Factors Affecting the Efficacy of Nematicides

As with any pesticide, the two factors that determine efficacy are concentration and exposure time. If toxic concentrations of a nematicide do not come in contact with the nematode for a sufficient period of time, then acceptable levels of control will not occur. Many factors can dilute the concentration of nematicide available in the soil and/or effectively shorten the time that nematodes are exposed.

Good site preparation is extremely important. The soil should be thoroughly tilled several weeks before application to break up clods and encourage the decomposition of plant debris. Large root pieces should be removed. Nematicides can adsorb to organic debris and thus reduce the amount of compound free in the soil. Large soil clods can interfere with the uniform distribution of nematicides and serve as protected areas for nematodes.

Fumigant nematicides such as Telone or Vapam volatilize quickly and move through the soil as a gas. These gases penetrate the nematode cuticle and interfere with vital processes, causing death. The mode of action is relatively quick. The movement of a fumigant through the soil is strongly affected by factors such as temperature, moisture, and soil texture. Fumigants tend to move upwards through the soil and will dissipate quickly unless the surface is sealed after treatment. Follow the label to ensure that you are applying the correct dose for your conditions.

Almost all nonfumigant nematicides such as Nemacur and Vydate are organophosphate or carbamate pesticides, which are potent cholinesterase inhibitors. These compounds are extremely water-soluble, and their redistribution in the soil depends on water movement. Excessive rain or irrigation creates a risk of diluting the nematicide below the level needed to be effective. On the other hand, too little water may prevent the nematicide from being distributed effectively in the root zone. Unlike fumigant nematicides, contact nematicides act relatively slowly. Although high concentrations are lethal, the lower concentrations in soil generally kill by behavior modification. The affected nematodes typically are unable to move, find a host, feed, or find a mate. Eventually they die. If exposure to the nematicide is too short or at a low concentration, however, these behavioral modifications can be reversed and the treatment is not effective.