According to Dr. Tony Shelton, professor
of entomology at Cornell University in New York, biological control is defined as the use of predators, parasitoids or
pathogens to reduce pest populations. Such agents
are commonly called natural enemies.
Shelton says, "While biological control will not solve all our pest problems, it should be a recognized component of any integrated pest management program. Both small and large-scale farms can use biological control to reduce traditional pesticide inputs and make the systems more sustainable and less likely to experience pest outbreaks."
Since lady beetles consume other insects, they can be used as biological controls as part of an integrated pest management program. Parasitoids, such as Diadegma insulare, develop on or inside an insect host, acting as a biological control of damaging insect pests.
Why biological control?
Some hear a term like "biological control" and automatically dismiss the concept as representing exotic and narrowly applicable approaches to controlling insect pest populations on farms. In fact, biological control has been a part of agriculture since the beginning of recorded time. Advanced biological control, using techniques made available by modern science, has taken on increasing importance in America and elsewhere for as much as a century.
A 2008 paper by Shelton discusses the natural bacterium Bacillus thuringiensis (commonly called Bt), a biological control important to the nation's corn and cotton industries. Shelton wrote, "One of agriculture's best defenses against plant-eating insects is Bt, which either can be sprayed on the surfaces of crops to provide temporary protection or can be genetically engineered into the crops to protect against insects throughout the life span of the plants. Bt has allowed growers to avoid applying large quantities of potentially toxic insecticides."
Biological control technologies are also becoming more important as the move toward organic farming gains momentum, but beyond organic farms, Shelton says, "Biological control is an essential component in all cropping systems in use today, but is more effective in some than others. It is most effective in perennial systems, like citrus and forests and in greenhouses, but natural enemies help suppress pest populations in virtually all systems."
The natural enemies Shelton speaks of include predators like ladybugs (lady beetles), which consume other insects over their lifetimes; parasitoids, including wasps that complete their larval development on or inside an insect host; and pathogens, disease-causing organisms including bacteria, fungi and viruses.
Biological control as part of an IPM system
In promoting biological control of the various pests bedeviling growers, Shelton is not suggesting the adoption of a wholesale switch to biological control. He is promoting the idea that biological control is a pest reduction strategy capable of enhancing the effectiveness of other approaches, reducing the negative aspects of a pest control regimen based strictly on chemicals and, important to any agriculturalist, improving profitability.
According to Shelton, effective management of insect pests rarely relies on a single control practice. Instead, a variety of tactics can be used as part of an integrated pest management (IPM) system to attack the pests preying on valuable crops.
"The goal of IPM is not to eliminate all pests. Some pests are tolerable and essential so that their natural enemies remain in the crop. Rather, the aim is to reduce pest populations to less-than-damaging numbers. The control tactics used in IPM include pest-resistant or tolerant plants, and cultural, physical, mechanical, biological and chemical control. Applying multiple control tactics minimizes the chance that insects will adapt to any one tactic," Shelton explains.
Approaches to biological control of pests generally fall into one of three categories, with considerable overlap at the boundaries of the techniques: conservation, classical biological control and augmentation.
Conservation of natural enemies
According to Shelton, the conservation of natural enemies is probably the most important and readily available biological control practice available to growers. Natural enemies to pests can usually be found wherever the pest is found, whether in the backyard garden, the 2-acre berry patch or in the larger commercial field. It is important to consider preserving natural enemies when developing strategies to manage pests in the fields. For example, if pesticides are used carelessly and end up killing pest predators, the ability to control pests long-term takes a hit.
Classical biological control of imported pests
About 40 percent of the insect pests in the U.S. are nonnative, either to the U.S. or to the area they infest. Introduced pests, without their natural enemies, can have a tremendous impact on farms and forests; witness the European corn borer, one of the most destructive insects ever introduced to North America.
Classical biological control consists of determining the origin of an introduced pest and then collecting that pest's natural enemies from the region of origin. After a rigorous quarantine regimen to assure unwanted pests are not being introduced, large quantities of the pest's natural enemies are reared and released. Ongoing monitoring is then important to assess the effectiveness of the control.
Shelton developed a biological control website (www.biocontrol.entomology.cornell.edu), where he discusses classical approaches to pest control. According to the site, "There are many examples of successful classical biological control programs. One of the earliest successes was with the cottony cushion scale, a pest that was devastating the California citrus industry in the late 1800s. A predatory insect, the vedalia beetle, and a parasitoid fly were introduced from Australia. Within a few years the cottony cushion scale was completely controlled by these introduced natural enemies." Other examples include the successful introduction of biological controls for the alfalfa weevil and the importation of a small wasp from China to help control the European corn borer.
