1. BIOLOGY

Figure 1. Large Heterorhabditis females feeding on a dissected host decomposed by the bacterial symbiont of the nematode.

A. Introduction

Steinernema and Heterorhabditis species (order Rhabditida) are nematodes parasitic on insects. They transmit bacteria which are lethal to their host, a characteristic which makes them more suitable for biological control of insects than any other nematode group. Over the last two decades steinemematids and heterorhabditids have become increasingly popular in insect control. Among their useful attributes are (a) a wide host range, (b) the infective juveniles can be readily and cheaply cultured, either on hosts or on artificial media, (c) cultures can be stored for extended periods, and (d) they can be easily applied in the field. Provided the humidity of the air is within a suitable range, the nematodes are resistant to a variety of environmental conditions, can actively find and penetrate susceptible hosts, and cause up to 100% mortality within a few days (Figs. 1 and 2). They are harmless to higher organisms.

Twenty years ago probably no more than five or six scientists worldwide were committed to the study of this group of nematodes. At present their number is well over 50 and growing (Poinar, 1985a). This development is largely due to pressure from environmental lobbies for biological means of pest control and bans on persistent insecticides.

Figure 2. Large Heterorhabditis females showing through the cuticle of an infected host.

The American R. W. Glaser (1932a) was the first person to publish on the biological control potential of steinemematids. In a field in New Jersey, he observed that Steinernema glaseri killed large numbers of Japanese beetles, Popillia japonica Newn. In his laboratory, in a salve can of sterilized soil, a heavy suspension of this nematode killed the beetles in the manner of a true infectious disease; however, houseflies, silkworms force-fed with nematodes, and plants all remained uninfected (Glaser 1932a,b). Glaser did not understand the mode of action of this disease but did not discount the possible presence of a bacterium. His first field experiments were extremely promising and a program for extensive treatment against Japanese beetles throughout New Jersey was organized. It had to compete, however, with Dutky's milky disease organism (Bacillus popilliae), introduced in 1941, and which showed astonishing results, and World War 11 intervened (Briand and Welch, 1963; Stoll, 1973). The discovery of two other species of this genus, S. feltiae Filipjev, 1934 and S. bibionis Bovien, 1937, attracted little attention. Filipjev (1934) and Bovien (1937) considered that their species were able to multiply within a host but did not suspect pathogenicity. Bovien assumed that the infective juveniles accumulated in the host's intestine and penetrated the body cavity after the host had died.

Dutky and Hough (1955) developed a practical rearing method using larvae of the greater wax moth, Galleria mellonella L. as hosts. Steinernema feltiae was made widely available and soon showed promise when used against a large number of insect pests in many parts of the world.

Only Dutky's S. feltiae DD-136 strain was known to transmit a lethal symbiont, and most work concentrated on that strain. More literature is available on this strain than on all other Steinernema species combined. However, all steinemematid and heterorhabditid infective juveniles carry symbiotic bacteria. The bacterial species belong to one genus, Xenorhabdus, but are specific for each nematode species (Akhurst and Boemare, 1988; Poinar and Brooks, 1977; Thomas and Poinar, 1979).

The number of insect species susceptible to steinernematids and heterorhabditids seems unlimited. Few, however, cause plant damage in an environment sufficiently damp to allow the nematodes to reach them. In general, soil insects occupy habitats conducive to nematode survival; however, in undisturbed agricultural soils, nematode mobility is very limited (Reed and Came, 1967). Steinernematids and heterorhabditids can be very effective agents of control against hosts associated with glasshouse crops, especially those grown in pots, and also against hosts which tunnel in the tissues of live plants.