Some use it to refer to the use of predators and parasites to control nematodes, a meaning often spoken of as "classical" biological control.
At the other end of the spectrum, any control methodology not utilizing "chemical" nematicides is considered biological control by some nematologists.
For the purposes of this discussion, we will include predators and parasites, soil amendments which are thought to affect predators and parasites, toxins and metabolites produced by bacteria and fungi, killed microbials, and "natural" products.
Although classical biological control is a promising and active area of nematological research, at the present time, there are no predators or parasites of nematodes commercially available for use in California.
A number of "natural" products and soil amendments have been or are being marketed. These products are not registered as nematicides but rely on testimonials or subtle wording to imply that in addition to improving soil conditions for plant growth, they inhibit or kill nematodes.
In the past few years, three "natural" products have received U.S. EPA registration and one is registered in California for use on a number of crops.
A possible explanation for this phenomena is that over millions of years, balanced relationships have evolved in nature.
Scientists studying natural systems may find it difficult to perturb this balance in favor of increased parasitism.
A host specific organism runs the risk of becoming extinct if it kills off all of its hosts.
This has at times been cited as a reason why "natural" biological control will never be practical for nematodes.
Although it may not be feasible to effect control with a single organism, combinations of organisms could be developed or organisms utilized in combination with cultural controls which might provide control comparable to chemical nematicides.
In contrast, no viruses have been documented to cause harm to nematodes, and only a single bacterial parasite has been extensively studied.
Each group utilizes a different type of structure to adhere to or to attack nematodes. For nematode trapping fungi, this is thought to be mainly a passive activity with the fungus waiting for the nematode to pass by and become stuck to knobs , networks, or conidia. The fungi which form rings have been shown to form more rings in the presence of nematodes than in their absence. It is not well understood why nematodes put their heads through the rings. However, when they do they either become wedged, or stuck on an adhesive substance, or the rings constrict. Fungal hyphae then penetrate the nematode and utilize it for food.
Some nematophagous fungi have been shown to be poor competitors in the natural environment. They utilize the nematode cuticle for protection from other organisms.
Many of the nematode parasitic fungi can be raised in the laboratory on various media.
Some have been encapsulated in pellets made from calcium arginate to aid in their dispersal in the field and to provide a food source until a nematode is encountered.
HIrsutella rhossiliensis is one example of a fungus with conidia which adhere to nematodes which may accidentally contact them. Fungal hyphae then penetrate the cuticle, utilize the internal contents for food, and the cuticle for protection. When the food source has been utilized, adhesive conidia grow out of the cuticle to await passage of another host.
Zoosporic fungi have swimming spores which can seek out nematodes, attach to the cuticle, and develop hyphae which feed on nematodes.
Some fungi parasitize vermiform adult or larval nematodes, while others parasitize eggs, or cysts. Egg parasites include fungi from a variety of taxonomic groups.
A number of different strains have been isolated and tested. In spite of extensive work, this bacteria still cannot be reared on artificial media and this has greatly hindered commercial development.
Spores of this bacteria adhere to the cuticle of a nematode and a penetration tube develops which penetrates the nematode. Once gaining access, the bacteria reproduces inside of the nematode utilizing the cuticle as protection. Two million spores have been shown to be formed within a single nematode. Typically, the nematode survives and feeds following infection but reproduction is greatly reduced.
It is thought that one or more biological organisms in the soil are suppressing nematode populations. Trying to document that a soil is suppressive is no easy task.
There are indications in the literature that crops grown under monoculture for many years in nematode infested fields will have high nematode populations for several years which will then fall and remain at low levels. It is thought that some biological organisms are functioning to maintain nematode populations at a low level.
It is not uncommon in chemical control field trials for chemically treated plots to have significantly higher nematode populations at harvest than untreated controls. At times, this has been interpreted as indicating the field is naturally suppressing nematode populations in the controls. In reality, the difference is likely due to the chemically treated plots having healthier root systems which are able to support larger populations than can be supported by the roots of the untreated controls.