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Insecticidal Nematodes

July 26, 2000

Insecticidal nematodes are effective in controlling insects. Nematodes are very tiny, unsegmented worms that are best known to landscapers, farmers, and other agriculturists by the diseases they cause. There are 10,000 nematode species known to science, but the number of species that exists has been estimated at 500,000. There are nematodes that feed on plants, others that feed on animals, but probably most species are scavengers, feeding on dead plant and animal material.

Insecticidal nematodes, also called entomopatho-genic nematodes, do not attack plants but only attack insects and their relatives. The infective juvenile stage of the nematode usually enters a natural opening of the insect such as the mouth, anus, or spiracle. Spiracles are openings used by the insect for respiration. Some nematodes, such as Heterorhabditis spp., can make their own hole through the insect’s body wall. Once inside the insect, the nematode penetrates the gut lining or air tube, releasing bacteria into the haemocoel, the cavity inside the insect that contains the blood.

The bacteria that are released attack and feed on the blood and other body tissues of the insect, causing its death within 2 days. The nematode feeds on the broken-down tissues of the insect as well as the bacteria. This allows the nematode to grow, develop, mate, and produce infective juveniles within the dead insect. There are usually three generations of nematodes produced within the dead insect before the insect’s body wall breaks down, releasing infective juveniles that carry the bacteria within their intestines to new hosts.

The bacteria species that occur within insecticidal nematodes are not found anywhere else, and insecticidal nematodes that do not contain these bacteria are ineffective in killing insects. In this mutual arrangement, the nematode transports the bacteria into a suitable host, and the bacteria provide food for the nematode by breaking down the host insect’s tissues.

These bacteria produce antibiotics that inhibit the development of fungi and other bacteria. Thus, insects killed by these nematodes do not rot and disappear as quickly as those killed by natural causes or insecticides. The presence of dead larvae in treated areas makes it likely that they were killed by the nematodes rather than by some other means.

Insecticidal nematodes are fragile animals, with most stages protected within the cadaver of the attacked insect. The infective juvenile stage is the most resistant stage, but it is still very susceptible to drying and to the ultraviolet rays in sunlight. For this reason, insecticidal nematodes are most effective in the soil and tunnels of boring insects, where they are protected from dry air and sunlight. In the environment, insecticidal nematodes use one of two methods to locate their hosts: ambushing or cruising.

Ambushing nematode species sit and wait for a suitable host by nictating. That is, they stand on their tails waiting to attack a host as it comes by. Because infective juveniles are only a couple of millimeters long, a host that is even a few millimeters away will not be attacked. In addition, these nematodes are most effective in soil near the surface where there are sufficient spaces between the soil particles to allow this nictating behavior. For this reason, ambushers are most effective against very active insects near the soil surface such as cutworms, armyworms, sod webworms, and other soil-living caterpillars and mole crickets. <Steinernema carpocapsae and S. scapterisci are examples of ambushing insecticidal nematodes.

Steinernema carpocapsae is sold under the trade names Biosafe, Savior, Millenium, and BioVector 25. It is labeled for cranberry girdler, black vine weevil, strawberry root weevil, strawberry girdler, mint root borer, and mint flea beetle. Steinernema scapterisci has been shown to be effective against mole crickets. It is not available commercially.

Cruising nematodes search out their hosts. Although they can nictate, the infective juveniles also tunnel through the soil looking for their hosts. They are attracted to the carbon dioxide and other chemicals that are exuded by potential hosts. Because of their searching behavior, they are more effective than ambushers against less active insects such as white grubs, black vine weevil larvae, and fungus gnat larvae. Heterorhabditis bacteriophora and Steinernema glaseri are examples of cruising nematodes. There are also some species of insecticidal nematodes that are intermediate in activity between ambushers and cruisers. These include Steinernema feltiae and S. riobravis.

Heterorhabditis bacteriophora is commonly called Hb nematode in trade literature and is sold under the trade names Cruiser, Heteromask, and Gardens Alive Hb Nematodes. This nematode has a shorter shelf life than most others. It is labeled for white grubs, billbugs, cutworms, sod webworms, cranberry girdler, armyworm, black vine weevil, strawberry root weevil, pine weevils, fungus gnats, and flea larvae. The bacteria in this nematode are luminescent, which causes killed insects to glow slightly in the dark. Killed insects turn brick red instead of the medium brown color typical of insects killed by other nematode species.

