Using Beneficial Nematodes for G

Using Beneficial Nematodes for Grub Control and Japanese Beetles

Nematodes Biology fact sheet

Nematodes are microscopic, non-segmented worms which occur naturally in soil all over the world. Thousands of strains exist with different lifestyles. Beneficial nematodes attack only soil-dwelling insects and leave plants alone. Beneficial nematodes and the bacteria they spread are not known to be harmful to humans, animals, plants, earthworms or other non-target organisms, but they do aggressively pursue insects like grubs. When they sense the temperature and carbon dioxide emissions of soilborne insects, beneficial nematodes move toward their prey and enter the pest through its body openings. The nematodes carry an associated bacterium (Photorhabdus species) that kills insects fast (within 48 hours). Several generations of nematodes may live and breed within the dead pest; they emerge and seek more pests in the soil. Beneficial nematodes have been shown to be as much as 96% effective against Japanese beetle grubs in field studies. Although many species of beneficial nematodes are available, Heterorhabditis bacteriophora (Hb) nematodes are most effective against Japanese beetles, European chafers and other grubs that are lawn pests. They are more efficient than the Steinernema species. Hb nematodes work better because they are cruiser nematodes that burrow down in the soil searching for deep soil-dwelling pests. They also have a special "tooth" that helps them get into the grub.

Life Cycle

Diagram courtesy of H. Kaya

Steinernema and heterorhabditis have similar life histories. The non-feeding developmentally arrested infective juvenile seeks out insect hosts and initiates infections. When a host has been located, the nematodes penetrate into the insect body cavity, usually via natural body openings (mouth, anus, spiracles) or areas of thin cuticle. Once in the body cavity, a symbiotic bacterium (Xenorhabdus for steinernema, and heterorhabditis) is released from the nematode gut, which multiplies rapidly and causes rapid insect death. The nematodes feed upon the bacteria and liquefying host, and mature into adults. Steinernema infective juveniles may become males or females, where as heterorhabditis develop into self-fertilizing hermaphrodites although subsequent generations within a host produce males and females as well. The life cycle is completed in a few days, and hundreds of thousands of new infective juveniles emerge in search of fresh hosts.

Thus, entomopathogenic nematodes are a nematode-bacterium complex. The nematode may appear as little more than a biological syringe for its bacterial partner, yet the relationship between these organisms of one of classic mutualism. Nematode growth and reproduction depend upon conditions established in the host cadaver by the bacterium. The bacterium further contributes anti-immune proteins to assist the nematode in overcoming host defenses, and anti-microbials that suppress colonization of the cadaver by competing secondary invaders. Conversely, the bacterium lacks invasive powers and is dependent upon the nematode to locate and penetrate suitable hosts.

How often do I need to use biological control nematodes?

Contrary to what you may hear, in most cases biological control nematodes do not persist for a long time after they are applied. Most of these nematodes can live for only a few weeks at most without their host insect. Biological control nematodes occur naturally in most environments. They kill a few insects and help reduce insect epidemics. If they killed off all of their host insects, the nematodes would die off too. When we apply commercially available biological control nematodes, we are attempting to overload the natural system and kill many more insects than would be killed by nematodes naturally. If the treatment is successful, and most of the pest insects die, the nematode populations decline as well. Soon the natural balance is restored. When insect populations begin to build back up another nematode application is required.