Use of Commercial Biological-Control Agents for Insects and Mites
Small Farm Center, University of California, Davis, CA 95616
By L.E. Ehler
University of California, Davis
Department of Entomology
Biological control is a key element in integrated pest management.(IPM) and will continue to be in the foreseeable future. It is therefore imperative that biological control be exploited to the fullest extent. This can be accomplished in at least three ways: 1) importation and release of exotic natural enemies (i.e., classical biological control), (2) conservation of natural enemies (native and introduced) that occur each year in agroecosytems (e.g., through selective use of chemical insecticides,), and (3) augmentation of natural enemies, either through environmental modification (e.g., providing overwintering shelter) or augmentative release of enemies in required numbers. Augmentative release of insectary-produced natural enemies is not a new approach to biological control; however, this method has not been fully exploited over the years. Fortunately, this situation is rapidly changing. In other words, with the ever-increasing number of commercially available biological-control agents and the dwindling supply of ecologically suitable, registered chemical insecticides, it is evident that the time has come for broad-scale use of augmentative release in California. The purpose of this paper is to provide some tips for those planning to us commercially available natural enemies, either in production agriculture or in non-agricultural environments. The discussion will emphasize predaceous and parasitic arthropods which are available for control of pest insects and mites.
Commercially available pest-control agents can be divided into two categories: those which must be registered with both the Environmental Protection Agency (EPA) and the California Department of Food and Agriculture (CDFA) vs. those which are exempt from registration. Agents which require registration include chemical insecticides and acaricides, plus microbial biological-control agents such as viruses, bacteria, protozoa, and fungi. Authoritative information on efficacy safety, etc., for these agents can be found on the label for a particular commercial product. Ordinarily, this information is the result of several years of research and a considerable financial investment on the part of the company involved. Agents which are currently exempt from registration include multicellular animals, such as predaceous insects and mites, parasitic insects, entomopathogenic nematodes, predatory snails, insectivorous fish, etc. Because these organisms are exempt, there is no label that a potential user can turn to for authoritative information. For this reason, users should exercise an appropriate amount of caution when choosing such an agent for release. In other words, just because an exempt biological-control agent is commercially available, this does not necessarily mean that the agent is effective.
Holistic Approach Needed
To ensure success, users of commercially available predators and parasites are urged to adopt a "holistic" approach. Holism implies an understanding of the target system as a whole, including how the many components interact to produce the particular system that we observe. (The other end of the continuum is "reductionism", such as that practiced by molecular biologists concerned with one small part of only one organism in the systems.) In a given ecosystem, it is important to consider not only the target pest population, but also the attributes of the natural enemy to e released, and the ecological structure of the target system. Each fields, orchard, or landscape should be viewed as a unique situation, such that recommendations for one particular site will not necessarily be appropriate for another. This is hardly the case with traditional chemical control of arthropods pests, and may require some getting used to. The discussion which follow is designed to illustrate the need for a holistic approach, with particular emphasis on commercially available, biological-control agents which are exempt from EPA/CDFA regulation.
Prior to actual release of a natural enemy, some consideration should be given to choice of species, biological attributes of the chosen species, obtaining the agent, and quality of the purchased product. In choosing a species, it is critical to ask why this particular agent is available. Is it because the agent is effective? Easily collected and stored? Easily cultured? A fad? Many commercially available predators and parasites have been shown to be effective; however, this does not mean that the same agent will be effective in a new situation. As discussed below, numerous factors in a target system can mitigate an otherwise effective species. To further complicate matters, for many target pests species, there are two or more commercially available agents for use (e.g., aphids, spider mites, filth flies). Unfortunately, we have insufficient data in most cases concerning the relative merits of a particular multiple-species release as opposed to a given single-species release.
The attributes of a given agent can be critical to success, particularly when there is a choice between two or more species for a given pest problem. For example, release of a predator which is predaceous in the adult stage could provide a greater immediate impact, compared to release of adult parasites which must first lay their eggs in hosts so as to allow their progeny to eventually kill the pest. Also, with such predators, both adult males and females can destroy numerous pest individuals, whereas only the female parasite actively seeks and oviposits in hosts. Host-specific agents will be highly dependent on their prey, whereas less specialized or more generalist enemies may feed on a number of other species following release. In the latter case, the impact of the agent must be considered (see below). Whether the agent is native or exotic (i.e., introduced) can be important. Exotic species may not be well adapted to California conditions, particularly if they originated in a tropical or otherwise humid environment. For example, the introduced predaceous mite Phytoseiulus persimilis is an effective control agent for spider mites in greenhouses, but it is poorly adapted to outdoor conditions in the Central Valley. In recent years, pesticide-resistant parasites and predators have been developed in the laboratory and made available commercially. Such agents might be appropriate in certain situations, but unnecessary in others.
