.

Controlling an invasive species is a challenge. If it’s a plant, we might be able to weed it, at least until it covers too much acreage. For unwanted plants and insects, chemicals (herbicides or insecticides) might be another option.

But one effective way to get rid of a pest is to have a natural predator kill it. In fact, that is the definition of “biological control,” or “bio-control” for short.

Natural controls for pests

“Biological control is the use of living organisms to suppress pest populations, making them less damaging than they would otherwise be,” said Kimberley Stoner, an entomologist with the Connecticut Agricultural Experiment Station in her paper, “Approaches to the Biological Control of Insect Pests,” published by the Connecticut Agricultural Experiment Station. According to Stoner, biological control can be used against all types of pests, including vertebrates, plant pathogens, weeds and insects.

The use of bio-controls is not new. One of the earliest success stories dates back to the late 1800s when cottony cushion scale (Icerya purchasi) threatened California’s citrus industry. This scale is an insect that feeds on plants and citrus trees in particular. Fruit growers imported the vedalia beetle which feeds on cottony cushion scale. Within a few years, the scale problem subsided.

Another example of the successful use of a bio-control is a wasp from China that was introduced in the United States to prevent the European corn borer from decimating corn crops. The wasp fed on the corn borer, and this stemmed the threat without the heavy use of pesticides.

There are three types of bio-controls: predators, parasitoids and pathogens.

Predators

Insects, like beetles and wasps, are the most common forms of predators used as bio-controls, often to get rid of other insect pests. Vertebrates—birds, amphibians, reptiles, fish and mammals—might also feed on unwanted bugs. But not all of them serve as effective bio-controls. The problem is that invasive species may not be their top food choice or only food choice. What’s more, these vertebrates probably don’t exist in large enough numbers to have the needed impact.

But insects that naturally feed on an unwanted interloper often do exist in large enough numbers. This can work when the insect has a narrow diet, a short lifecycle that’s tied to the invasive species and population density that ebbs and flows with the invasive’s population. When the undesirable bug is gone, the bio-control often disappears, too.

Parasitoids

A parasitoid is an insect that spends part of its lifecycle developing inside a different “host insect,” killing the host. This is an ideal type of bio-control if the parasitoid requires a specialized host that is an invasive species. When the host species is gone, the bio-control dies off.

Pathogens

Pathogens, such as bacteria, fungi, protozoa and viruses, can reduce the rate at which an invasive species feeds, grows and reproduces. Or they might simply kill the pest. The risk is that the pathogen might infect native species as well. That said, pathogens can be an effective form of bio-control.

For example, the fungus Entomophaga maimaiga has proven an effective way to control the invasive gypsy moth in New England. The spores of the fungus rest on the forest floor over the winter, then germinate when gypsy moth caterpillars are present. The caterpillars crawl through the fungal spores, then carry them into the trees, where rainfall triggers the production of more spores. In June, the infected caterpillars die and fall to the forest floor, where the fungus remains, waiting to germinate again the next spring.

Once a bio-control is identified, then researchers determine how to introduce it. Some of the choices are 1) establish a permanent population or not; 2) import the species or breed it in the U.S., and 3) release the species all at once or intermittently.

However, the problem with bio-controls is that they can become problems in their own right. It’s a tricky balance since the control is often non-native, too.

Regulating bio-controls

Since the vedalia beetle saved California’s citrus trees over a century ago, biologists have sought bio-controls with varying degrees of success. We know more now than even a few decades ago.

Today, when the U.S. Department of Agriculture and a state agency approve a bio-control, a tremendous amount of research and testing has already been conducted to ensure it will work—without becoming a problem itself. Biologists must first identify the potential control species, then understand how it will impact native species. If it passes that test, then the bio-control has to be quarantined to ensure it isn’t carrying unwanted diseases or parasites. Finally, the bio-control must be released in a limited way, then studied to ensure it effectively does what it’s supposed to do.

This process can take years. But if successful, the results can be dramatic and positive. One example is the parasitoids that were introduced from Europe to control the destructive alfalfa weevil, also from Europe, that had spread across the United States during the 1900s. One of the parasitoids attacked the weevil’s larvae. Another struck the adult, and yet another targeted the weevil’s eggs. Thanks to this combination of bio-controls, alfalfa remains a viable crop.

Bio-controls that backfire

Unfortunately, bio-controls can backfire. Take invasive carp, which include bighead, black, grass and silver carp, all of which outcompete native fish in the Mississippi River system and have spread to other bodies of water in the country.

Native to Russia and China, these carp were first brought to the United States about 50 years ago to control aquatic plants in reservoirs and aquaculture farms in Arkansas and other locations near the Mississippi River. Other states soon stocked them, too. When flooding gave them unintended access into the Mississippi River system, they found their way into many of the nation’s waterways, wreaking havoc as they ballooned in number.

Carp grow quickly and feed prolifically, outcompeting native fish for food. The longer carp are present, the more they alter the make-up of a waterway’s food web by lowering its phytoplankton and invertebrate populations. They reduce the amount of sheltering flora used by native fish, reduce the populations of sportfish like bass and generally change the trophic structure of a lake or river.

According to the Illinois Department of Resources, which recently renamed Asian carp “Copi” (as in “copious”), 20 to 50 million pounds of Copi could be harvested from the Illinois River alone. As a result, that state is embarking on a PR campaign to encourage anglers to eat Copi, which have a mild taste and contain heart-healthy omega-3 fatty acids. Underlying the “eat Copi” campaign is the hope that eating more of them will reduce their impact in the Great Lakes region and help restore aquatic systems downstream.

Another example of a bio-control that got out of control is the plant, kudzu, an edible, semi-woody vine with pleasant-smelling purple-red flowers. Initially imported into the U.S. in 1876 from Japan and China as an ornamental vine, it was used from the 1930s to the 1950s in the southeastern U.S. to reduce erosion. Now called “the vine that ate the South,” kudzu has spread to the Midwest, Northeast and Oregon. It can grow up to a foot per day, creeping over and covering literally everything in its path.

Native plants deprived of sunlight under a carpet of kudzu often die. The insects that depend on native plants also disappear. That, in turn, affects bird populations, which depend on insects to feed their young. Other native animals that can’t adapt don’t survive either. Over time, as native flora and fauna disappear, biodiversity steeply declines.

On the bright side, many bio-controls help to effectively control pests in our back yards, croplands and wildlands, often reducing the need for chemical controls. The key to successful use of bio-controls is understanding the full impact of a species in an ecosystem to ensure that it’s a solution that doesn’t become a new problem. When we get bio-controls right, the results can be long-lasting and inexpensive, with little or no ongoing human effort.

Top photo: Kudzu is sometimes called the vine that ate the South. Credit: Plant NoVa Natives.