LIBRARY December 1945 STATE PLANT BOARD E-677 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine PROTECTING STORED SEED FROM INSECT ATTACK R. T. Cotton and J. C. Frankenfeld Division of Cereal and Forage Insect Investigations The protection of seed from insect attack is of considerable importance to the seedsman faced with the problem of preserving bulk seed in storage prior to packaging and surplus seed that must be carried over from one season to another. Stored seeds provide a supply of concentrated food that constitutes an ideal diet for a large group of insects known popularly as stored-produce insects. These insects have adapted themselves to living on foods low in moisture content without recourse to other sources of moisture. The germ of the seed, with its high food value, is particularly attrac- tive to these insects. Therefore, its destruction is often the first injury resulting from the attack of such species as the Indian-meal moth (Plodla interpunctella (Hbn.)), the cadelle (Tenebroides mauritanlcus (L. ) ) , or the flat grain beetle (Laemophloeus minutus (Oliv.)). Information on the protection of seed from insect damage is by no means complete. However, the increasing demand for information on this subject makes it desirable to present the best information avail- able at this time. Factors Affecting Development of Grain Insects To preserve the viability of seed for the longest possible time, it should be kept in a dormant condition. High moisture content and high temperature speed up the life processes of seed and unduly shorten the period of viability. For this reason the fundamental requirement for the preservation of seed is low moisture. Low temperature is also helpful but not indispensable. The same factors that speed up the l'ife processes of the seed similarly affect the seed-infesting insects. Within certain limits, the higher the moisture content of the seed and the higher the tempera- ture the faster these insects reproduce. Seed that is cool, dry, and free from dockage (dust or broken kernels) is unfavorable for insect development. - 2 - The rice weevil (Sltophilus oryaa (L.)) and the granary weevil (S. granaria (L.)), the most serious pests of stored grains, cannot "breed in seed with a moisture content of 8 percent or less, and they soon die if restricted to such seed for food. Mites, which are not true insects "but are usually classed with this group of seed pests, require food with a rather high moisture content; they are not trouble- some in seed unless the moisture content is above 12 percent. Moths such as the Indian-meal moth and the Mediterranean flour moth (Bphestia kuehniella Zell.), insects of the "bran "beetle type— e.g., the saw-toothed grain "beetle (Oryzaephilus surinamensis (L.)) — and the flour "beetles (Tribolium spp.) are capable of "breeding in seed almost devoid of moisture. Flour "beetles are not troublesome in dry seed, however, if the seed is clean and free from "broken kernels, since the young larvae find it difficult to attack the undamaged seed. Temperatures above 95° F. are not favorable for the development of most seed-infesting insects. Temperatures below 70Q also greatly retard their development. A seed temperature of 65 prevents the reproduction of flour "beetles, and 60 is approximately the lowest temperature at which the rice and granary weevils reproduce. Tyrobly- phid mites are somewhat more hardy; they are reported to breed in seed kept between U0° and 50°, but only when the seed moisture is above 12 percent. Within the limits shown in figure 1, reproduction of the confused flour beetle (Tribolium confusum Duv.) in wheat increases with the increase in temperature, grain moisture, and amount of dockage. Moisture Changes in Seed In the dry climate of the Southwest seeds lose moisture rapidly, and therefore retain their viability longer than they do in the South and Southeast, where the high relative humidity of the air tends to maintain high seed moistures. In the North, where storage temperatures are lower, a low seed moisture content is not so vital. The relation between seed moisture and the relative humidity of the atmosphere is indicated in table 1. - 3 - Table 1. — Estimated moisture contents of seed (fresh "basis) attained at different relative humidities of airi/ Kind of seed Estimated moisture content of seed stored at relative humidity of — percent 63 percent 73 percent 80 percent Bean, kidney 9.0 12.0 14.0 lb.O Bean, lima 9.0 TO ft i )i 0 ic r\ .U Beet inn lor i ^ n Cabbage £ n D.U •f n q n i n ft xu.u Carrot 7 f\ TOO J.VJ.U 11 n id. 0 Celery- q n IPO 17 K Corn, sweet n o J-^-O iU 0 In #V Cucumber 7 0 St R inn Lettuce ft 0 O . V/ q n inn iv> U Okra 10.0 12.0 13.0 1H.5 Onion 9.0 „11«0 12.0 130 Pea, garden 9.0 12.0 13.5 l?.5 Peanut (shelled) U.5 6.0 7.0 g.O Pepper 7.5 9.5 10.5 12.0 Spinach 10.0 .12.0 13.0 lb. 5 Tomato g.o 10.0 11.0 12.0 Turnip 6.0 g.O 9.0 10.0 Watermelon 7.5 9.0 10.0 11.0 From Toole, E. H. , U. S. Dept. A^r. Leaflet 220, g pp., illus. 