Obtaining enemies can be problematic at times. An agent that is listed as commercially available may not be available at a certain time, or in the numbers required for a given release. Thus, some advance preparation may be necessary. Legal requirements for importation should be e investigated for those agents offered by companies out of state and(or) in foreign countries. The USDA and CDFA have rather strict regulations concerning importation of exotic species, including beneficial ones. Finally, the cost per unit should be compared among sellers, to help distinguish between producers/suppliers and companies which only supply agents. The price differential can be considerable.
Once the natural enemies have been received, some attention to quality of the lot is required. It may even be necessary to determine if the correct species has been received. There are reports indicating that some companies have at times supplied the wrong species, either inadvertently or because their stock culture was misidentified to begin with. Intraspecific variation in an enemy can also be important, and where different biotypes or races of a commercially available species exist, using the appropriate one can be critical to success. Fro example, there are two host races of the scale parasite Comperiella bifasciata: one parasitizes yellow scale whereas the other parasitizes California red scale. Considerable intraspecific variations also exists in North American green lacewings (Chrysopa carnea), such that it might be helpful to know the source population from which insectary stock was originally obtained. In parasites, body size can be a measure of quality (i.e., larger females generally have greater fecundity). Sex ratio should also be observed, especially in parasites because males do not kill pests. Ideally, the number of males in a shipment of parasites should approximate the number required to inseminate all of the females. Finally, viability should be checked whenever possible. For example, the rate of hatch (e.g., predator eggs) or emergence (e.g. hosts containing parasites) may be relatively low in some shipments.
Several factors must be considered for successful release of commercially available predators and parasites, including characteristics of the target pest population and the release strategy to be employed. The population density of the pest (i.e., number per unit of habitat) can be useful in determining how many agents to release. As the number of agents required will vary with the number of pests present, an enemy-to-pest ratio may be of value in standardizing release rates. Unfortunately, our current knowledge of the required ratios for most commercially available agents is limited at best. The dispersion or "spatial pattern" of the pest population can also influence release strategy, particularly where the pest is patchily distributed in a field or first appears at the edge. The age structure of the pest population should be assessed in those cases where parasites are to be released. Parasites are greatly restricted with respect to host stages in which they can successfully develop (e.g., Trichogramma spp. vs. host eggs, Metaphycus helvolus vs. immature black scale). In some parasitic species, sex allocation is related to host size: unfertilized eggs (which give rise to males) tend to be deposited in smaller (younger) hosts, whereas fertilized eggs (which give rise to females) tend to be placed in larger (older) hosts. Thus, the distribution of body sizes in a pest population can be important; in some cases, it might be prudent (for example) to delay the release until a greater proportion of suitable hosts for the parasites are available.
There are several strategies for augmentative release. An "inundative" release simply floods the target system with massive numbers of agents and is designed to give rather immediate pest suppression. At the other end of the continuum is the "inoculative" release, in which a smaller number of agents (inoculum) is released at a critical time with the hope that their progeny will ultimately provide pest suppression. Cost will ordinarily play a decisive role in which approach is taken. In some cases, it may be expedient to release not only the enemy, but the pest as well. For example, Encarsia formosa, an effective agent for control of greenhouse whitefly, parasitizes the fourth stage (pupa) of its host; in greenhouses where whiteflies will surely colonize anyway, the crop can be artificially infested with whiteflies at strategic locations early in its development. These infestations can then be monitored to determine when the bulk of the pest population has reached the appropriate developmental stage for release of the parasite. In the European literature this is often referred to as the "pest-in-first" technique. Releases of both enemy and pest could of course be simultaneous in those situations where the enemy's preferred host stage can also be released.
Release methods are numerous and varied. At present, there is a critical need for innovative research, particularly with respect to application technology. Perhaps in the near future, more University of California researchers will begin to address these critical problems. In the meantime, users of commercially available predators and parasites must either rely on information from the producer and(or) supplier, or devise their own application procedures.
