The San Joaquin County and Delta Water Quality Coalition — made up of irrigating landowners — is charged with monitoring sub-watershed drainage areas in San Joaquin County and parts of Calaveras and Contra Costa Counties for compliance with surface water pollution laws and regulations. Among compounds found in drainage waters from agricultural areas, chlorpyrifos (the active ingredient in Lorsban, Lock-On, and other products) has often been detected at levels above allowable limits.

These "exceedances" have occurred in samples collected during winter as well as summer months. Exceedances are of concern: if they cannot be prevented by voluntary pesticide use practices undertaken by growers, then mandatory measures must be imposed to eliminate them.

About 50 percent of the chlorpyrifos applied in San Joaquin County is used on walnuts. Chlorpyrifos moves easily in water and has only moderate affinity for plant and soil particles. It also has long residual activity; this is good for pest control but bad for runoff potential.

Chlorpyrifos applications in walnuts begin in May and can continue through August. Any irrigation runoff during this time period can cause chlorpyrifos to enter surface waters. Significant residues have been found in irrigation runoff collected as long as two months after application. Fortunately, most walnut orchards in this area are irrigated with sprinklers, microsprinklers, or drip. Good water management, including turning the system off before any runoff occurs, can help ensure that off-site movement of pesticide residues will not occur

Residues can also enter waterways through spray drift. Avoid spraying in conditions or locations where irrigation supply and drainage ditches pass near orchards.

Another option for reducing the potential for off-site movement of chlorpyrifos is simply not to use products that contain it. In many situations, alternative materials such as mating disruption or non-organophosphate insecticides and insect growth regulators can be substituted for chlorpyrifos. Substitute products can be used in the entire orchard or just in portions near waterways.

Among several products available, Delegate (spinetoram), a relatively new product from Dow AgroSciences, offers a significant level of aquatic safety compared to organophosphate products. Other effective alternatives are available. Consult your pest control advisor for materials that will work for you without running the risk of triggering further restrictions on chlorpyrifos use.