The third major approach to biological control is augmentation: the release of natural enemies to the pests being controlled to supplement the work of already existing natural enemies.
Augmentation can mean the release of small numbers of natural enemies at strategic times, called an inoculative release, or as many as millions of natural enemies to inundate an area. Inoculative releases are routinely used in greenhouses to control pests. Inundative releases of insect enemies, like ladybugs, lacewings and others, are more often used to control field crop pests.
Photo courtesy of Joseph Berger, .
Augmentation can also be accomplished through habitat manipulation. According to Shelton's biological control website, "Mixed plantings and the provision of flowering borders can increase the diversity of habitats and provide shelter and alternative food sources. They are easily incorporated into home gardens and even small-scale commercial plantings, but are more difficult to accommodate in large-scale crop production. There may also be some conflict with pest control for the large producer because of the difficulty of targeting the pest species and the use of refuges by the pest insects as well as natural enemies.
"Examples of habitat manipulation include growing flowering plants (pollen and nectar sources) near crops to attract and maintain populations of natural enemies. For example, hoverfly adults can be attracted to umbelliferous plants in bloom."
Ongoing work is leading to new understanding of the potential habitat manipulation has for biological control. For example, the website points to California, where it has been demonstrated that planting prune trees in grape vineyards can provide winter habitat for an important parasitoid that preys on pests that damage grapes.
Approaches to an IPM system
According to Shelton, IPM requires an understanding of the ecology of the cropping system, including that of the pests, their natural enemies and the surrounding environment. Knowledge about the ecological interrelationships between insects and their environment, like that available on the Cornell website, is critical to effective pest management.
An important beginning step in developing a pest management strategy is to assess the amount of damage to the crop in relation to the economic impact of possible controls. In some cases, application of a pesticide may be necessary, but in other cases biological controls may make more sense.
Once assessment of the potential for both short and long-term damage to a desired crop has been made, a variety of strategies can be brought to bear on the problem. Some of the more common strategies are described below.
Shelton says, "One of the mainstays of integrated pest management is the use of crop varieties that are resistant or tolerant to insect pests and diseases. A resistant variety may be less preferred by the insect pest, adversely affect its normal development and survival, or the plant may tolerate the damage without an economic loss in yield or quality. Disease-resistant vegetables are widely used, whereas insect-resistant varieties are less common but nonetheless important. An example of where the approach has worked remarkably well is the use of resistant cabbage varieties to control thrips, a small but devastating insect pest."
Advantages of this tactic, Shelton says, "include ease of use, compatibility with other integrated pest management tactics, low cost, and cumulative impact on the pest (each subsequent generation of the pest is further reduced) with minimal environmental impact."
Downsides include the time and money needed to develop resistant plants and, as is the case with many strategies, the ability of insect populations to adjust to the new environment over time.
Shelton points out that there are many practices that make the environment less favorable to insect pests. Examples he points to include cultivation of alternate hosts that may be more preferable to pests than the crop to be protected, crop rotation, selection of planting sites, trap crops, and adjusting the timing of planting or harvest.
The selection of a strategy depends on the type of crop and the pest attacking it. For example, planting a crop at a time that is out of sync with a pest's natural life cycle can be an effective control mechanism, and in some cases an economic advantage.
Physical and mechanical control
In days gone by, flypaper hanging from a room's ceiling was a standard "cultural control" for houseflies. The same technology is used today in greenhouses to capture flying pests. Other means of control might include physical barriers to prevent insect travel, including row covers, trenches or various kinds of traps.
Insecticides, Shelton points out, can be reasonably inexpensive, are easy to apply, act quickly and are usually effective when needed. However, when chemical controls are needed, it is important to select the product based on more than just how well the pest is controlled. One chemical, for instance, might destroy the targeted pest as well as beneficial critters. Shelton says, "Ideally only the target pest(s) should be affected. The goal is to maximize pest mortality while minimizing harm to natural enemies."
Examining the options might reveal an alternative that will be equally effective in terms of pest control, but doesn't destroy natural enemies that should be preserved to kill off future generations of the targeted pest. A process leading to the choice of the least-toxic pesticide possible can lead to more effective long-term attacks on pests, reduced environmental impact and reduced cost over time.
Shelton makes no claim that biological control technologies are capable, for the present at least, of replacing chemical approaches to controlling the pests that prey on crops. On the other hand, Shelton points to the cost savings, reduced environmental impact and improved effectiveness biological control can bring to the grower's toolbox as part of an integrated pest management system.
Shelton concludes, "Biocontrol agents are valuable for integrated pest management and should be preserved. They can reduce the use of other pesticides; they can help stabilize systems and help prevent pest outbreaks; and their effectiveness will depend on the system in which they are used."
The author is a longtime freelance contributor to Moose River Media.