The pros and cons of biological control Part I: Bugs for hire
For managing landscape pests, many people view the use of biological control favorably. Thus, when appropriate, this tactic can generate good will with clients. Though this approach is not without disadvantages, several factors contribute to its positive image:
Biological control includes the use of existing natural controls-enemies of pests (predators, parasites and diseases) that already are present in the landscape.
Although the point is debatable, many people perceive "natural" as good.
Under ideal conditions, biological control has a sustainability that is lacking in some other management options.
Some disadvantages include:
You must tolerate some level of infestation to sustain beneficial insect populations.
The shipping, storage and application techniques necessary to accommodate living organisms can be relatively complex.
Control is sometimes erratic for environmental or ecological reasons that may not be apparent.
Cost often is high relative to conventional controls.
As these lists indicate, biological controls have strengths and weakness like any other method. You must weigh the benefits and drawbacks of any control measure to decide if it is appropriate. Generally, the strict demand for efficacy in the landscape-maintenance industry limits the use of biological controls in this field. However, if you carefully consider the available biological controls, you may find a few that fit into your existing pest-control program without sacrificing quality. This article discusses the pros and cons of "bugs for hire"-predators and parasites you can buy and then release in the landscape. "Part II" will cover microbes and their derivatives, insect growth regulators and plant extracts.
What, exactly, is "biological control," and how can you best utilize it in landscape management? Let's look at the three general types of biological control we recognize.
Classic biological control Classic biological control involves bringing into a region new natural enemies that you hope will become permanently established and provide long-term suppression of the target pest. Scientists have known the value of this approach for over 100 years-ever since the vedalia beetle was brought here from Australia in 1888. This introduction saved the California citrus industry in spectacular fashion from destruction by the cottony cushion scale. A more recent example involved the ash whitefly. This insect exploded onto the scene in the early 1980s, seriously threatening a wide variety of shade trees and shrubs in California and the Southwest. Yet, the successful introduction of a small parasitic wasp checked the infestation rapidly, reducing the ash whitefly to a minor pest within a couple of years.
Classic biological control almost always is targeted at insect and mite pests that were themselves previously introduced from foreign lands. In a new environment, a lack of natural enemies may allow an introduced pest to increase its numbers exponentially, rapidly creating a serious outbreak. When researchers study the pest in its original habitat, they often are able to identify a natural enemy they can import to successfully control the pest.
Government agencies typically have sole management over such operations and for good reason. Extensive testing and careful study are necessary whenever a new organism is considered for release. This is to ensure that this new beneficial species does not cause ecological harm after release in its new home. However, classic biological control for landscape pests has received little government support compared to agricultural crops. This is unfortunate because many important landscape pests originated from foreign lands-the elm leaf beetle, oystershell scale, black vine weevil and European chafer, to name but a very few.
Enhancing existing controls A less well-recognized approach to biological control is conserving and enhancing the activity of existing natural enemies. You can achieve this with several simple measures, starting with learning how to identify the biological controls already present in the landscape. Reducing pesticide use or switching to more selective products can help conserve these natural enemies. However, you can take more proactive steps. For example, many natural enemies of pests supplement their diet with nectar or pollen from various plants. This can be important for sustaining their numbers during periods when their usual prey are lacking. Thus, designing landscapes in a manner that promotes existing natural enemies may include steps such as using a range of plant material that provides season-long bloom. Such diversity also will increase the kinds of natural enemies present.
Bugs for hire When most people think of biological control, they think of a commercial product-something they can purchase to help manage landscape-pest problems. Several of these are available-"bugs for hire" that have either been reared in special facilities (insectaries) or have been field collected for sale. A 1994 compilation of sources of biological-control organisms (see "For more information...," below center) listed almost 100 companies in the United States that sell over 80 species to control insect or mite pests. Most of these work best for control of insects and mites in greenhouses, around livestock barns and for certain agricultural pests. However, a few are appropriate for landscape pest management. See "Pros and cons," below, for a discussion of the benefits and drawbacks of some beneficial organisms.
Many suppliers of biological-control organisms are in operation. Some produce their own product and some are middlemen. Thus, it is common to find a two- or three-fold price range for specific biological-control organisms among suppliers. Garden-catalog prices can be as much as 10 times higher than other sources! So, if you are considering purchasing biological controls, contact several suppliers, compare prices and discuss your needs with them.
Although purchased biological controls will never solve all your landscape pest problems, you may find a few products that fill significant niches in your pest-management program.
Dr. Whitney Cranshaw is professor of entomology and extension specialist at Colorado State University-Fort Collins.
Suppliers of Beneficial Organisms in North America is a listing of biological-control organisms for sale in North America. The California Environmental Protection Agency, Department of Pesticide Regulation, produces it as publication PM 94-03. For a free copy, call (916) 324-4100 or write to: 1020 "N" Street, Room 161, Sacramento, CA 95814-5604.
GREEN LACEWINGS Commercial insectaries rear green lacewings (Chrysoperla spp.), which are widely available through biological-control suppliers. The larvae of green lacewings are general predators that eat large numbers of a variety of insect pests. Their tremendous appetite has led to their dubbing as "aphid lions." However, their powerful curved jaws allow them to subdue many other insects, even fairly large caterpillars. Adults do not feed on insects but consume the nectar and pollen.
You can purchase lacewings in many different stages, but eggs are the most common and by far the cheapest form. The insects which emerge from the eggs-the voracious larvae-are the form most desirable for pest management.