Once the release has been made, it is critical to assess ecological impact of the agent on the pest population. This can be difficult, particularly where no untreated control or "check" plot is available. Even in replicated experimental plots designed to gather efficacy data, agents may spread from one plot to another, thereby compromising the experiment. Nevertheless, every effort should be made to determine efficacy of commercially available parasites and predators so that their use in applied biological control can become a more predictive science.
Users should also be cognizant of environmental impact of introduced agents. In this context, environmental impact is defined as any affect on a nontarget organism brought about by the introduced predator parasite. Thus, most commercially available biological-control agents can be expected to have an environmental impact of one sort or another. The critical issue is just how serious the impact will be. Enemies which are not host specific are of particular concern. For example, mealybug destroyer (Cryptolaemus montrouzieri) has interfered with a cochineal scale used for control of Opuntiacactus in South Africa, and Trichogramma minutum has parasitized eggs of a moth used to control purple nutsedge in Hawaii. More recently, Bacillus thuringiensis var. kurstaki had a considerable disruptive effect on oak-feeding Lepidoptera in southern Oregon following its use for eradication of gypsy moth. In general, we know very little about potential environmental impact of commercially available agents under California conditions. Let us hope that such impacts will be minimal. Nevertheless, we cannot expect to replace broad-spectrum insecticides with broad-spectrum biological-control agents without some attendant environmental effects.
Use of Biological Control Agents
In reviewing evidence from the literature and listening to the experiences of pest control advisors, a major theme that emerges is the inconsistency of results in field trials. In view of this, it seems appropriate to end this discussion with an account of some factors which can affect the efficiency of predaceous and parasitic arthropods following release. (This assumes that environmental conditions such as temperature were appropriate during the release phase.) In some cases, the problem may reside in the material received (e.g., poor viability or male-biased sex ratio (in the case of parasites)). In addition, species which have been maintained for long periods of time in commercial insectaries may become so adapted to these conditions that they cannot perform (or even survive) in the field. We now know that certain parasites become "conditioned" to search for and parasitize the host species on which they were reared; how this might affect performance of (say) Trichogramma (reared on eggs of a flour moth) when released for control of corn earworm is in need of further investigation. Behavior of the agent may in itself be a limiting factor, e.g., convergent lady beetles (Hippodamia convergens) collected from winter aggregations are physiologically programmed to disperse. Release strategy may influence efficacy, e.g., inadequate numbers released, poor placement of released material, etc. The nature of the target system should be not overlooked. For example, how do nematophagous mites in the soil affect entomopathogenic nematodes applied for control of a soil-dwelling insect? Similarly, general predators above ground can feed on released material prior to hatch/emergence of the agents. Supplementary foods for adults of certain species may be necessary for proper nutrition and egg development, e.g., honeydew for Chrysopa carnea nectar for many parasitic wasps. Finally, production practices can influence the outcome of a release, the obvious one being application of broad spectrum chemical pesticides. As a general rule, natural enemies are more susceptible to a given dose of a pesticide than are plant-feeding insects, and this must be borne in mind when integrating chemical and biological control.
The "take home message" from this discussion is that users of commercially available predators and parasites can enhance their success by adopting a holistic approach, with proper attention to the attributes of the enemy, the characteristics of the pest population, and the nature of the target system. At the same time, University of California researchers must strive to develop the new knowledge required for sophisticated application of these biological tools in IPM. Frequent meetings of users, producers, researchers, and other interested parties to share information and personal experiences will help immensely during this critical phase in the development of augmentative release of commercially available biological-control agents.
For More Information
Natural Enemies Handbook: The Illustrated Guide to Biological Pest Control by Mary Louise Flint. Publication 3386, University of California, 6701 San Pablo Ave., Oakland, CA 94608-1239. Telephone: (800) 994-8849. 164 pp.
Pest of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide by Mary Louise Flint. Revised 1999. Publication 3332, University of California, 6701 San Pablo Ave., Oakland, CA 94608-1239. Telephone: (800) 994-8849. 288 pp.
Suppliers of Beneficial Organisms in North America by Larry Bezark. Biological Control Services Program, 3288 Meadowview Rd., Sacramento, CA 95832. Telephone: (916) 4274590. 16 pp.
Association of Natural Biocontrol Producers, PO Box 44537, Blue Jay, CA 92317.
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