Predatory mite feeding on immature insect
The regulation of pests by their natural enemies is termed biological control. Natural enemies or "beneficial" arthropods (insects and mites) are termed predators or parasitoids. Nematodes and diseases also attack arthropods. All living organisms have natural enemies which often regulate their numbers. Typically, populations of the prey and natural enemies cycle in the following manner. The prey, in the presence of adequate food supply, increases to high numbers followed by an increase in natural enemies in response to the increased prey population. Natural enemies then reduce prey numbers to levels below that which will sustain them. The natural enemies then die or disperse and the cycle begins again. Both prey and natural enemy populations are also regulated by abiotic factors such as climate and weather. The host plant also plays a role, in that plant chemistry and morphology may contribute negatively or positively to the population dynamics of prey and natural enemies. We call these tritrophic interactions. By necessity there is almost always a lag period between occurrence of the pest and the arrival of natural enemies.
Because of the effect of tritrophic interactions and biological and ecological considerations, biological control in nursery and landscapes must be closely linked with other plant care and IPM tactics, particularly, pesticide selectivity, plant selection, e.g. host plant resistance, plant diversity and culture and management practices. The aesthetic nature of nursery production and landscapes coincident with high pesticide use have delayed the implementation of biological control tactics.
The practice of biological control takes several approaches. Many pests have been introduced into the U.S. from foreign countries without their accompanying natural enemies. Classical biological control is the search for, importation, evaluation and release of natural enemies from the pest's country of origin into the new host country in an attempt to bring the pest species under control. This method has worked best against exotic pests but may also be effective against some native pests. Ideally, following establishment, the natural enemies maintain pest numbers below damaging levels in perpetuity.
Natural enemies may be enhanced through conservation and augmentation. Conservation concerns the manipulation of the host plant, culture and management practices or selective use of pesticides in ways that minimize harm to natural enemies. Augmentation is either the release of laboratory reared natural enemies to enhance the natural populations or use of other cultural practices such as cover crops to provide needed resources, alternate hosts, nectar sources, etc., to aid natural enemies so their populations may increase naturally.
Because of the aesthetic nature of nursery and landscape plants, most current pest management schemes rely on chemicals. Most agricultural insecticides and acaricides are toxic to beneficial arthropods, as are fungicides to naturally occurring entomopathogenic fungi. Therefore, to fully use or enhance the mortality to pest populations from natural enemies, pesticide selectivity is critical. This problem becomes most difficult in attempts to manage multiple pest species when one pest is under biological control. Unfortunately, we have very few selective, conventional pesticides. Use of "biorational" or other selective pesticides is part of the answer.
The path to increased biological control begins with the principles of IPM, using as many mortality factors as possible against the pest population in order to minimize pesticide use. Biological control will not work exclusively in all situations, even for pests that have many effective natural enemies. Monitor (scout) pests and natural enemy populations and apply controls only when absolutely necessary to pest populations that are increasing above the economic threshold. Elimination of pest populations will cause the natural enemies to starve or disperse. Use biorational or selective pesticides whenever possible to conserve natural enemies.
Classical biological control is being pursued throughout the world for many introduced pests including insects, mites, diseases, and weeds. Many nursery and landscape pests are important and wide spread in the U.S., some are also pests of other commodities. As a result these pests may be the target of classical biological control. Several lady beetles have been introduced against euonymus and other scales (Chilocorus spp. ) and aphids (Harmonia axyridis and Coccinella septempunctata) in the Southeast. Nematodes, flies and wasps have been successfully introduced for control of mole crickets in Florida turf and pastures. One major Florida biological control success is the control of the citrus blackfly by introduced parasitoids. Citrus whitefly is also controlled by parasitoids (Encarsia spp.) in some areas of the state. Many other pests are the targets of biological control research.
Conservation of natural enemies will not always improve biological control enough to meet producer needs. Natural enemy populations may need augmentation in the form of alternate hosts, food sources such as nectar, or shelter. Provision of such factors may still not enhance natural enemies to the level required by man. Therefore, it may be necessary to augment natural enemy populations by inoculation (release in small numbers) or inundation (mass release) to achieve successful biological control. Research has demonstrated that pests are less successful in areas with plant diversity while natural enemies often perform better. The ability of the pest to find and remain on the host plant and the addition of alternate hosts and food for natural enemies are important. Plant assembly and diversity, i.e. landscape design, can have significant impact on pest populations and the success of biological controls. Host plant resistance (HPR) is the use of plant cultivars that have characteristics that work against pests. Such characteristics may enhance or reduce the success of natural enemies. They are also dynamic and may diminish in effectiveness over time. Associational resistance derived from plant diversity works against the pests in several ways, through HPR and by enhancing the effects from natural enemies.
Augmentative or mass release of natural enemies is a tactic that has been and is the subject of much research. This tactic requires repeated releases of natural enemies as they may not permanently establish in the crop. This approach to biological control has been successful on many pests of vegetables in glasshouse culture in Europe. While potential is great for this tactic, current reality is somewhat different! The efficacy and economics of specific natural enemies have not been demonstrated and use in unenclosed ornamentals and landscape environments are rare. Multiple pest complexes are challenging and few methods to determine proper release rates, timing or expected outcomes are available.
