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.