Using biological-control strategies for turf. Part 1: Insects
Many turfgrass and landscape managers seek alternatives for managing insect and mite pests. The use of beneficial organisms including predators, parasitoids and insect pathogens as part of an overall IPM program offers an environmentally sound approach for reducing pest infestations. The greatest obstacle to date has been lack of consistent effectiveness from biological controls.
Predators One way to implement biological control is by releasing beneficial organisms into turf or the landscape, or by modifying cultural, chemical and other management practices to conserve existing natural-enemy populations. Insectaries now rear several species of beneficial insects, which you can purchase from commercial suppliers. In general, this approach is risky because success requires detailed knowledge (of predator/prey or parasite/host relationships), accurate releases and careful management. For example, if you release lady beetles to control aphids, and few aphids are present at the time of release, most of the beetles will simply fly elsewhere.
In the long run, it generally is more practical to conserve existing natural-enemy populations through wise pest-management practices. Natural populations of predators (such as lady beetles, big-eyed bugs, ground beetles, lacewings, predaceous thrips and mites) and parasites (for example, parasitoid wasps and tachinid flies) are already present in most landscapes and can be valuable in reducing insect and mite infestations. In a biocontrol program, however, you may need to tolerate low levels of pest infestations to attract and maintain natural enemy populations.
If chemical pest control becomes necessary, you should use corrective measures that minimize injury to beneficial organisms. You can achieve this by learning to accurately identify both pest and beneficial insects, and by applying insecticides only when pests have reached their treatment threshold. When insecticides are necessary, use them as selectively as possible. For example, while a broad-spectrum insecticide may kill the insect pest, it will affect beneficial natural enemies as well. A Bacillus thuringiensis (Bt) application, conversely, is highly pest specific and generally is non-injurious to beneficial insects. You may not always have such clear-cut options, but when you do, you should use them.
Planting patterns and the diversity of plant species in the landscape will influence the natural-enemy complex. A diversity of plants increases the likelihood that some will harbor low levels of pest insects. This allows predators and parasitoids to survive periods of low pest populations. In addition, many natural enemies feed on pollen, nectar or plant sap either as an essential part of their diet or as an alternate food source in the absence of insect prey. Providing flowering plants with different blooming periods in the landscape can increase survival of these beneficial insects. Ornamentals such as chrysanthemum, daylily and salvia can provide an important food source-you should incorporate them into landscape designs when possible.
Pathogens Insect pathogens or their by-products also can suppress insect populations. Bacteria, fungi, viruses, protozoans and nematodes all attack turf and landscape pests (see table, page XX). However, most insect pathogens are slow-acting compared to conventional insecticides, and you must time their application with some precision. They also tend to suppress, rather than eliminate, pests and have a narrow host (target-pest) range. The latter characteristic may be a drawback in some situations, but it also has the benefit of preserving populations of beneficial insects. For this reason, insect-pathogenic products can be good fits with IPM programs, with almost no potential environmental hazard.
* Bacillus thuringiensis, commonly called Bt, is marketed under several trade names. When susceptible insects ingest this common soil-inhabiting bacterium, a bacterial toxin acts on the host's digestive tract causing the insect to stop feeding, sicken and die within 4 to 7 days. Until recently, control with Bt was limited to caterpillars, and mosquito and fungus-gnat larvae. However, strains of Bt may soon be available to control white grubs, billbugs and other insect pests. B.t. japanensis 'buibui', first discovered in Japan in 1991, is a strain that's active against certain white grubs.
Bt has been a remarkably effective product since its introduction. Bt strains are highly selective, which can be an advantage, as we just mentioned. However, this is a drawback at times because you must obtain different strains to control different types of pests. Further, Bt is useful only on insects that ingest the poison, making it generally ineffective against pests such as borers, leaf miners and sucking/piercing insects.
* Bacillus popillae. A moderately successful microbial-control program employed in some parts of the United States involves the use of Bacillus popillae, a commercially available bacterium that causes milky-spore disease in Japanese-beetle grubs. Unfortunately, this bacterium is not consistently effective in the field. In one study at the University of Kentucky, commercially produced B. popillae spores were able to infect Japanese-beetle grubs in the laboratory but were ineffective (less than 20-percent reduction) in the field. Further, B. popillae does not affect all species of white grubs (including the Cyclocephala and Phyllophaga grubs, which are some of the most serious grub pests), and it takes several years to reduce grub populations. Plus, certain pesticides can adversely affect B. popillae. Nevertheless, it may be a useful preventive measure in certain situations because infected grubs re-release B. popillae back into the soil, giving it relatively long-term environmental persistence.