Selection of a natural enemy that is capable of controlling the target pest on the target host plant under variable culture and management practices is difficult. In order to make mass releases of natural enemies, mass rearing methods are necessary. For many natural enemies, no method of culture except on the living host is available. Therefore, the arthropod host together with its host plant must also be cultured. The required rearing schemes are complicated, labor intensive and wrought with problems that are often expensive and difficult to solve. The living product is highly perishable and requires special handling. Cultures of natural enemies that are needed only a few months of the year often must be reared year-round. Furthermore, laboratory rearing of natural enemies often results in populations that become "domesticated" to laboratory conditions which reduces their ability to function in the natural habitat. Mass rearing methods are available for only a few of the myriad of existing natural enemies. These exotic species are purchased and released with little regard to biological factors necessary for successful establishment and pest reduction. This approach has taken precedent over detailed biological investigations of the native natural enemy species that occur in most habitats. Such investigations would lead to the better understanding and use of native natural enemies more adapted to nursery and landscape conditions.
Assuming natural enemy selection, mass rearing and quality control problems are overcome, the next questions concerning natural enemy/pest release ratios, proper timing of releases, methods of release, numbers required for establishment, the effects of different host plants, effects of various culture and management practices on establishment, etc., must be answered.
As an example of what is necessary, let's use predatory mites against spider mites. Several species of predatory mites are commercially available. Phytoseiulus persimilis is the most common. Originally described from Chile, it has been introduced to hundreds of crops worldwide. This wide spread use is due to the predator's broad host range and adaptation to a wide range of temperature and moisture regimes, i.e., it is a generalist predator. It has been used successfully in glasshouse culture in Europe and Israel and on several crops such as strawberries in Florida and California. P. persimilis supposedly has several strains that are more or less adapted to specific humidity and temperature ranges. In the nursery and landscape a wide range of such conditions exist with pest phenology and species varying across regions with different plants, climate and weather.
Predatory mites can be purchased from suppliers. The predators arrive in cold packs usually mixed with spider mite eggs and corncob grits. They are then distributed into the target crop. The proper predator/prey ratio that is required to achieve control in a given period of time has not been determined. Recommendations consist of releasing one to three predators per square foot of bed space, but this does not consider pest numbers, plant size and number of infested leaves per plant. It is also not known what effect the host plant morphology has on the predator. Plant leaf structure dramatically effects spider mite behavior. With normal overhead irrigation, it seems unlikely that predatory mites will do well on plants with flat or concave leaves that provide them no cover.
We do know that pesticide residues are often very toxic to natural enemies for long periods. For example, pyrethroids such as Mavrik and Tame may be toxic to predatory mites for 4-6 weeks after application. Organophosphates and other classes of pesticides are also toxic and cannot be integrated with predatory mites. A commitment to reduce pesticide use, conversion to use of biorationals and tolerance of more damage is required for success. If you release P. persimilis against spider mites and then get an outbreak of whitefly or aphids what do you do? These pests also have natural enemies. Often the best strategy is to use neem products, growth regulators, soaps or oils at low rates to conserve the natural enemies and allow them time to develop. Even soaps and oils will kill some of the predators they contact, although the residues are harmless.
Assuming you get the predators established, will they remain on the host plants as the pest numbers decrease? Some predators are able to use alternative food sources such as pollen, honeydew or other prey species, but many will leave the plants as pests are reduced or eliminated. Therefore, the next infestation may require new predator introductions. Suppose that you do maintain the predators on the plants until the shipping date. The plant inspector or the buyer inspects your plants and finds mites. It may be difficult to prove that the organisms are natural enemies. Predatory mites may have the most potential for use on plants which are not going to be shipped in the near future.
Integration of chemical and biological controls would seem like a reasonable strategy to use in transition from complete reliance on chemicals towards reliance on non-chemical tactics. Selective use of pesticides will help conserve natural enemies and reduce the need for chemicals. Selective pesticides might also be used to reduce pest populations to levels which fewer predators could control. Lacewings and lady beetles are the best known of the generalist predators commonly occurring in the landscape. It has been shown that insect families and species within families respond differently to pesticides. Different life stages of individual predator species respond differently to specific pesticides. In general acaricides are mostly safe to natural insect enemies, but with the exception of Vendex and Ornamite, are toxic to predatory mites. Most of the biorationals such as neem products, Bacillus thuringiensis (BT), growth regulators such as Enstar, insecticidal soaps and oils are safe to insect and mite natural enemies. Many new BT products consist of the toxin produced by the bacteria rather than the bacteria itself. These products can be toxic to natural enemies and to mammals. Pyrethroids are generally safe to lacewings but other classes of chemicals show mixed effects. Organophosphates are toxic to most natural enemies. Chlorinated hydrocarbons such as lindane are generally less harmful. Most fungicides are selective of insect natural enemies, but not of entomopathogens. There are exceptions to most of these statements but all natural enemies have not been evaluated against pesticides on a systematic basis. A list of registered pesticides and a rating of their impact on natural enemies is available.
Recommendations: Implementation of IPM strategies and tactics can be a big step for growers. IPM requires extensive knowledge and a strong commitment to the process. Biological control along with host plant resistance and culture and management practices are the cornerstones of IPM. Most growers recognize that pest and natural enemy arthropods exist, but are less likely to accept that low populations of pests should be tolerated and are needed to sustain natural enemies. Implementation of biological control must be preceded by reduction in pesticide use and commitment to the economic threshold concept. Reduction in pesticide use will conserve natural enemies and enable implementation of other biological control techniques.
Many natural enemies are available from commercial insectaries. We do not recommend the indiscriminant release of these natural enemies. Research on many pertinent issues is in progress. Be skeptical of wild claims. Contact your local county agent or extension specialists for advice and information. Private consultants are also available for consultation.
For additional information visit the Florida IPM and Biocontrol Web site at http://biocontrol.ifas.ufl.edu/.