Grains And Protectants

Several insecticides are available for postharvest application to grains in the United States: malathion, chlorpyri-fos-methyl (Reldan), pirimiphos-methyl (Actellic), methoprene (Diacon), Bacillus thuringiensis (Dipel), synergized pyrethrins, and diatomaceous earth (Table 1). Malathion, Reldan, and Actellic are most the commonly used insecticides; however, Reldan and Actellic can be applied only to specific grains.

Malathion, an organophosphate (OP) insecticide, has been labeled for application to grain since 1957. It can be applied to all major grain commodities in addition to some minor crops (1-4). The labeled rate for application to stored grains is 10.4 ppm and the established Environmental Protection Agency (EPA) tolerance is 8.0 ppm. In addition to its use as a grain protectant, malathion has been widely used in control programs for production agriculture and disease vector control. However, many companies have voluntarily withdrawn their formulations of malathion that were registered on stored grains, and there are doubts as to whether this product will be reregistered. Many insect pests of stored grain have developed resistance to malathion (5-13), which is also affecting decisions regarding registration.

In 1985, almost 30 years after the introduction of OPs as protectants, Reldan (chlorpyrifos-methyl) received a registration for direct application to grain (14). Reldan can be applied to barley, oats, sorghum, rice, and wheat, but not to corn. In addition, Reldan can be used as a prebin spray only if wheat, sorghum, rice, barley, and oats will be stored in the bin. Pirimiphos-methyl (Actellic) was registered in 1986 (1-4). It is labeled for corn and sorghum only and does not have a label for use as a prebin spray.

Acceptable daily intake (ADI) calculations are used by the EPA to regulate nononcogenic compounds. The ADI is an estimate of the daily exposure dose that can be consumed without potential for any harmful effects, and the intake of commodities treated with a particular pesticide is represented as a percentage of the ADI. Although in future actions by the EPA, ADI will be referred to as reference dose (rfd), it was not used at this writing because no EPA documents have used this terminology for pesticide registrations. The total ADI for all registered uses of an active ingredient should not exceed 100%. The calculated ADI for malathion is about 507% for the general population (1-4). In addition, in the most susceptible age group (nonnursing infants aged one to six years) the ADI exceeds 1100%. Malathion has broad use patterns and was registered prior to the EPA's use of ADI as a regulatory tool. Neither Reldan nor Actellic exceed the 100% calculated ADI value.

Synergized pyrethrins have been registered as a protectant for many years but have not been used extensively due to the limited availability and high cost. Synergized pyrethrins have a high acute toxicity to insects but low toxicity to mammals and are generally used as preventative surface applications instead of treatments to the entire grain mass. The base pyrethrum compounds are extracted from chrysanthemum flowers, and while the natural products give quick knockdown, insects often re-

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George G. Khachatourians Adrienne E. Woytowich University of Saskatchewan Saskatoon, Saskatchewan Canada

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