19U2. - u - An estimate "based on the assumption that insect attack will not "be a factor in the maximum safe seed-moisture content of different kinds of vegetable seeds for satisfactory storage for a year is given in tahle 2. Tahle 2. — Estimated maximum safe seed moisture contents for storage for 1 year at different mean temperatures of storage (approximate guide only).!/ Maximum safe seed moisture content for A.XI1U. Oi SCCU average temperature of storage indicated ( U i • OU J! • P dtp on 4" x "x w en Bean, kidney g (J Bean, lima 1R 11 g Beet Ik 11 9 Cabbage 9 7 5 Carrot 13 9 7 Celery 11 9 7 Corn, sweet 10 g Cucumber 11 9 g Lettuce 10 7 5 Okra lU 12 10 Onion n g 6 Pea, garden 15 13 9 Peanut (shelled) 6 5 3 Pepper 10 9 7 Spinach. 13 11 9 Tomato 13 11 9 Turnip 10 g 6 Watermelon 10 g 7 1/ From Toole (see footnote 1, table 1). 2/ — Special precautions needed when removed to higher tempera- ture. (See Section on Cold Storage of Seed.) _ c; _ Sources of Infestation Most of the insect pests of seed are good fliers, and infesta- tion may occur in the field before harvest. This is particularly- true in warm climates, where the insects can survive the winter in the fields. Infestation in the field is not serious with most kinds of seed, hut is of considerable importance in the production of seed of leguminous crops. These seeds are attacked in the field by bruchid beetles, end most of the snecies continue to breed after the seed is harvested and placed in storage. Usually, however, infestation of seed occurs after it is harvested and placed in storage. The insect pests of stored seed are r-ther general feeders and occur everywhere. They breed in feed and grain supplies on farms and in nearly every type of food commodity and storage place used by man. Infestation results from temporary storage in bins or warehouses that are not thoroughly clean or that are near sources of infested materials. Packaged seed that is crrried over from one season to another is particularly susceptible to infestation by the Indian-meal moth. This moth, which is one of the most troublesome pests of stored seed, lays its eggs on or near the packages. The caterpillars, or "worms," that hatch from the eggs are exceedingly small and easily enter the packages at top corners where the gummed flap does not completely sea.1 the pack- age. The caterpillars "web up" the contents and when fully^ grown cut their way out through the sides of the package. Preventive Measures Prompt harvesting of seed will greatly reduce the opportunity for field infestation. Seed subject to infestation in the field should be treated as soon as possible after harvest, to prevent further dajnage to the seed already infested and to check the spread of infestation. Seed should be clean and should have0as low a moisture content as possible. Storage at temperatures below ~(0 F. is desirable. Storage bins and warehouses should bo kept scrupulously clean and free from old infested seed. Returned stocks of seed should be segregated and treated before being taken into storage warehouses. Fumigation Fumigation offers a rapid and effective method of destroying in- sect life in stored seed. If the proper precautions are taken, no de-mage to germination need be feared. Under certain circumstances, how- ever, most fumigants are likely to lower the viability of some types of seed. If seed moisture is over 12 percent, or if the dosage or the exposure period is excessive, many fumigants will cause injur;/. Expos- ure periods should not exceed 2U hours. If bulk seed is treated, provision must be made to aerate it after 2l- hours unless the fumigant used is known to be harmless under all conditions. Most bulk seed has - 6 - the faculty of absorbing and retaining fumigants for long periods; therefore, unless it is aerated the exposure period is automatically extended and serious germ injury may result. The effect of excessive dosages and exposure periods on the viability of seed corn fumigated with chloropicrin is indicated "by the data in table 3. Table 3» — Germination injury to seed corn of different moisture content when fumigated with different dosages of chloropicrin at 75° P. for various lengths of time Dosage of fumigant per 1,000 bushels of seed Period of exposure Germination of corn with indicated seed-moisture content 