A Note From the Project Director. The environmental safety of biological control has come under closer scrutiny in recent years. People are concerned about possible effects of exotic or mass-released natural enemies on non-target species. While there are a few examples of problems associated with biological control attempts that were made before, or circumventing, modern regulatory practices, modern biological control programs are generally perceived to have a good track record of safety. But there is every reason to carefully consider the usage of new biological control organisms to make every possible assurance that there will be no unacceptable environmental impacts. Because of this, the editors of BioScience, the journal of the American Institute of Biological Sciences, devoted much of the June 1996 issue to this topic. In a previous issue of MBCN we discussed the host range of parasitoids. In this issue our feature article is on host specificity of predators. We have drawn much of the information from the following article:
We thank Dr. John Obrycki of Iowa State University for reviewing this article.
Although predators of insects include such things as birds, rodents, and frogs, all of which are important in natural regulation of many types of insects, the most important predators in pest management are other insects, spiders and predatory mites. Three general factors that characterize predators are (1) they are usually free-living and often very mobile, (2) they consume many prey (pests) during their lives, and (3) they are usually as large as, or larger than their prey. Predators can be predacious as immatures, as adults, or both, depending on type. Predators are often generalist natural enemies although a few types are specialized. A large percentage of insects are predators; they occur in about 20 different insect orders, although those that are most beneficial in crops, forests, home gardens, and the landscape belong to a much smaller number of major groups. A few common examples include lady beetles, lacewings (aphidlions), praying mantids, syrphid (hover) flies, assassin bugs, minute pirate bugs, spiders, and predatory mites (phytoseiids).
Various terms are used to describe feeding tendencies. The terms "monophagous" (feeds on only one species) and "polyphagous" (feeds on many species) are often used to describe the range of prey a predator will accept. Terms such as "aphidophagous" (feeding on aphids), "coccidovorous" (feeding on scale insects) or "ambush feeder" are also used to categorize some predators.
Host range refers to the set of species that can serve as prey for a predator. No predator attacks all types of insects in a given area, and those that it does attack often share certain characteristics that may be as simple as general size range and capturability. The life histories of very few species of predators have ever been studied in sufficient detail to adequately categorize the host range. But the studies that have been conducted provide insight into the factors that influence host range. Aspects of predator life history that influence host range include foraging behavior, feeding habits, egg-laying decisions, and development of offspring.
The process of locating and accepting prey is called host selection. The act of prey location (foraging) can be influenced by many factors, such as travel time between patches of prey, the risks of mortality while traveling (predators may be prey to other predators!), competition between predators (of the same species or others), and physiological condition of the predator (such as level of satiation and nutrient balance). Host selection behavior also varies depending on circumstances in the environment. The probability that a given prey will be accepted by a predator may depend on the quality of other prey in the environment. Within the total host range of a given predator, the actual range of prey attacked will be narrower when there is an abundance of high-quality prey, and broader in habitats containing fewer such prey. For example, Delphastus pusillus, a small lady beetle that attacks whiteflies, will eat whitefly nymphs and pupae, but requires whitefly eggs to reproduce well. If no whiteflies are available the beetle will eat spider mites to survive. Some specialists eat only a certain type of prey; host quality may vary with the developmental stage or age of the prey. By contrast, a generalist predator such as the Chinese mantis feeds on many prey types, probably based on size and capturability.
Biological control scientists have traditionally divided the host selection process into four components that often blend together: host habitat location, host location, host acceptance, and host suitability. Throughout this process, the predator uses a combination of physical cues (vision and touch) and chemical cues (odor and taste). Host habitat location is the process of finding a likely habitat that will include appropriate prey. For example, the larvae of the green lacewing Chrysoperla carnea feed primarily on aphids, many of which occur on cotton plants. Caryophyllene is a volatile chemical released from cotton that attracts C. carnea adults. Host location is the process of actually finding suitable prey within the appropriate habitat. Continuing with our example, after arriving in a cotton field, adult lacewings orient to the honeydew excretion of aphids because such sites likely have suitable prey for their offspring. Host acceptance may be based on several characteristics of the prey that is located by the predator. For example, predators often reject prey that are much larger than themselves. Some prey may undergo elaborate defensive behaviors they result in rejection. Other physical characteristics, such as dense hairiness of some caterpillars deter some predators. Host suitability defines whether or not an insect is truly suitable to be utilized as prey. Nutritional and physical characteristics of the prey are involved here. For example, a diseased prey may be acceptable to a generalist predator but be rejected by a specialist. Suitability of a given prey species can change by life stage of either the prey or the predator. Specialist predators, by definition, have more exacting requirements than do generalists; the latter have a much broader range of suitable hosts.
Many factors influence host selection by predators and few species have been studied in great detail. However, there have been some good studies that provide insight on the host ranges of natural enemies. These studies indicate that very few predators are strictly monophagous, and some are highly polyphagous. Many can be classified as "oligophagous," that is they attack a narrow range of prey. These prey share similar life-history traits or exist in a common habitat. In some cases the narrow host range is based on taxonomic relatedness of the prey. For example, in the Coccinellidae (lady beetles) and Syrphidae (hover flies) the different subfamilies tend to feed on taxonomically related groups of prey. One explanation is that related prey types often share similar biological characteristics, while unrelated groups often do not.
Ambush predators, such as assassin bugs, praying mantids, and crab spiders, or trapping predators such as orb-weaving spiders, are less likely to be selective about their prey. Prey selection is based more on what is available in the habitat the predator occupies. In such cases, it is more difficult to predict the likely host range of the predator. However, even amongst these predators there may be some predictability. Many generalist ambush predators restrict their hiding sites to particular habitats, plants, or plant communities.
Natural enemies may restrict their foraging activity to particular habitats yet feed across different trophic levels. For example, damsel bugs and bigeyed bugs often feed as primary predators of aphids, but may also prey on the lace-wings that come to feed on the same aphid colonies.
Because of the great number of species of predators, we will probably never have a comprehensive knowledge of their host range. However, it is essential that natural enemies used in biological control not be unacceptably damaging to non-target (non-pest) species. When evaluating the potential safety of a given species of predator for a biological control program, it is essential that we conduct reasonable studies on host range. Such studies should be conducted both in the laboratory and the field. Laboratory studies, however, must be viewed cautiously. Several studies have indicated that host range demonstrated in laboratory research often is greater than that actually expressed in the field. When evaluating a new exotic predator for classical biological control, field studies should be conducted in the predator's native geographical range. It is important to conduct such field studies throughout the year and within the various habitats occupied by the predator, as host range can change with season and habitat. When evaluating the potential for inundative releases of a native natural enemy, we should first evaluate the potential impacts on non-target species, again by conducting both laboratory and field studies. When using high release rates in inundative biological control, if insufficient target prey are available, the potential exists for a temporary broadening of the usual host range. However, such situations would likely be short-term in those habitats where the predator is native.
In conclusion, the safety, acceptance, and ultimate success of predator-based biological control programs will be based in part on a solid understanding of the predators' host ranges.
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