Refrigeration to provide cold storage is impractical or too expensive for protecting most seeds. The cost may be justified for limited quantities of valuable seed stock or if the infestation hazard is unusually high. Even in cold storage, the moisture content must be low, and the relative atmospheric humidity must be kept down to avoid damage to germination.

Upon removal from cold storage, there can be difficulty from condensation. Seed with high moisture removed from cold storage and subjected to high summer temperatures will deteriorate so rapidly that in a few weeks the benefit of cold storage will be lost. Unless the seed is to be planted at once, the moisture content should be determined when it comes out of cold storage. If it is above the safe limit for the temperature to which it will be exposed, it should be dried carefully to a safe level.

SEED CLEANING can be of great benefit in preventing insect infestation and indirectly in avoiding fungus attack.

The young larvae of the bran beetles do not feed readily on sound, unbroken seeds. It is difficult therefore for an infestation to get started in clean seed. They can, however, feed on broken seeds and floury dust that may be present as dockage.

After the infestation gets started, the larger larvae and adults may be able to attack sound seeds, making available additional food for young larvae. In filling bulk bins, the dockage tends to concentrate in layers or columns, which are especially susceptible to insect attack. The typical sequence of increased moisture, rise in temperature, and subsequent growth of fungi is quite likely to center around these concentrations of dockage.

The dense, compact nature of the dust and dockage, plus the moisture and results of fungus activity, create a mass that is hard to fumigate. The gas penetrates it poorly or not at all and tends to channel around the spot. If aeration is being used for cooling or the forced distribution of fumigants, interference with airflow reduces the effectiveness. When dockage is scattered throughout bulk seed, the fine material absorbs a great deal of any fumigant gas that may be applied and reduces the effectiveness of the treatments.

It is apparent that the more thoroughly seed is cleaned the less hazard there will be of insect infestation. A concentration of 0.5 percent of dockage may be critical at a seed moisture content of 9 or 10 percent. A smaller amount is critical at 12 percent moisture or higher.

Thorough seed cleaning will remove most or all of the external forms of insects. It will not remove the insects, such as weevil larvae, that develop inside the seeds.

SEED PROTECTANTS and coatings of pesticides often are used to prevent damage by fungi or insects after planting. As we said, the fungicides are of no value against storage fungi. They have little or no effect on insects.

Dieldrin as applied to seeds for protection against insects after planting is also effective in preventing infestation during storage before planting.

Other insecticides, such as malathion, synergized pyrethrum, lindane, methoxychlor, and DDT, may be applied to seeds as a dust, slurry, or spray treatment to protect it against insect damage during storage.

Tolerances have been established for pyrethrum and malathion under the Food, Drug, and Cosmetic Act to permit their use as protectants on food and feed grains. Surplus or reserve seed stocks treated with these materials therefore can be diverted for use as animal feed or human food if the tolerance level has not been exceeded. Seeds treated with dieldrin, lindane, methoxychlor, or DDT must not be so diverted.

A NUMBER of chemically inert dusts can also be used effectively and economically on seeds to prevent infestation. These treatments are less effective as the moisture content of the seeds increases over 12 percent. The kinds of inert dusts found satisfactory include silica gel, magnesium oxide, diatomaceous earth, rock phosphates, precipitated chalk, and aluminum oxide. The particle size should be 1 micron or less. A common application rate is 1 part of dust per 1,000 parts of seed by weight, or about 1 ounce per bushel.

Any one of several methods of application or kinds of equipment may be suitable for treating the seed. The important consideration is to insure uniform coverage. None of the inert dusts or pesticides we have mentioned has been found to have any adverse effect on germination.

INSECT CONTROL in warehouses may vary somewhat, depending on whether the seed is stored in bulk or in bags and according to the type of storage structure. The basic principles are the same in all cases. Best results are obtained by emphasizing preventive measures.

