Fig. 1. Entomopathogenic nematodes reproducing inside a recently killed white grub.

Entomopathogenic nematodes (Steinernema and Heterorhabditis) are microscopic roundworms that parasitize and kill insects in the soil. These nematodes have demonstrated potential for biological control of insect pests.

The entomopathogenic nematodes occur naturally in almost all soils and reproduce in dead hosts (see Fig. 1). The nematodes cause widespread mortality of insects in the soil and are often seen as crashes in insect populations or conspicuous epizootics. More than 30 of these nematode species have been discovered worldwide. Due to the ease in nematode mass production, several nematode-based products have been developed for use as biological insecticides. Entomopathogenic nematodes are well suited for pest control in turfgrass because they attack a broad range of pests and can be easily mass-produced and applied using conventional spray equipment.

Fig. 2. Generalized life cycle of Steinernema and Heterorhabditis nematodes.

Life cycle Under suitable environmental conditions, the infective juvenile nematodes seek insect larvae and pupae in soil. They penetrate host insects through natural body openings (mouth, anus, and spiracles) and release a bacterium that kills the insects within a day or two. Insects killed by the nematodes are flaccid, do not give off foul smell, and have conspicuous colors. For example, insects killed by Steinernema carpocapsae are yellow and those killed by Heterorhabditis bacteriophora are reddish brown.

After the death of the host, nematodes feed on the bacteria, and insect body contents, and reproduce. Within two to three weeks, hundreds of infective juveniles are released into the environment to seek out new insect hosts and continue their life cycle. For commercial uses, they are mass-produced either in live insects or in fermenters.

Table 1. Target pests of commercially available entomopathogenic nematodes

Species, strains and searching behavior Nematode species and strains differ in their activity against different insect pests. These differences are due to the different search behaviors of nematodes, and also the type and number of bacteria carried by the infective juveniles. Steinernema carpocapsae will kill more mobile insects that live in the upper soil or thatch layer, such as billbugs, sod webworms, cutworms, and armyworms. This nematode species uses an ambush approach to find insect hosts.

The infective juvenile nematodes stand on their tails and wait for long periods for insect to come into range. Steinernema glaseri and H. bacteriophora on the other hand use a more active search strategy called cruising, to find insect hosts. These nematodes are therefore, more effective against less mobile hosts such as white grubs. This distinction should be used as an overall guide for matching the right nematode species with the target pest. Turfgrass pests that can be successfully controlled by the nematodes are listed in Table 1.

Pests controlledArmyworms The common armyworm, fall armyworm, and yellowstriped armyworm most commonly damage home lawns, and only occasionally are pests on golfcourse turf. Armyworms are also very susceptible to nematodes, as all larval stages and the pupae may be infected. Tests on home lawn turf have shown that S. carpocapsae is the most effective nematode species for the control of armyworms.

Cutworms The black cutworm is found throughout North America and is a perennial problem on bentgrass turf of golf course greens, tees, and fairways, but rarely damages lawns. The bronzed, variegated, and glassy cutworms are pests of home lawn turf. Cutworms are semi-subterranean pests and usually dig a burrow into the ground or thatch and emerge at night to clip off grass blades and shoots. S. carpocapsae can be used effectively to manage all cutworm species. Black cutworm larvae can be controlled on golf course greens by applying nematodes at a rate of 1.0 billion per acre.

Sod webworms The bluegrass, larger, western, striped, elegant, and vagabond sod webworms, along with the closely related cranberry girdler sometimes damage cool season grasses. The tropical sod webworm is the most damaging pest of warm-season grasses. Both S. carpocapsae and H. bacteriophora are effective against sod webworms in turfgrass.

Weevils The annual bluegrass weevil or Hyperodes weevil is an important pest of Poa annua and annual bluegrass on golfcourses in the northeastern United States. S. carpocapsae, H. bacteriophora, and H. megidis have shown good results for the control of weevil larvae in golfcourse turf.

Two species of billbugs are most important in turfgrass. The bluegrass billbug damages most cool-season grasses, but mainly Kentucky bluegrass and perennial ryegrass. The hunting billbug causes damage to warm-season turfgrasses including bermudagrass and Zoysiagrass. Nematodes infect both adult and larval billbugs, but treatments against larvae are generally more effective. Trials often show nematodes to be more effective than standard insecticides when larval stages are treated. In fact nematodes are the most effective control method for the hunting billbug on golfcourses in Japan. Both S. carpocapsae and H. bacteriophora are equally effective.

Fleas Larvae of the cat flea are highly susceptible to nematodes The cat flea is a cosmopolitan parasite on dogs and cats, and has also been reported feeding on humans. Flea adults spend most of their time feeding on mammal hosts where mating and egg laying also occur. The eggs eventually drop off the animal and the emerging larvae feed on organic debris in pet beddings on lawns, carpeting, or upholstered furniture.

Nematodes have been extremely effective at controlling flea larvae and pupae in home lawns. In tests performed in North Carolina, S. carpocapsae applied at 1 billion per acre caused more than 90% mortality of flea larvae within 24 h. Steinernema carpocapsae also caused 91 to 97% mortality of flea pupae in cocoons in a test in Louisiana. Nematodes are most effective against flea larvae in turf and soil when the outdoor temperatures are above 14° C and the soil is moist. S. carpocapsae-based products Interrupt and bio Flea Halt became extremely popular in 1994 and 1995.

Crane flies or leather jackets European crane fly is considered a pest of turfgrass in British Columbia, Nova Scotia, Oregon, and Washington. Substantial research conducted in Europe shows that the crane fly larva is highly susceptible to heterorhabditid nematodes.