Criteria for Choosing Entomopathogens for Microbial Control of Insects

Many, if not all, insect pests are susceptible to a variety of infectious diseases. This fact raises the question of how to choose species or even strains of entomopathogens to target for research and development in microbial control. This choice is not trivial, because the overall development cost for a bioinsecticide has been estimated at up to US$1-10 million.

Underlying the choice of entomopathogen is the approach with which it will be used. The choice of an entomopathogen for microbial control can be based on two major types of criteria: those related to the entomopathogen; and those related to the target insect pest, the host plant, and the ecosystem. These criteria are applicable to microbial control in row-crop agriculture, medical and veterinary entomology, and forest entomology.

Approaches to Microbial Control of Insects

Microbial control can be defined simply as the use of microorganisms or their by-products by humans to suppress insect pest populations, implying that the microorganism is subject to some sort of manipulation. There are four approaches to microbial control of insects: short-term insecticide, introduction and establishment, seasonal colonization, and environmental manipulation. Generally, commercial development is limited to the first two of these approaches. The four approaches differ fundamentally in the manner in which the entomopathogen is manipulated and in the expected results.

In the short-term insecticide approach, large numbers of the entomopathogen (the "microbial insecticide") are produced and released for relatively quick suppression of the pest population. The pathogen does not replicate or "recycle" in the environment sufficiently to suppress subsequent infestations of the pest. Thus, repeated applications are necessary, which is beneficial for commercial development. They generally are applied by means of technology developed for agricultural chemicals and often are formulated as aqueous concentrates or wettable powders to be applied as sprays. Relatively rapid cessation of pest damage is often beneficial in this approach but is difficult to achieve with most entomopathogens.

The introduction and establishment approach also is referred to as "classical biological control." An entomopathogen species or strain is released in an area where it did not previously occur. It becomes a permanent component of the ecosystem in which it is released, recycling and often spreading from the release site, resulting in permanent pest population suppression. Commercial development is virtually nonexistent, because the one-time release of a pathogen does not lead to monetary profit. Due to the one-time application and spread, the cost of production and application is a consideration but is not nearly as critical as with commercial approaches. This allows much more flexibility in the choice of the pathogen and release method. The entomopathogen does not necessarily have to kill the pest, only to reduce pest reproduction. Also, a release does not necessarily have to reduce pest populations below economic injury levels, for two reasons: first, releases of multiple species are possible; second, if partial suppression even occasionally eliminates the need for application of a chemical insecticide, this still can be economically worthwhile because the permanent level of control is cost-free after the initial release.

The seasonal colonization approach can be thought of as a "booster shot" of an entomopathogen. The technology is usually similar to that for the short-term approach. Large numbers of the pathogen are produced as a microbial insecticide and usually applied by means of conventional agricultural technology. This approach usually, but not always, is a commercial venture. The difference from the short-term approach lies in the activity of the entomopathogen after its application. The pathogen again suppresses the target pest population, but in this approach it also recycles and significantly suppresses at least one subsequent generation of the pest insect. Population density of the entomopathogen eventually declines, however, necessitating additional applications of the microbial insecticide, usually once in each growing season. Since more than one pest generation is suppressed by one application, this approach can be more cost competitive with chemical insecticides than are short-term microbial insecticides. On the other hand, seasonal colonization demands some capability for causing natural epizootics, which in turn requires some degree of pathogen replication, persistence, and efficient transmission in the target insect and ecosystem.

The final approach is environmental manipulation, including conservation. This is the only approach that does not involve release of pathogens, although it can be used to supplement the other three approaches. Instead, the usual agricultural or resource management practices are altered to conserve or create a more favorable environment for an entomopathogen population. This approach reduces the pest population without significantly interfering with normal management practices. Because this approach does not involve environmental application or release of an entomopathogen, it will not be discussed further in terms of criteria for microbial control.

Criteria for Choosing Entomopathogens

Criteria for choosing entomopathogens to develop for microbial control can be complex, and differ somewhat for the short-term insecticide, introduction and establishment, and seasonal colonization approaches. These criteria can be separated into three categories: criteria that are difficult to circumvent, characteristics that can be improved with research, and additional factors to consider. The possible use of an entomopathogen against a particular pest can be evaluated quickly by the "criteria difficult to circumvent," because any negative responses in this category will weigh heavily against the development of microbial control. The criteria or factors can be further subdivided into "target pest and ecosystem" and "entomopathogen" characteristics. It is not possible to evaluate an entomopathogen for microbial control without doing so in the context of the target pest and ecosystem. The final category, "factors to consider," offers further criteria after the person considering development of microbial control has narrowed a list of candidate entomopathogens and requires more specific considerations. Any list of criteria represents some degree of personal opinion of the person writing the list, and other people could easily categorize the criteria differently. The criteria serve primarily as a foundation that can be further developed.

Criteria for Choosing Short-term Microbial Insecticides

As an example, the criteria developed for short-term microbial insecticides approach are presented here.

The first group of criteria are the "Difficult To Circumvent" category, starting with those that concern pest and ecosystem characteristics:

The following set of criteria address entomopathogen characteristics:

The next group of criteria are in the "Characteristics That Can Be Improved With Research" category, beginning with pest and ecosystem characteristics:

The following criteria address entomopathogen characteristics:

And finally, we come to the last group of criteria in the "Factors To Consider" category, beginning with the pest and ecosystem:

The following criteria address entomopathogen characteristics:

Similar criteria for choosing entomopathogens to be used in introduction and establishment and seasonal colonization approaches have also been developed. These are outlined in the publication listed below.

- Jim Fuxa, Louisiana State University, Baton Rouge

Adapted from

Fuxa, J. R., R. Ayyappath, and R. A. Goyer. 1998. Pathogens and microbial control of North American Forest Insect Pests. USDA Forest Health Techonology Enterprise Team Publication 97-27.

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