The importance of temperature, moisture content, relative atmospheric humidity, and clean seed have been discussed. Add to these thorough cleaning of the warehouse or storage structure, inside and out, before bringing in new seed. One should get rid of spilled seed, grain, or animal feed. Any old stocks of these materials that may be infested should be removed or fumigated. A heavy-duty commercial vacuum cleaner should be used. Walls, window sills, beams, ledges, floors, and other places where old seed and dirt can lodge should be swept. All parts of elevators and conveyors should be cleaned. Attention must be given to places under loading platforms and to buildings that are off the ground. One should burn, bury, or haul away all refuse and cleanings, so that infestation does not spread to new seed when it is brought in.

It is well to apply a residual spray of premium-grade malathion or DDT to kill out any infestation remaining in the structure after the cleanup. It should be done 1 or 2 weeks before new seed is stored. DDT should not be used if the seed may be diverted to food or feed channels.

The application of a protestant as seed goes into storage is another highly desirable procedure if the storage period is to be more than just a few weeks. The premium-grade malathion, synergized pyrethrum, or one of the inert dusts may be used.

The periodic application of a residual spray over the surface of stacks of bagged seed is another desirable preventive measure. This will be useful to avoid infestation by beetles or moths.

The seed must be free of infestation when the stacks are formed, and the treatments must be started immediately for best results. With malathion, a second application in 2 weeks and monthly thereafter while temperatures are above 70 is usually adequate. With pyrethrum, a 2-week continuing schedule should be maintained during warm weather. Similar treatments on the surface of bulk-stored seed are useful where there is a hazard of moth infestation.

Space treatments with aerosols or mist sprays appear to give rapid and spectacular results, especially against moths. Their real value is limited as a control measure. They can be useful as a supplemental preventive procedure.

SEED THAT is infested at harvesttime must be fumigated before, during, or immediately after it is placed in storage. Fumigation is the most effective corrective measure to apply when an infestation develops in storage when preventive steps have been lacking or inadequate.

Fumigation can reduce or destroy germination. The effect involves a complex relationship of dosage, time, temperature, and moisture. It is wise not to fumigate seed having a moisture content of more than 12 percent or a temperature of more than 85 . Recommended dosages should be used.

Mixtures containing ethylene dichloride, carbon tetrachloride, or carbon disulfide are safest, but the seed should be aerated after 3 to 5 days. Phosphine is safe on the cereal grain seeds and apparently most of the others, except the legumes. Normal dosages of hydrogen cyanide or methyl bromide within the temperature and moisture limits we named are safe with a fumigation period of not more than 24 hours.

Chloropicrin, acrylonitrile, and some other fumigants can be highly injurious to germination.

INSECT-RESISTANT PACKAGING is useful for protecting seed during storage and until it is ready for planting. Even closely woven fabric bags offer little protection against insect invasion. Multiwall paper bags, laminates, and plastic films are somewhat more resistant but fall short of being insect-proof. Paper bags treated with synergized pyrethrum or methoxychlor can be highly effective for one season of storage in protecting against invasion or penetration. The bags must be well constructed and have tight closures. DDT could be used on bags for seed that will not be diverted for use as food or feed. None of the treatments is completely effective on fabric bags.

PREVENTING DAMAGE TO SEED by insects and fungi after harvest can be accomplished by the following procedures:

Harvest promptly.

Clean and dry seed before storage.

Clean up the storage structure and apply residual spray before bringing seed in.

Apply a protectant to seed as it is placed in storage.

Keep the seed and the warehouse cool and dry.

Make frequent culture tests to be sure storage fungi are not invading seed.

Apply residual sprays on a periodic preventive maintenance schedule. Fumigate if and when necessary.

Use protective packaging.

LYMAN S. HENDERSON is Chief of the Stored-Product Insects Branch, Agricultural Marketing Service. He has been with the Department of Agriculture since 1938. He has administered research programs in insect control since 1945.

CLYDE M. CHRISTENSEN is a professor in the Department of Plant Pathology and Botany at the University of Minnesota. He was on special assignment with the Rockefeller Foundation agricultural program in Mexico during 1959. He conducts research on the deterioration of grains caused by storage fungi.