The six compounds listed above vary in their use by the grain and food industries due to variation in specific uses, cost, practicality, and efficacy. Historically, malathion was by far the most frequently used grain protectant and residual surface treatment for stored products in the U.S.Many populations of the major pest species have developed resistance to this insecticide (Subramanyam and Hagstrum 1995, Arthur 1996). Ironically, stored grain insect populations have been reported to actually increase following treatment with malathion, relative to untreated controls (Reed et al. 1993). The most important OP grain protectants in the U.S. are chlorpyrifos methyl and pirimiphos-methyl. Despite some drawbacks with efficacy (e.g., Reldan is ineffective against lesser grain borers, and many other insect species are already resistant to this compound [see Subramanyam and Hagstrum 1995]), these compounds are used successfully by knowledgeable farmers and elevator managers throughout the country. reassessment of chlorpyrifos-methyl, EPA has asked that the registrant generate $2-6 million worth of new toxicology data to determine the risk of occupational exposure, which is estimated to be high for applicators treating empty grain bins. The registrant (DowAgrisciences) has made a business decision to cancel the registration on chlorpyrifosmethyl; thus, it is very likely that grain uses for this compound will not be supported in the future. Tolerance reassessment for pirimiphos-methyl has not been completed.

    Stored grain users of chlorpyrifos methyl and pirimiphos-methyl will be left without any effective, practical substitutes if these OPs are canceled. Malathion’s regulatory status in the future remains uncertain, and it is not a viable option because it breaks down rapidly on grain, and many stored-product insects, including a mold feeder (hairy fungus beetle), have developed resistance to it (see Subramanyam and Hagstrum 1995). Another labeled grain protectant that might be considered a substitute for OPs is diatomaceous earth (DE). DE can be effective against grain insects (Golob 1997, Korunic 1998), but the level of application required (300 ppm or above) results in a significant loss in test weight (bulk density) so that its use is prohibitive (Korunic et al. 1998). However, DE is an effective product for use in empty bins (Subramanyam, unpublished data).

    Phosphine gas is widely used by certified grain managers, but should not be used by farmers who are untrained in its safe and effective application. Additionally, fumigation has no residual effect and may be unsuitable for high-risk climates. Similarly temperature management via ambient aeration, which does not kill insects but slows their population growth, is an option only if storage systems are equipped with aeration equipment. New protectant treatments for stored grain that can effectively replace OPs are necessary to maintain quality and prevent economic loss. Once such alternative is Spinosad, an insecticide containing bacterial fermentation metabolites, labeled by EPA for use on vegetables, turf, cotton, and tree crops (Bret et al. 1997, Nolting et al. 1997). Spinosad is extremely effective on stored wheat against five stored product insect species (Subramanyam et al. 2000). Potential replacements to the fumigant phosphine include the ozone gas, which at a low rate of 5 ppm is effective against insects and molds (Mason et al. 1997).