10 percent 12 percent Ik percent 16 percent Pounds Hours Percent Percent Percent Percent p 6 Ik Ik 72 68 12 71 75 69 70 2k 79 77 71 76 US Ik 70 63 65 72 7f» (u 7U b3 °3 u 6 76 77 70 70 12 75 71 6l 65 2k 71 61 60 52 Us 76 56 55 56 72 73 60 57 59 6 6 76 76 57 57 12 70 75 6l 58 71 67 56 k7 us 71 59 53 55 72 72 52 its 1*8 8 6 73 72 56 59 12 71 69 5k 55 2k 73 60 k5 ks us 71 56 UU U6 72 68 ko k5 U6 Check (un- treated) 72 82 85 80 76 - 7 - Fumigations m^' "be conducted in "bin?, vaults, or warehouses. For warehouse and vault fumigation hydrocyanic acid, methyl "bromide, or chloropicrin can "be used at the rate of 1 pound to 1,000 cubic feet of space. No damage to germination need "be feared from the use of hydrocyanic acid. If the moisture content is not over 12 percent, and if fans are employed to keep the vapors from forming layers near the floor, methyl "bromide or chloropicrin may "be safely used. For the treatment of "binned seed a 3:1 mixture of ethylene dichloride and carbon tetrachloride is recommended, at a dosage of h to 5 gallons per 1,000 "bushels of seed, depending on the tightness of the "bin and the type of seed "being treated. This mixture does not appear to injure the germination of "bulk seed regardless of the seed moisture, the dosage, or the exposure period. The data in table U show that many of the common grain fumigants are safe to use if the moisture content of the seed is not over 12 percent and the dosage and exposure period are not excessive. Direc- tions for applying these fumigants and nrecautions to "be taken in handling then are given in Circular 369 and Farmers' Bulletin 1811 published "by this Department. Table k. — Effect of various fumigants on the germination of wheat with a 12-percent moisture content when treated with the recommended dosages and exposed for 2h hours Fumigant Dosage per 1,000 "bushels Germination Calcium cyanide Ethylene dichloride 4 carbon tetra- chloride (3sl mixture) Same plus methyl "bromide 10 percent Carbon disulfide Same plus carbon tetrachloride (l:U mixture) 1,1-Dichloro-l-nitroethane 3 lb. 4 carbon tetrachloride to make 1 gal. Chloropicrin 3 lb. 4 carbon tetra- chloride to make 1 gal. Check (untreated) Percent 15 pounds 9*+ Gallons 6 2 3 3 l 93 9U 95 95 39 98 - g - Use of Naphthalene and Paradichlorobenzene Napthalene and paradichlorobenzene crystals have "been used extensively for the protection of seed. The recommended dosages vary greatly. A popular dosage is about 1 ounce of the crystals per "bushel, although much heavier dosages are sometimes recommended. According to Roark and Nelson ,i/ the maximum weights of naphthalene and of paradichlorobenzene that can exist in vapor form in 1,000 cubic feet of space at 77° P. are 0.0U and 0.5 pound. Therefore, the small dosage recommended is more than sufficient to provide a saturated atmosphere. The crystals mixed with the seed give off vapors that are toxic to insects. The effect of saturated atmospheres of these materials on the germination of seed corn of different moisture content was deter- mined by storing seed for various lengths of time in 1-quart glass jars together with Ijk ounce of the crystals. The results of this experiment, as shown in table 5» indicate that little injury to ger- mination need be feared from naphthal ene vapors if the seed-moisture content is not over 12 percent. Paradichlorobenzene vapors, however, cause considerable damage to germination even in ve^ dry seed. Seed treated with either of these chemicals is rendered unfit for animal feeds, since there is an obnoxious odor and taste imparted to the flesh of animals and poultry fed treated grain and to the eggs laid by poultry so fed. ""' Roark, R. C, and ITelson, 0. A. Maximum weights of various furcigants which can exist in vapor form in a 1,000 cubic foot fumiga- tion chamber. Jour. Econ. Ent. 22: 3SI-3S7. 1929- - 9 - Table 5* — Effect of atmospheres saturated with paradichlorobenzene and naphthalene on germination of seed corn containing different amounts of moisture when exposed for various lengths of time Chemical Period of exposure Germination of com with indie seed-moisture content a u ea. j.u percent 12 "Percent lU "percent 16 percent Months Pprp pnt. P p Y> r> pi*\ +• A "J- ^ oil V Percent Paradichlorohenzene 1 86 85 H7 0 2 62 59 19 I 61 38 12 0 ^9 35 23 0 5 ^3 29 9 0 6 17 10 0 U 7 3pi 1^ 1 0 8 r 1 lU 0 0 9 lU lU 0 0 10 0 0 0 0 11 0 0 0 0 Naphthalene 1 90 Sh 9^ 93 2 93 Sk 91 91 3 98 97 9? 79 k 95 9H 81 51 5 93 88 6U 0 6 91 79 56 0 7 95 88 53 U g 96 82 32 U 9 95 75 29 U 10 95 75 U 11 97 75 19 r\ 0 Check (untreated) 1 93 Sk 9U Ai 91 2 96 95 96 73 ? 