Augmentation: The Periodic Release of Natural Enemies

Editor's note: This is the second in a series of articles on the three primary approaches to insect biological control. The importation and permanent establishment of foreign natural enemies was covered in the last issue; conservation of existing natural enemies will be covered in the next issue.

To many people, "biological control" means the purchase and release of beneficial natural enemies to control insect and mite pests. This approach is known as augmentation of natural enemies. The underlying reason for the widespread recognition of this technique is that it relies on the use of commercial products that are advertised in farming and gardening magazines and publicized in the media. Further, the historical use of pesticides has trained us to think about pest management in the context of purchased products. However, of the three general approaches to insect biological control, augmentation is the least sustainable because it does require the regular or periodic investment in purchased inputs. Nonetheless, in some pest situations it is a highly efficacious, cost effective, and environmentally sound approach to pest management.

The practice of augmentation is based on the knowledge or assumption that in some situations there are not adequate numbers or species of natural enemies to provide optimal biological control, but that the numbers can be increased (and control improved) by releases. This requires a readily available source of large numbers of natural enemies which has fostered the development of companies to produce and sell these. Many companies (called insectaries) produce a variety of predatory and parasitic insects; other companies produce and market insect pathogens for use as microbial insecticides.

There are two general approaches to augmentation: inundative releases and inoculative releases. Inundation involves releasing large numbers of natural enemies for immediate reduction of a damaging or near-damaging pest population. It is a corrective measure; the expected outcome is immediate pest control. Because of the nature of natural enemy activity, and the cost of purchasing them, this approach using predaceous and parasitic insects is recommended only in certain situations, such as the mass release of the egg parasite Trichogramma for controlling the eggs of various types of moths. The utilization of some microbial insecticides (such as those containing Bacillus thuringiensis) is also inundation. Inoculation involves releasing small numbers of natural enemies at prescribed intervals throughout the pest period, starting when the pest population is very low. The natural enemies are expected to reproduce themselves to provide more long-term control. The expected outcome of inoculative releases is to keep the pest at low numbers, never allowing it to approach an economic injury level; therefore, it is more of a preventive measure. Two examples are the release of predatory mites to protect greenhouse crops, and the inoculation of soils with the milkyspore pathogen (Bacillus popillae) to control Japanese beetle grubs.

Targets of augmentation. Augmentative biological controls have not been developed for all pest problems. Indeed, relatively few situations are amenable to this approach. One of the most frequent uses of augmentation is to protect greenhouse crops, a practice that was started in Europe over 30 years ago in response to widespread occurrence of insecticide resistance in greenhouse pests. Today, commercial natural enemies are available for controlling aphids, mites, scale insects, mealybugs, leafminers, thrips, caterpillars, and other greenhouse pests.

Another situation that uses augmentation is the control of filth flies in livestock manure. Several parasites are commercially available; their impact is heightened when used in conjunction with appropriate manure handling practices.

Augmentation, other than the use of microbial insecticides, has not been widely used in Midwest orchards and vineyards. It is heavily used in some areas of California, where cooperative, non-profit citrus protection districts have their own insectaries for natural enemy production. In row crops, generalist natural enemies are frequently used, such as the egg parasite Trichogramma, green lacewings, and microbial insecticides. In the United States, augmentation has probably been used the least on field crops, partly because of the lack of a complex of effective natural enemies, and partly because the expenses may not be acceptable on low-value crops. Bacillus thuringiensis is commonly used for controlling European corn borer, and considerable research is aimed at making the releases of Trichogramma, also for corn borer, a viable option. Home gardeners are increasingly using natural enemies to protect food crops and landscape plants. There are several other areas where commercial natural enemies may be used, and some companies target specialized markets, such as gypsy moth, fire ant, and stored product pests.

Types of natural enemies available. There are over 100 types of commercially available natural enemies, including predatory insects and mites, parasitic insects, insect-parasitic nematodes, and insect pathogens. Although this sounds like a high number, it is small compared to the total number of pests in the United States. Further, many of these natural enemies are specialized for pests on crops such as cotton and citrus that are not grown in the upper Midwest. Other commercial natural enemies, such as lady beetles and praying mantids, are of questionable value even though they have been highly popularized.

Efficacy. "But do they work?" This is a frequently asked question about commercially produced natural enemies. The short answer is "Yes..., and no." The long answer requires a few hundred more pages than the editor is willing to allocate to this article. There is no doubt that well-researched applications of natural enemies can be very effective. This includes the use of microbial insecticides as well as many specific uses of predators and parasitic insects. There is also no doubt that many natural enemies that are sold do not control the intended target pest(s). The reasons for the latter scenario are multiple and complex. They range from the ridiculous (my favorite example involves a community that purchased and released lady beetles for mosquito control) to the obscure. Probably the common thread that exists with "failures" is a lack knowledge. This encompasses both a lack of research needed to make recommendations for successful implementation, and a lack of needed knowledge on the part of the pest manager about the biology of the pests, the natural enemies, and their environment, all of which is crucial to making augmentation work. In this short space, my best advice for pest managers interested in embarking on a new augmentation program is to first get as much information as possible to assure a reasonable chance for success.

Cost effectiveness. Some natural enemies are much easier and less expensive to produce than others; this is reflected in their prices. Because of the differences in prices and usage patterns, it is hard to generalize on the cost effectiveness of purchased natural enemies. Other less obvious factors also have to be considered, especially when comparing the release of natural enemies to the use of pesticides. These include pesticide resistance management, worker protection, impacts on non-target pests, environmental considerations, and marketing practices (such as conventional vs. organic). Another problem is that, for many commercial natural enemies and their potential target pests, there is not adequate research to recommend specific release rates based upon pest population levels. There are, however, many situations where augmentative biological control is cost competitive with the use of pesticides or other pest management practices. The high value of many specialty crops reflects high production costs, including pest management. In such crops, the expense of biological control may be relatively low when compared to overall production costs. On low value crops, the use of natural enemies must be inexpensive to be justified. This does not preclude the use of augmentation in field crops; inundative controls such as Bacillus thuringiensis and Trichogramma may be cost effective, as can be inoculative releases that rely on relatively low numbers of natural enemies. The cost of natural enemy releases should be carefully evaluated, as with any other production cost.

In summary, we in Extension get more questions about the release of purchased natural enemies than all other approaches to biological control. And in some cases, it is the area where we have the fewest answers. Many augmentation programs do work and are cost effective. But augmentation can not be considered "the silver bullet" of biological control. It is not foolproof, and it requires a certain level of knowledge and understanding to make it work. Additionally, we have effective commercial natural enemies for a relatively small percentage of all the types of pests we must manage. It is the most costly and least sustainable form of biological control. However, where it does work and is cost effective, augmentation can be a very useful pest management method.

For further reading, refer to the book Biological Control by Augmentation of Natural Enemies, by R. L. Ridgway and S. B. Vinson, 1977, Plenum Press.

- Dan Mahr, University of Wisconsin - Madison

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