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Diatomaceous earth

Grains

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"ADI, the average daily intake, is an estimate of the daily exposure dose to a pesticide, and the intake of commodities treated with a pesticide is represented as a percentage of the ADI.

"ADI, the average daily intake, is an estimate of the daily exposure dose to a pesticide, and the intake of commodities treated with a pesticide is represented as a percentage of the ADI.

cover from this initial knockdown (15). Addition of the synergist piperonyl butoxide increases the toxicity of the compounds.

Diatomaceous earth is a silicaceous earth composed of the cell walls of diatoms. This insecticide kills insects by removing the waterproof layer, or exoskeleton, of the insect, causing a continuous loss of lipids from the exoskeleton (16). It is available in dust formulations, and many older products were physically irritating to exposed workers and were suspected of causing pulmonary fibrosis. Many new formulations have been registered in recent years that are not as irritating to humans as the older compounds.

A biological compound, Dipel (B. thuringiensis), was registered for use on grain and other stored commodities in 1977. B. thuringiensis is a naturally occurring pathogen isolated from insects and is exempt from an EPA tolerance and ADI consideration (1-4). The current application rate calculates to 3.17 ppm with respect to traditional pesticide tolerances, which equals 46.02 billion International Units of B. theringiensis per million pounds of grain. The grain surface application is for Lepidopteran pests (eg, Indian-meal moth, Plodia interpunctella, Angoumois grain moth, Sitotroga cerealla, Mediterranean flour moth, Ephestia kuehiniella), and almond moth (Cuda cautella) because this is the only group of grain pests affected by B. thurin-ginensis. Indianmeal moths can develop resistance to B. thuringiensis (17).

Diacon (methoprene) is an insect growth regulator (IGR) that was registered in 1988 (1-4). It affects insects by interrupting the molting process. Diacon has a low mammalian toxicity and a tolerance of 5 ppm on all of the major grain crops as well as uses in vector and household insect control. As indicated, Diacon affects insect molting and egg hatch in some species (Fig. 1). This product does not eliminate existing adults, but has a secondary effect on the Fj generation by limiting population explosions of most stored grain insects. The rice weevil, Sitophilus ory-zae, granary weevil, Sitophilus granarius, and lesser grain borer, Rhyzopertha dominica, are difficult to control with methoprene because the larvae develop inside the grain kernel. In countries other than the United States, additional compounds labeled as grain protectants include res-methrin, bromophos, fenitrothion, bioresmethrin, delta-methrin, and permethrin (18).

Figure 1. Effects of insect growth regulators (IGRs) on stored product insects (effected stages shown in shaded areas).

In addition to those protectants and grain surface treatments labeled in the United States, several chemicals are labeled for application to grain storage structures (19). Methoxychlor is a chlorinated hydrocarbon that has been registered for more than 30 years. As mentioned earlier, Reldan can be used as a prebinning treatment if wheat, oats, sorghum, rice and barley will be stored in the bin. Actellic is not labeled as a prebinning spray. Cyfluthrin (Tempo) is a pyrethroid insecticide that can be used as a residual spray treatment for all storage structures, including grain bins.

Currently, protectants formulated for direct application to grain are emulsifiable concentrates (EC) or ready to apply as dry material. The ECs are diluted in water or FDA-approved mineral or soybean oil. Liquid solutions are applied through gravity-flow or pressurized systems. In commercial facilities pressurized pump systems are used to treat from 30 t/h to 60,000 t/h (1,000 to 200,000 bu/h) (Fig. 2). The dry formulations are metered into the auger with mechanical applicators or dispersed on the grain surface, or cut into the grain with a scoop before unloading from a truck. As the grain is augured to the bin, distribution throughout the grain is adequate for intended protection. Only one application of a protectant is recommended

Adults

Belt conveyor

In the auger

Drop from auger or conveyor

Belt conveyor

In the auger

Drop from auger or conveyor

One pint of wheat + 50 pairs of weevils x 150 days = 13,551 weevils

Figure 2. Suggested points at which chemical should be applied.

to ensure that residues remain below allowable tolerances. Protectants should be applied as the grain is loaded into the bin (Fig. 2). Organophosphate insecticides degrade more rapidly as temperature and moisture content increase (20), but evaluations for protectant registrations are made at the full application rate, which helps minimize the risks of exposure associated with protectant insecticides.

To illustrate the challenge of maintaining grain quality, a study on rice weevils (Fig. 3) shows how 50 adults can multiply to more than 13,551 adults in five months (21). Low moisture has a negative effect on weevil populations. The reproductive potential of several important species is shown in Table 2 (22). Studies conducted at the Gustafson Seed and Grain Technology Laboratory (Fig. 4) show that the percentage of insect-damaged kernels (IDK) is directly related to the insect population-to-grain ratio. The percentage of IDK was 18 and 97% with 2 and 50 insects per 200 g of wheat, respectively.

In May 1988, revised Federal Grain Inspection Service (FGIS) grading standards went into effect (23). These standards were more stringent; in wheat, rye, and triticale, one live weevil or lesser grain borer and one or more other live insects (OLI) injurious to stored grain in a 1-kg sample can cause that load of grain to be graded infested (Table 3). In corn, barley, oats, sorghum, soybeans, sunflower seed, or mixed grain, the infested grade is assigned when two or more live weevils or lesser grain borers, or one live weevil or lesser grain borer and five or more OLI injurious to stored grain, or ten or more OLI injurious to stored grain (Table 3).

An additional FGIS revision established an IDK limit of 32 damaged kernels per 100 g of wheat. Prior to the FGIS revisions when IDK was not included as part of the grade, a load of wheat could have significant damage and still have graded U.S. #1. Under the new standards, if a

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