96 97 92 59 97 95 7U 21 5 99 96 85 58 6 98 96 1+6 3^ 7 97 9^ 33 5 g 92 92 15 9 9 98 96 55 6 10 98 92 36 0 11 96 95 2U 0 - 10 - Use of Dusts for Protecting Seed Of the several methods available for protecting seed from insect attack, mixing seed with a dust provides the most economical long-time protection. The treatment is simple, nonhazardous to the workman, and noninjurious to the seed. Many dusts have "been proposed and employed for this purpose. Some of these dusts are known to he active insect poisons, while others apparently affect the insects physically rather than chemi- cally. In the following discussion the poisonous dusts are designated as chemically active, and those which anpear to have only a physical effect on the insects are referred to as chemically inert. Chemically inert dusts are those which are thought to he effective "by causing "breaks in the waterproof fatty covering of insects, so that the dusted insect dies as a result of the evaporation of excessive amounts of "body moisture. Owing to their mode of action, the effectiveness of inert dusts decreases as the moisture content of the seed increases over 12 percent. The effectiveness of chemically active dusts is not reduced by seed moisture. Of numerous chemically inert dusts tested, magnesium oxide has "been found especially well adapted for -protecting seed from insects. Its insecticidal value against stored-prcduct insects was first recorded "by P. Zacher in Germany in 1929. It is a nonpoisonous dust used medicinally as a remedy for stomach acidity. Many types of magnesium oxide are available on the market, and extensive laboratory tests have shown that those having a particle size of 1 micron or less afford excellent protection when mixed with. seed at the rate of 0.1 percent by weight. In general the finer the dust the more effective it is. In addition to its killing effect, magnesium oxide has a distinct repellent property and seeds treated with it are rarely invaded by insects. Treatment with this dust is not only effective and inexpensive, but also imparts a clean and attractive appearance to seed (fig. 2). Magnesium oxide can he purchased in large quantities at a cost of approximately 3 cents per pound, and in smaller lots at correspondingly higher prices. A list of firms handling magnesium oxides will he found on page 13 • Of chemically active dusts tested, those containing technical DDT are the most promising. They are effective against most of the common pests of stored seed, regardless of its moisture content. DDT is best used in combination with a carrier dust, such as magnesium oxide or pyrophylllte. The carrier, by increasing the volume, insures adequate distribution over the seed. An effective and inexpensive dust consists of 3 percent of DDT in magnesium oxide. By using this mixture advan- tage may be taken of the repellent effect of the magnesium oxide as well as the insecticidal though nonrepellent qualities of the DDT. Neither product has any deleterious effect on germination, as indicated hy the data in table 6. Commercially prepared mixtures containing 3 percent of DDT in pyrophylllte or other carrier will probably soon be available, and it may be more convenient to use such mixtures. - 11 - Table 6. — Effect of magnesium oxide and DDT dusts on the germination of wheat of different moisture content Dust Period of exposure Germination of wheat with indicated seed-moisture content 12 percent lU percent l6 percent Percent of dust mixed with seed 0.05 0.1 0.2 0.05 0.1 1 0.2 0.2 Months Percent Percent Percent Percent' Efiraani' Percent Percent Magnesium oxide DDT, 3 percent in pyro- phyllite Check 1 2 I 5 6 7 g 9 10 li 12 1 2 3 U 5 6 7 g 9 10 n 12 91 91 90 95 gg 90 11 93 90 90 89 S6 90 95 90 89 92 95 89 96 9U 95 90 93 93 91 90 96 96 95 gg 97 9U 96 g6 97 9^ 96 g6 98 9^ 96 92 9U 96 (No dust of any concentration anr>lied) g7 gg' g6 g2 93 g7 87 75 9^ 71 93 65 93 6U 93 60 95 65 Sk 65 Sk 6g 9^ 68 1 92 90 89 gg 90 93 92 2 92 92 93 91 90 87 gh i 90 90 90 85 g7 92 77 gg 93 91+ g6 gu 83 78 5 96 93 95 76 gh 83 75 6 95 95 90 g2 83 gh 6g 7 96 98 90 79 89 g2 66 g 97 93 93 go gl 77 66 9 96 93 91 7g 81 77 6g 10 96 93 9U 7g gl 77 71 li 97 97 97 g6 g6 g7 12 97 97 97 g6 g6 87 89 7^ 78 78 65 67 6k P 60 59 91 g6 70 62 51 Ul 38 31 - 12 - For the treatment of seed in "bulk a seed- treating machine will be found most practicable. With this method it is suggested that magnesium oxide be applied at the rate of 1 ounce per bushel of seed, or the 3-percent DDT dust at the rate of 1/2 ounce per bushel. Seed that has been treated with any of these dusts should not be used as food for man or livestock. DDT in particular is poisonous to warm-blooded animals as well as to insects. Cold