* Serratia entomophila. This bacterium is a natural pathogen of the New Zealand grass grub Costelytra zealandica. Similar bacteria occur in masked-chafer grubs in the United States, so potential exists for the development of products that affect various grub species. Currently, however, commercial formulations are available only for the Japanese beetle.
* Beneficial nematodes. Manufacturers are now producing several species of beneficial, or entomopathogenic (literally "insect-infecting"), nematodes that are available from many commercial suppliers. These nematodes enter the body of the insect and release bacteria, which then kill the insect host. Efficacy against insect pests varies widely with different nematode species and strains.
* Viruses and protozoans. These pathogens currently find only limited use in turf and landscape situations. However, a few such products are available.
The following table lists some of the more common insect pathogens, their insect hosts and, if available, their commercial formulations.
Because many microbial insecticides have a short shelf life and narrow target range, it is critical to evaluate the effectiveness of each application and identify factors responsible for its success or failure. This is especially important with biological products because environmental conditions are critical to their effectiveness. For example, high or low temperatures, excessive or inadequate soil moisture, improper timing, incorrect product or formulation selection, outdated product or failure to follow the label directions can reduce the effectiveness of an application. Consult with other turfgrass and landscape managers, manufacturer representatives or cooperative-extension staff for assistance with pesticide-performance review and determination of the responsible limiting factor(s). Once you've identified them, take steps to correct them. Manufacturers are working to improve their understanding of favorable conditions for biological controls as well as developing formulations and application methods adaptable to a wider range of conditions.
Microbe-derived insecticides Biologically active products (toxins) derived from microorganisms are valuable tools for suppressing pest populations (see table, page XX). These products use substances produced by bacteria or fungi (not necessarily the organism itself), even though the microbes may not be pathogenic.
Microbe-derived products offer several advantages. First, the biologically derived products currently available for turf-and-ornamental insect control generally pose minimal hazard to most non-target organisms. They also tend to possess low acute toxicity (EPA classifies many as reduced-risk pesticides) and therefore pose little threat to the environment in the worst-case scenario of a pesticide spill.
Second, public perception-accurate or not-is that using biopesticides derived from living organisms is safer and more environmentally responsible than synthetic pesticides. Thus, due to their perceived safety, clients often specifically request biopesticides. Opportunistic turf and landscape managers can capitalize on this by offering services centering on biological controls. Finally, these products usually are compatible with conventional application equipment and techniques. Therefore, they fit easily into existing pest-management programs.
Plant extracts Conceptually no different than microbe-derived insecticides (in that they simply are toxins produced by some living thing), available plant extracts include two significant products (see table, page XX). Most notable is pyrethrin, extracted from certain Chrysanthemum plants. This broad-spectrum insecticide has been available commercially for many years. Azadirachtin is another extract gaining in popularity. It is derived from the Azadirachta tree and, like pyrethrin, has a broad spectrum of activity.
Control starts with good culture In any pest-management program, decisions you make during the establishment and maintenance of a turf or landscape area can significantly influence future pest problems. Among these key decisions are selection of well-adapted plant species and cultivars, proper siting, irrigation and fertility programs, weed- and disease-control strategies and other practices that affect plant health and vigor. As a general rule, stressed or poorly maintained plants are more susceptible to pests and can even be especially attractive to them. Thus, maintaining healthy, vigorous plants is probably the best preventive insect-management strategy.
The future of biocontrol in urban landscapes looks promising. As EPA restrictions and consumer preferences dictate, manufacturers will continue to invest in research and development of biopesticides and eventually bring them to market. Key to the success of a biological-control program is the understanding that natural enemies survive better in stable environments, moderate pest levels help maintain natural enemies and control by beneficial organisms takes time (but once established, pest reduction may be long-term). Putting biological control to work can be difficult but rewarding. It requires a thorough understanding of the biology of both the target pests and natural enemies.
John C. Fech is an extension educator and Dr. Fred P. Baxendale is professor of entomology at the University of Nebraska (Lincoln, Neb.).
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