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:
Direct or Indirect Pest. Microbial control generally has a higher success rate against indirect pests (ones that damage parts with no inherent commercial value) than direct pests (ones that damages parts of a resource that are marketed or otherwise used by humans).
Extent of Damage. Market size is of primary concern for a commercial microbial insecticide product, and is related to a great degree to the extent of the pest problem. A large potential market is preferable over a small one.
Pest Feeding and Behavior. Entomopathogens differ in the sites where they invade or damage the host and in how well they function in different components of the environment. Insects that chew broad vegetative areas or that live in soil usually offer more microbial control opportunities than aquatic insects or those with sucking mouthparts. Insects that quickly bore into some substrate other than soil can be particularly difficult to target with a microbial insecticide.
Member of a Pest Complex. In most cases, microbial insecticides are not likely to be successful for a pest of a crop or resource that has a complex of pestiferous arthropods because of host specificity. Even the more generalist pathogens usually are not effective against a wide range of pests and users prefer a control agent that will suppress all pests in a complex.
Economic Injury Level (EIL) Not Too Low. The EIL is the pest population density at which damage costs begin to exceed the costs of pest control. Due to the relatively slow speed or necessity for ingestion of virtually all entomopathogens, crops or resources with a low EIL (i.e., can tolerate little damage) are less likely to be conducive to microbial control than those with a moderate or high EIL (can tolerate some degree of damage).
Competing Controls. There are numerous examples of microbial insecticides that were efficacious but not successful as commercial products, because--from the users' perspectives--they were not advantageous over competing products. Host specificity (i.e., environmental safety) is a general advantage possessed by entomopathogens, but it usually is not enough to offset higher costs without government intervention. Thus, the microbial must have some advantage over competing controls (primarily chemicals) to succeed in the insecticide market.
The following set of criteria address entomopathogen characteristics:
Host Range. An entomopathogen with a relatively broad effective host range is better for commercial development over one with a narrow host range due to potential market size and, perhaps, effectiveness in certain pest complexes. A broad host range can be something of a disadvantage, however, in rare cases where a beneficial or endangered insect population might be affected.
Cost of Production. A major reason for failure in the market of certain, past microbial insecticides has been their cost-competitiveness with other pesticides, primarily chemicals. Obligate pathogens which must be produced in living insects are more costly to produce than facultative pathogens.
Speed of Kill or Damage Cessation. A major weakness of most entomopathogens developed as microbial insecticides is the length of time required to kill the insect or at least terminate its damage-causing activity. The viruses, protozoa, fungi, and certain bacteria and nematodes mostly develop as parasites in their hosts; they usually do not debilitate the insect for at least several days. Those that damage the insect very quickly--generally within 1 day and usually with toxins, symbiotic bacteria, or massive damage from invasion--are more likely to prevent pest damage and be more acceptable to users who expect a "knockdown" action. Genetic engineering is expected to improve some of the "slow" entomopathogens.
Suppresses Pest Population Below EIL. For a microbial insecticide to succeed, the entomopathogen must possess characteristics that enable it to reliably suppress the pest population below the EIL.
Environmental Safety. In view of public opinion environmental safety is essential. Fortunately, environmental safety is the major strength of entomopathogens and the major rationale for developing them for insect control. Natural strains of entomopathogens are safe to non-target organisms, although occasionally there is concern that a pathogen with a wide host range might harm an endangered insect species or upset an ecosystem by killing a nonpest species.
Cost Effective. Cost effectiveness encompasses more than just production cost versus damage reduction. It also includes environmental costs, infrastructure costs, and so on.
Advantage Over Competing Controls. The entomopathogen in a particular pest and ecosystem must not only be cost effective, it must have a relatively good degree of cost effectiveness compared with other controls. Environmental safety alone often is not sufficient to offset disadvantages such as high cost or unreliability in competition against chemicals.
Patentable. Short-term microbial insecticides usually are developed by private industry, which in turn requires protection of its investment. Exclusive rights to a strain of an entomopathogen depend on a patent, which requires that the strain be novel in some respect and patentable. One that is not patentable is at a serious disadvantage.
Registerable. The great majority of entomopathogens present no particular difficulty in meeting government guidelines for registration as a pesticide. Although costs are low compared with those for chemicals, they are still prohibitive for a small company developing a microbial insecticide for a niche market. Registration also is still difficult for most recombinant entomopathogens.