Storage of Seed Since insects are inactive below 50° » storage of seed at temperatures between k0° and 50° will prevent insect damage. Seed moisture should be as low as possible. As mentioned previously, mites may cause damage at such temperatures if the moisture content is over 12 percent. According to Toole, ^* "even at low temperatures contaminating fungi may develop at humidities approaching 20 percent and cause further injury to the seed. **** the seeds of onion, sweet corn, celery, watermelon, and pepper showed appreciable loss of viability in 6 to 9 months when stored at 80 percent humidity at a temperature of 50° )" l^ie same author suggests that precaution should be taken in the use of cold storage, since "It has been found that seeds removed from cold storage with a high moisture content and subjected to high summer temperatures deteriorate so rapidly that in a few weeks the benefit of holding in cold storage may be lost." He recommends that "Unless the seed is to be used at once after removal from storage, the moisture content should be determined and, if the moisture is above the safe limit (see table 2) for the expected tem- perature, the seed should be dried carefully to a safe moisture content." Use of Heat Insect infestation in seed may be destroyed by high temperatures. A temperature of lUO P. for 10 minutes is fatal to all stored-seed insects exposed to it. Such a temperature is not likely to injure germination, unless the seed is low in vigor or high in moisture con- tent. Newly harvested seed with a high natural moisture content should not be subjected to high temperatures until the excess moisture is removed. Any method of applying the heat that will provide for the uniform heating of all the seed is satisfactory. 2/ See footnote 1, table 1. - 13 - Procurement of Magnesium Oxide The accompanying list of manufacturers and distributors of magnesium oxide is included for the information of the users of this circular, without given or inferred guarantee of the reliability of the firms or endorsement of their individual products. No attempt has "been made to make the list complete, and no discrimination is intended or implied against firms v.-hose names are not listed. When ordering magnesium oxide, it is essential to specify that the particl size should not exceed 1 micron. American Cyanamid and Chemical Corp. , 30 Rockefeller Plaza, New York 20, N. Y. Dow Chemical Co., Midland, Mich. General Magnesite and Magnesia Co., 2960 East Venango St., Philadelphia, Pa. G-oldwynne Chemical Corp., U20 Lexington Ave., New York, IT. Y. Marine Magnesium Products Corp., South San Francisco, Calif. Michigan Chemical Co., St. Louis, Mich. Westvaco Chlorine Products Corp., U05 Lexington Ave., New York, N. Y. Summary Stored seed should be kept cool and dry in order to protect it from serious insect damage and to preserve its viability for the longest possible period. Storage of seed at temper- tures of below 60 P. will prevent serious insect injury, and heating of dry seed to ihO for 10 minutes destroys insect infestation. Fumigation with any of the common grain fumigants also destroys insect infestation, and no material damage to germination need be feared if seed moisture is not over 12 percent, exposure periods are not longer than ?X hours and the seed is aerated immediately thereafter, end dosages are not ex- cessive. A magnesium oxide dust mixed with the seed at the r te of 1 ounce per bushel or a dust containing 3 percent of DDT in a suitable carrier, applied at the rate of 1/2 ounce of the mixture per bushel of seed, affords effective and inexpensive long-time protection against infestation by insects. Seed treated with any of these dusts should not be used as food for man or livestock. DDT is poisonous to warm-blooded animals as well as insects. - Ik - Figure 1. — Rfftct of variation* in temperature, moisture, and dockage preeeat in wheat on the reproduction of 25 confused flour beetles over a 19-week period. Treated Untreated Figure 2. — Two lots of sunflower seed after exposure to insect infes- tation for 1 year. Seed on left was treated with magnesium oxide and remained undamaged. Untreated seed on right was almost com- pletely destroyed. United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant quarantine CORRECTION E-677. Flease substitute the attached sheet for page 15, recently mailed to you. \ / Treated Untreated Figure 2.— Two lots of sunflower seed after exposure to insect infes- tation for 1 year. Seed on left was treated with magnesium oxide and remained undamaged. Untreated seed on right was almost com- pletely destroyed. UNIVERSITY OF FLORIDA 3 1262 09238 7462