Know Your Friends

Nematodes

Editor's Note: This is the second of two articles on nematodes. Go to Part 1

Insect-attacking nematodes are naturally occurring tiny worms which live in the soil. Nematodes from two familiesSteinernematidae and Heterorhabditidaeare available commercially from numerous sources for biological insect control (see The IPM Practitioner Directory of Least-Toxic Pest Control Products, Nov./Dec. 1994 issue). Understanding their biology and life cycle will increase the likelihood that they will provide effective biological control.

The third stage larva (infective juvenile) is the stage sold commercially for application in the field. This is the stage which is adapted for host finding and survival in the soil. Effective use of nematodes depends on providing proper conditions for their survival after application and proper selection of nematode species or strains for the desired target pest. Insect-attacking nematodeslike plant-parasitic nematodeslive in the water-coated spaces between soil particles. They are dependent on the availability of a water film for their movement and survival. Nematodes may be killed by dry conditions, temperature extremes or exposure to ultraviolet light from sunlight. For these reasons, application of nematodes to foliage is not usually recommended, even though many above-ground insect pests on plants are susceptible to nematodes.

Nematodes should be applied to moist soil. Irrigation or watering before application is advisable if soil conditions are dry. Maintaining adequate soil moisture after application is also important to increase the persistence of nematodes.

Most nematode species are not very active when soil temperatures are less than 60F. If nematodes are applied to cool soil, they may remain on the soil surface and dry out or be exposed to ultraviolet light and die. Nematode efficacy is decreased if applications are made when soil surface temperatures are above 86F. Nematodes survive best if applied late in the day, when the intensity of ultraviolet light and temperatures are usually lower. Irrigation immediately after application can minimize nematode mortality by washing them off leaf surfaces and down to the soil. Also, irrigation may decrease soil surface temperatures.

In addition to damaging effects from lack of moisture, temperature extremes, and UV radiation, there are various natural enemies of nematodes. These include small insects such as springtails, mites and fungi, among others. Because of all these sources of natural mortality, application of nematodes one year often does not provide effective control of pests for more than one season.

Industry and public sector scientists continue to make advances in mass rearing procedures which will lead to lower prices and wider availability of nematodes in the future. Currently the most cost-effective rearing procedure for producing large numbers of nematodes is the use of liquid culture in large fermentation tanks. New formulations are being developed that will improve shelf-life and application ease. Most current formulations are designed for liquid applications of nematodes. Granular formulations are being researched. For example, USDA-ARS scientists have developed a granular formulation (Pesta) based on durum wheat flour, clay, and peat moss.

There are many different nematodes species which vary in their behavior and efficacy against different pests. More research is needed to define which nematode species works best against particular pest species. Steinernema carpocapsae (sometimes also referred to as Neoaplectana carpocapsae) is currently the most common nematode sold commercially. It is one of the easiest to rear, and some formulated products can be stored at room temperature. However, it may not always be the most effective species against certain pests.

There are undoubtedly many new species of steinernematid and heterorhabditid nematodes which will be discovered in the future that will be more effective against particular pests. For example, two Steinernema species have been described in recent years: S. riobravis was isolated from south Texas and has tolerance to warm temperatures; and S. scapterisci was isolated from South America, and is effective against mole crickets. A new Heterorhabditis species, H. megidis was isolated from Japanese beetle grubs in Ohio.

Adapted from:

- Bob Wright, University of Nebraska


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