Heterorhabditis magidis is a cruiser nematode sold as Nemasys II. This very large nematode is labeled for the control of black vine weevil. Its large size makes it too big to fit through many other insects’ body openings. Steinernema feltiae is both a cruiser and an ambusher sold under the trade names Nemasys, Enton-em, X-Gnat, and Magnet. It is labeled for the control of sciarid flies and fungus gnats. Unlike most insecticidal nematodes, it is infective against insects in soil temperatures below 50°F.

Steinernema riobravis is both a cruiser and an ambusher. It is sold as BioVector 355 and Devour. It is labeled for the control of sugarcane rootstalk borer, citrus root weevil, and blue-green weevil. In the laboratory, S. riobravis has been shown also to be effective against mole crickets. It is active in drier soils than other insecticidal nematodes but needs high soil temperatures. It is most effective at temperatures above 95°F. Although this greatly limits its usefulness in Illinois, this nematode may prove very useful in the southern United States.

Application of insecticidal nematodes is effective with most chemical-application equipment. Application is typically made through sprayers and irrigation systems. Some drip irrigation systems have a slow enough flow that the infective juveniles can settle out in the lines. However, increasing the flow during nematode application can overcome this problem. Infective juveniles are small enough to pass through most sprayer nozzle orifices but are large enough to catch on pump and nozzle screens. For this reason, it is recommended that these screens be removed before application. They can survive tank agitation. In fact, without tank agitation, they may die because of lack of oxygen in the spray tank.

Formulations of insecticidal nematodes include water suspension, gel, water dispersible granule, and vermiculite. The water-suspension formulation stor-age time may be limited to only a few days under refrigeration because of the oxygen needs of the infective juveniles. The gel formulation is usually soaked in water to remove the infective juvenile containing gel from the screen matrix. The gel, vermiculite, and breakdown products of the water dispersible granules can easily clog screens within the application system. However, their storage times are longer, usually from 1 to several months, particularly when refrigerated.

Application of insecticidal nematodes is normally recommended for late in the day, after 3 p.m., when the sun is low enough to reduce evaporation and intense sunlight. For turf applications, the turf should be wetted both before and immediately after application to reduce the chances of the nematodes drying out and dying. Similarly, sufficient irrigation, usually at least 1/2 inch, is needed to move the nematodes into the soil, where they are protected from both drying and ultraviolet light.

Transgenic research is being conducted with in-secticidal nematodes to make them less susceptible to severe environmental conditions. This will make them easier to work with in many practical application situations. So far, this research has produced nematodes that have shown increased resistance to high temperature exposure. Research is also being conducted to determine whether the bacteria can be effective without the nematodes and whether there are substances within the bacteria that can be used with-out the use of either the bacteria or the nematodes. So far, it appears that both are necessary for control to be achieved.

As living organisms, insecticidal nematodes are exempt from many of the USEPA’s pesticide regulations. Tests have shown them to be harmless to mammals. This allows them to be brought to market much more quickly than chemical insecticides. However, this also allows the avoidance of much of the consumer protection provided by pesticide registration. When using a new insecticidal nematode or one under an unfamiliar label, try it out on a small area to be sure you will be satisfied with the results before applying to a large area.

Levels of control with insecticidal nematodes can be quite high, but 60% to 70% control is more common in turf applications. This level of control is usu-ally enough to reduce pest numbers below damaging levels. Although insecticidal nematodes are living organisms and reproduce in attacked insects, they should be used as conventional insecticides are used; apply them when control is needed. Do not expect the nematodes to survive in the soil from year to year in sufficient numbers to provide a high level of control.

Insecticidal nematodes are expensive, perhaps costing 10 times more per unit area than conventional insecticides. These costs should decrease with increased production in the future. They also demand extra care in storage, length of storage, timing of application, field conditions, and application to avoid clogged equipment. On the other hand, there are situations where conditions or clientele may demand nonchemical controls. In these situations, the extra cost and trouble may be worth the benefits.

Author: Phil Nixon


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