Nematodes are microorganisms that inhabit soil. Some invade the root systems of plants and reduce crop yields. Traditionally, chemical agents, called nematicides, have been used to control parasitic nematodes in vegetable plants, but the recent move away from chemical solutions in gardening and agriculture has generated interest in possible biological control agents. Different species of bacteria and fungi kill nematodes, and their presence in the soil may protect plants from nematode infection. However, biocontrol systems are difficult to select and implement.
Effectiveness as a Long-Term Solution
In a review of studies examining the effectiveness of biocontrols on nematodes, the UN Farm and Agriculture Organization found that large concentrations of biocontrol agents were needed to protect the plant from infection. Furthermore, biocontrols often must be applied directly to the affected area, such as the root system, which is difficult or impossible for many plants. They concluded that biocontrols were not effective long-term solutions and may prove uneconomical.
Complex Interaction with the Natural Environment
The UN Food and Agriculture Organization notes that biocontrols of nematodes interact in complex ways with the natural environment, and finding a biological agent that works well under most conditions may prove an insurmountable challenge. In many cases, selecting a biocontrol agent requires a level of expertise and training not available to the average gardener or to farming communities that would benefit most from a biocontrol program. In a study on the biocontrol of root-knot nematodes, Sharma and Pandey point out that dilution by water and the activities of other living and non-living components in the soil may reduce the benefit of biocontrols.
Creating the Ideal Soil Environment
Biological control agents often have very specific needs that are difficult to replicate in a garden or farm environment. Most research on the biocontrol of nematodes has been conducted in greenhouse conditions, where soil temperature and moisture levels may differ greatly from garden soil. For example, some bacteria used to control nematodes may not become active in dry soils that still support nematodes.
Interaction with the Host Plant
The efficacy of biological agents on nematode populations also depends on the structure of the plant itself, and not all biocontrol agents work well with all plants. For example, Bourne and Kerry found that biocontrols were less effective when nematodes remained inside the roots of the plant, as they do with some plant species, including tomatoes. Consideration of how the nematode and the biocontrol will interact with the host plant is essential, and one-size-fits-all solutions do not exist.
The UN's Food and Agriculture Organization set a series of standards for use in controlled experiments evaluating biocontrols of nematodes. In a subsequent review of the research literature, they found that less than 15 per cent of studies conformed to these standards. Much of our understanding of biological control agents are based on these studies, which are not replicating normal growing conditions or ensuring that nematode mortality is caused by the biocontrol agent.