Compatible. Another strength of entomopathogens is their compatibility with other insect controls and with agricultural and resource management practices in general.
The next group of criteria are in the "Characteristics That Can Be Improved With Research" category, beginning with pest and ecosystem characteristics:
Pest Population Quality. This refers to insect resistance to a pesticide. The threat of resistance can influence the pattern and extent of usage.
Suitable Environment. Entomopathogens generally do not persist for more than a few hours or, at best, days, when deposited on an exposed surface in the environment (e.g., sprayed onto a leaf). Some environments might be prohibitively harmful to certain species.
Knowledge Base. A good knowledge base of the pest, crop, and ecosystem, including effective interactions between users and extension personnel, is helpful for information distribution and user education as well as for research and development. For example, adapting an EIL for a chemical pesticide to a microbial insecticide generally requires less research than developing an EIL from scratch.
The following criteria address entomopathogen characteristics:
Amenable to Genetic Engineering. Genetic engineering increases the number of pathogen "strains" for possible development as insecticides or possible combinations of different traits. At this time, certain bacteria and viruses appear more amenable to recombinant-DNA research than fungi, nematodes, or protozoa.
Persistence in Storage. A certain time interval is necessary between the packaging of an insecticide and its application in the field. For certain entomopathogens, most notably nematodes, inability to persist in storage is detrimental to their competitiveness in the insecticide market.
Persistence of the Pathogen in the Environment. There generally is a time interval between application and invasion of or ingestion by the host insect, which may create serious problems, some of which can be alleviated by research into formulation, application, and strain selection.
Amenable to Appropriate Formulation. Appropriate formulation can counteract certainly difficulties, such as lack of persistence or problems with application to a precise location. Formulation has improved the performance of microbial insecticides, but it must do so at a low cost, or the product will not be competitive
Application with Conventional Equipment. It is helpful if commonly used agricultural equipment can be used to apply an the pathogen in an effective manner. Attempts to sell a product that would require additional capital outlay by the user probably would fail.
Reliable Efficacy. Under most circumstances, a user is not likely to repeatedly purchase a microbial insecticide that fails part of the time. This implies that the microbial insecticide must be amenable to some form of product standardization.
And finally, we come to the last group of criteria in the "Factors To Consider" category, beginning with the pest and ecosystem:
Severity of Damage. This can be either a positive or negative. If the damage is costly, or if there is a severe problem with human contact or disease, opportunities can arise in terms of research and development funding and market potential. On the other hand, these conditions often also require that the insect control method be rapid-acting as well as efficacious.
Pest Population Age Structure. In the great majority of cases, insects become increasingly refractive to infection in later instars. Thus, microbial insecticides generally are more efficacious when applied against populations consisting primarily of young larvae in discrete generations. It can be difficult to achieve acceptable control if an entomopathogen must be applied against older insects or, at times, against a mixed-age population.
Range of Pest Population Density. Certain insects reproduce quickly and in high numbers, rapidly building dense populations. It can be difficult to achieve an acceptable level of control of such pests with many entomopathogens, particularly the slow-acting ones.
Pest Rearing. Insect species vary greatly in their survival and reproduction in artificial settings. Some are easily reared on artificial diets, others require live host plants or animals, and still others cannot be reared at all. Rearing is a virtual necessity for entomopathogens that must be produced in living host insects.
Pest of Enclosed Resource. This can be a particularly attractive target for certain entomopathogens because enclosed systems increase the possibilities for manipulating the environment to favor the entomopathogen. For example, a high level of relative humidity or moisture can enhance the activity of fungi and nematodes.
Crop Value. Intermediate value crops generally provide the best situation for short-term microbial insecticides; however, exceptions to this rule are numerous.
The following criteria address entomopathogen characteristics:
Subspecies or Strains Readily Isolated. Strains increase the opportunity of discovering entomopathogens with different host ranges, greater virulence toward certain pests, increased persistence, source material for genetic engineering, and other characteristics. Strains with different biological characteristics generally are more readily isolated among bacteria and fungi than nematodes and viruses.
Portal of Entry. Most bacteria, viruses and protozoa must be ingested, while the route of entry for fungi and nematodes is penetration of cuticle or body openings. Everything else being equal, cuticular penetration or "contact action" is an advantage because timing and placement of application are not as critical as for ingestion.
Efficacious Outside North America. If an entomopathogen has been successfully developed for control of a pest somewhere other than North America, this success increases the probability that it will succeed through a similar approach in North America.
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
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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