Know Your Friends

Nematodes

Editor's Note: This is the first of two articles on nematodes. Go to Part 2

Insect-attacking nematodes in the families Steinernematidae and Heterorhabditidae have received a great deal of attention in the past 10-15 years because of their potential as biological control agents for a wide range of insect pests, especially those living in the soil. These nematodes attack a lot of different pest insects, can search for and kill their hosts rapidly, and are safe to nontarget organisms. Advances in rearing techniques have made it possible to produce them economically in large numbers, which has encouraged their commercialization. Numerous commercial suppliers of insect-attacking nematodes exist today (see Directory of Least-Toxic Pest Control Products, The IPM Practitioner, Nov./Dec. 1994 for a listing of commercial suppliers). These nematodes are being used commercially in certain specialized markets (e.g., citrus, turf, and ornamentals) and may be used more widely as costs of production are decreased.

Insect-attacking nematodes have a life cycle consisting of an egg, four juvenile stages and the adult. Both steinernematid and heterorhabditid nematodes have a specialized third juvenile stage, the dauer larva, or infective juvenile, which is the stage which attacks insects. It is nonfeeding, and thus can survive in the soil for extended periods until it is able to find a susceptible host. Third stage dauer larvae are the stage which is used for field applications of nematodes.

Nematodes find hosts by orienting to carbon dioxide, and host excretory products. Infective juveniles enter hosts through natural openings, such as the mouth, anus or breathing pores (spiracles). These nematodes carry specific species of bacteria in their intestines. Upon entering the host, the nematode releases the bacteria into the host, where they rapidly multiply. The bacteria kill the host through release of protein-destroying enzymes, usually within 24 hours. Nematodes feed on the host remains, and complete two or three generations inside the host. When the host resources are gone, large numbers of infective juveniles leave the host and begin to search for new hosts. At room temperature, it takes steinernematid nematodes about 7-10 days from infection to the emergence of new infective nematodes. Heterorhabditid nematodes take about 12-15 days for similar events.

Different species of bacteria are associated with steinernematid and heterorhabditid nematodes, which causes differences in the appearance of infected hosts. Insects infected by steinernematid nematodes are limp, and cream to dark brown in color, while those infected by heterorhabditid nematodes turn brick red and glow in the dark.

Many different species of these nematodes occur naturally in the soil throughout the world. There are currently 15 species of steinernematid nematodes and 5 of heterorhabditid nematodes that have been described. All vary somewhat in their behavior, host range, rate of development and other characteristics. Also, in some species, particularly Steinernema carpocapsae, different races, based on collections from different locations or different hosts have been identified, which also vary biologically.

Next month's Know Your Friends article will cover current uses of nematodes and future trends.

Source:

-Bob Wright, University of Nebraska


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