PROS Green lacewing larvae consume large numbers of a variety of pests. Unlike lady beetles, green lacewings will not wander far after you release them. The larval form-the beneficial predatory stage-is wingless.
CONS Suppliers typically mix the eggs with rice hulls or some other carrier that you then sprinkle over plants. This may be an aesthetic problem on ornamentals.
Predation by ants or cannibalism by hungry green lacewing brethren often cause large losses after release.
LADY BEETLES Also known as lady bugs or lady birds, more than 100 species of lady beetles occur in North America. One, the convergent lady beetle (Hippodamia convergens), dominates the lady beetle market. This is because it's easy to collect the convergent lady beetle at sites in the California foothills, where they periodically amass in huge numbers. The first of the commercially exploited biological controls (since the 1920s), it remains the only field-collected species for sale. The convergent lady beetle naturally occurs throughout North America.
PROS The convergent lady beetle is a highly efficient feeder, preying on aphids but also feeding on insect eggs and, occasionally, soft-bodied insects such as scale crawlers (an immature stage of scale insects), young caterpillars and beetle larvae.
A consistent benefit with the release of lady beetles is good public relations. Few insects are as widely recognized and held in such universal regard as lady bugs, and their release almost invariably provides "feel good" results. (Just don't bet your whole pest-management program on a few lady beetle releases.)
CONS The track record of purchased lady beetles is spotty and has long been controversial. The biggest problem is that they rarely remain where you release them. Collectors obtain them during dormancy, so they have few, if any, mature eggs and are poised to migrate. Thus, few of the insects remain at the site of release and rarely do they help control local pests. Some suppliers have tried to improve this situation by feeding the collected beetles before sale. However, the benefits of this practice remain debatable, while the costs of these "conditioned" insects typically are 10 times greater.
WHITEFLY PARASITES The greenhouse whitefly and sweetpotato whitefly produce serious problems on bedding plants, particularly in warmer climates. They also are important greenhouse pests and have been the target of some of the most extensive classic biological-control programs.
Two tiny parasitic wasps are the most common whitefly parasites: Encarsia formosa and Eretmocerus californicus. Both attack the immature "scale" stages of whiteflies, within which the immature wasps develop as they consume their hosts. If these parasites are not already present in a landscape where whiteflies are a concern, an early-season introduction may ward off later problems.
PROS You can use these wasps as an effective preventive measure-they provide season-long suppression if conditions are favorable.
Whitefly parasites have a long track record of success against the greenhouse whitefly.
Whitefly parasites are reasonably inexpensive and widely available.
CONS Whitefly parasites can only control greenhouse whiteflies if daily average temperatures exceed 72 to 75°F.
Available whitefly parasites do not control the sweetpotato whitefly, which recently has become a major pest in many Southern states.
INSECT PARASITIC NEMATODES Several nematodes commonly attack insects and are known as insect-parasitic nematodes or, more technically accurate, entomopathogenic nematodes. A few companies commercially produce a handful of these species, which are available through suppliers.
Insect-parasitic nematodes control pest insects first by penetrating their body, then releasing bacteria that rapidly kill the host. Then, young stages of the reproducing nematodes feast on the "soup" of bacteria and decaying insect tissues. Thousands may ultimately issue from an attacked insect.
The most commonly available species is Steinernema carpocapsae (LESCO's Vector TL), which controls various caterpillars (sod webworms, cutworms and certain borers), billbug larvae and various soil pests. A related species, Steinernema riobravis (LESCO's Vector MC), has recently become available for control of mole crickets in turfgrass. Nematodes in the genus Heterorhabditis (for example, Ecogen's Cruiser) are unique in their ability to penetrate certain insects and are particularly good for control of white grubs and black vine weevil.
PROS Effectiveness is well documented.
Insect-parasitic nematodes are highly selective, affecting few non-target organisms.
You can use conventional application equipment.
Insect-parastic nematodes can kill hosts fairly quickly, often within a few days of infection.
CONS Insect-parasitic nematodes will die if you store them too long or at excessively warm or cold temperatures.
They also are susceptible to exposure to light and desiccation. Therefore, they only are useful for controlling insects in soil or other protected sites, such as under bark (to control borers).
PREDATORY MITES Some of the most important natural enemies of mites and thrips are predatory mites of the family Phytoseiidae. About a half dozen species are in commercial production, with the primary markets being pest management in strawberries, tree fruit, greenhouse vegetables and interiorscapes.
Grounds managers seldom attempt to release them in landscapes. Although their potential usefulness in this situation seems clear, existing research has dealt primarily with greenhouse and interiorscape use.
If you attempt to use predatory mites, it is important to find the particular species that best fits your situation. Each has its own requirements for temperature and humidity, information which most suppliers provide. For example, optimal conditions for Phytoseiulus persimilis are 70°F to 100°F and 60- to 90-percent relative humidity. Mesoseiulus longipes can tolerate humidity as low as 40 percent, although its need for humidity increases with temperature.
Because predatory mites are perishable, it is important to find reliable suppliers. Some suppliers provide packaging that eases this urgency somewhat. One system encloses the mites with some food in a small "tea bag" sachet. Placed where needed, the mites work their way out and search nearby plants for prey.
PROS Healthy populations of predatory mites can provide adequate suppression of plant-feeding mites-pests that often require repeated chemical applications for control.
CONS Predatory mites are highly perishable. Express shipment often is necessary and you usually should release the mites as soon as possible after you receive them.
Cost can be high.
Adequate control may not occur until predatory-mite populations build up. This may take a while.
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