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Biological Control of Turf Insects
编辑:郭海滨   出处:南京农业大学杂草研究室   时间:2011-1-13 15:07:10

Many turf managers are looking for ways to reduce their dependence on traditional insecticides and to incorporate more "biopesticides" or other biological control options into their insect control programs. "Biopesticides" are living organisms which are handled and applied in a manner similar to traditional insecticides. Examples include entomopathogenic nematodes, bacteria, and fungi.

Nematodes

Entomopathogenic ("entomo" = insect; "pathogenic" = causes disease) nematodes are nematodes which carry bacteria inside their bodies. When the nematode penetrates an insect victim, it releases bacteria, which break down the internal tissues of the target insect, resulting in a massive infection ("septicemia"). As the insect dies, the nematodes reproduce within the cadaver. The juvenile nematodes pick up some of the bacteria, and move on in search of new target insects.

Entomopathogenic nematodes are available commercially and appear to be effective at suppressing populations of several turf insects. The most common commercially available nematode is Steinernema carpocapsae, available under a variety of trade names and formulations. This nematode appears to be effective against various turf caterpillars, such as webworms and cutworms, as long as the nematodes are applied when the caterpillars are still relatively small (less than half an inch long). The nematodes are sensitive to desiccation, so applications should not be made in the middle of the day and should be watered in immediately, either with irrigation or rainfall.

Other species of nematodes are under development for use against white grubs. The one which shows most promise is Steinernema glaseri, which has reduced grub populations 50 to 70% consistently over several years of testing. However, this nematode is a bit larger than its cousin, S. carpocapsae, and has proven difficult to produce in a form which can be applied through traditional application equipment.

Entomopathogenic nematodes are being used to suppress populations of many different kinds of insects. One species is proving very effective against mole crickets in Florida and the southeastern United States. Another is being used to control fungus gnats in greenhouses and mosquitoes in certain outdoor settings. Still another shows promise against black vine weevil larvae in nursery settings.

Bacteria

Some bacteria cause diseases in insects. The most common bacterium used against turf insects in Bacillus thuringiensis var. kurstaki, or "Bt," for short. It is effective against several kinds of caterpillars. Bt is much less sensitive to desiccation than the nematodes, so can be used as a foliar spray against caterpillars like the gypsy moth as well as cutworms and webworms in turf. Bt produces a toxin which paralyzes the insect gut, so the insect stops feeding shortly after ingesting treated plant material (although it may not die for several days). Bt works much more slowly than most traditional insecticides, and should be applied when the target caterpillars are still relatively small (less than half an inch). The strain of Bt used against caterpillars is quite specific, and so does not interfere with most of the beneficial insects in the area.

A new strain of Bt (var. japonensis and called "bui bui") appears to be effective against some species of white grubs. The product has not been tested widely in field conditions, and is not expected to be available commercially for several years. This strain of Bt appears to be on of the strongest candidates for biological control of white grubs.

Another species of bacterium causes a disease in Japanese beetle grubs. Bacillus popilliae causes "milky disease" in the grubs. Infected grubs take on a very milky appearance and are very flaccid. Use of milky disease in New England is somewhat controversial, because there are no data to indicate whether the disease actually suppresses grub populations. If the disease becomes established, it usually takes a few months to have a noticeable effect on the grubs in the area, but may remain "active" for three to five years in that area. B. popilliae is specific to Japanese beetle grubs and seems to have little effect on other grub species, such as the European chafer or oriental beetle larvae, which also damage turf.

Milky disease is available commercially, but there have been some problems with quality control in recent years. One company produces the bacterium by collecting grubs in the field, bringing them back to the laboratory, infecting them with Bacillus spores, and grinding up the infected grubs. This process is labor intensive and expensive, but works quite well, so the quality control is very good.

Fungi

Some fungi cause diseases in or on insect bodies. Several such fungi occur naturally in New England, and others are being developed in laboratory settings after having been located in other parts of the world. One of the "native" fungi is Beauveria bassiana, which attacks chinchbugs and billbugs in turf settings in New England. Particularly in cool, wet springs these insects may be found covered with tiny strands of materials that looks like cotton candy. These strands are mycelia of the fungus, which has invaded the insect body and attacked the internal tissue. At least one company is not marketing a strain of this fungus, which may suppress chinchbug or billbug populations in the field settings.

Another fungus, Metarhizium anisopliae, appears to be quite effective against white grubs in the soil. The main challenge currently is to find a way to "package" the fungus so that it is not sensitive to sunlight and can be applied through conventional application equipment. Several companies have shown interest in this fungus, and it should be available commercially by the end of the century.

Insect Growth Regulators

Insect growth regulators (IGRs) are chemicals which occur naturally within an insect, governing the natural molting process. In some cases an IGR will signal an insect that it is not time to make the final molt to the adult stage. As long as such an IGR is present, the insect will not molt to an adult. It may complete a few "extra" molts as an immature, but will not become an adult. As a result it is unable to reproduce, and ultimately the reproductive cycle is interrupted. Most IGRs of this sort are very specific, governing the actions of a particular species or group of closely related insects. Several IGRs have been developed for such insects as whiteflies and mosquitoes, and at least one is being tested for white grubs.

Other IGRs are more general in their action. Azadirachtin, a derivative of the neem tree, is a growth regulator which appears to interfere with a part of the molting process in which an insect develops new chitin for the exoskeleton ("outer shell"). Insects which have been exposed to azadirachtin exhibit some peculiar shapes, such as half larva and half pupa or half pupa and half adult. The end result is the same--healthy adults do not emerge.

Endophytes

Finally, there are some species of grasses which contain endophytes, which are fungi which produce substances which are toxic to certain insects. Endophytes occur in some perennial ryegrasses and fescues, and have been incorporated into some commercial cultivars. They are effective in the suppression of chinchbugs and billbug populations, and are active against cutworms and webworms as well. Turf managers who are looking at renovating turf areas should consider including some endophtic grasses in the renovation. Note that endophytes are also toxic to grazing stock, such as sheep and cattle, and so should not be used in pasture settings. See GreenShare Factsheet on Endophyte-enhanced grasses for more information.


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