U NIVERSITY OF CAFIFOBNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA The Preparation of Nicotine Dust as an Insecticide BY RALPH E. SMITH BULLETIN No. 336 November, 1921 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1921 David P. Barrows, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Dean. Edward J. Wickson, Horticulture (Emeritus). , Director of Resident Instruction C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station. B. H. Crocheron, Director of Agricultural Extension. James T. Barrett, Plant Pathology; Acting Director of Citrus Experiment Station. Hubert E. Van Norman, Dairy Management. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Ralph E. Smith, Plant Pathology. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Fruit Products. Warren T. Clarke, Agricultural Extension. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. Walter Mulford, Forestry. Fritz W. Woll, Animal Nutrition. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. L. D. Batchelor, Orchard Management. J. C. Whitten, Pomology. fFRANK Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. E. L. Adams, Farm Management. W. B. Herms, Entomology and Parasitology. F. L. Griffin, Agricultural Education. John E. Dougherty, Poultry Husbandry. D. R. Hoagland, Plant Nutrition. G. H. Hart, Veterinary Science. L. J. Fletcher, Agricultural Engineering. Edwin C. Voorhies, Assistant to the Dean. division of plant pathology R. E. Smith E. H. Smith W. G. Horne B. A. Rudolph * In cooperation with office of Public Roads and Rural Engineering, U. S. Department of Agriculture. THE PREPARATION OF NICOTINE DUST AS AN INSECTICIDE By EALPH E. SMITH The idea of a standardized nicotine dust, first suggested by the writer as an improvement on tobacco dust for the control of the walnut aphis, has aroused much interest in connection with the possibilities of destroying a number of other insect pests by dusting where liquid spraying is, for one reason or another, ineffective or impractical. Several other kinds of aphis, various leaf hoppers and thrips and a number of lepidopterous larvae, beetles and bugs are already being successfully combatted by this means, and it is believed that important possibilities have been opened for further development. The present publication has been prepared in order to make known the methods and results of the work thus far. HISTORICAL The indisputable advantages of the dusting method of applying insecticides and fungicides have, to a considerable extent, been offset by a lack of materials adapted to this method of application. The writer was attracted to dusting in an effort to find means of combat- ting the Walnut Blight, a bacterial infection (Pseudomonas juglandis Pierce) which annually causes great losses in California. In spite of many efforts, and the occurrence of a number of serious pests, the spraying of walnut trees with liquid materials has never become common on account of the large size of the trees, which makes spraying laborious, costly and slow, even though not impossible. The effort of the writer to control Walnut Blight by dusting consisted mainly in the application of sulphur. This did not control the disease but rather caused considerable injury to the trees by sul- phur burning during periods of hot weather. On account of the prevalence of the walnut aphis (Chromaphis juglandicola Kalt.) along with the blight, an effort was made to control this insect by dusting at the same application and this has resulted in a success which has quite overshadowed the original idea of controlling the walnut blight. The first experiments were made in May and June, 1917. A power blower was purchased from the Niagara Sprayer Co., of Middleport, N. Y., together with a supply of superfine (ground) dusting sulphur, mixed with tobacco dust in the proportion of 50-50. This tobacco contained about 1% nicotine. Applications were made at Goleta (near Santa Barbara), Santa Paula, El Monte and Santa Ana, using 262 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION from 3 to 5 pounds of dust per tree. Walnut dusting was also done at San Jose, using in this case pure tobacco dust. Some work was also done with a 50-50 mixture of tobacco dust and hydrated lime. In all this work the results on aphis were excellent. Not only were the trees which were actually dusted cleaned of the pest, but usually the trees for at least three rows to the windward of the last row dusted were equally well "de-loused." Wherever sulphur was used, how- ever, the unprecedented heat wave of June 15-20, 1917, caused great injury. Encouraged by this success in aphis control, a number of walnut growers ordered power dusters and a large quantity of tobacco dust for the season of 1918. This tobacco was mixed 50-50 with hydrated lime or kaolin, rather than with sulphur. In the spring of 1918 dusting was started on quite an extensive scale, but the results were disappointing and very different from those obtained in 1917. Inves- tigation soon showed that the tobacco dust which had been purchased had very little effect upon aphis and upon analysis it was found to contain only a small fraction of 1 per cent of nicotine. As a result of this failure work was begun upon an idea which had been growing ever since the tobacco dusting commenced. This idea was that of making an artificial nicotine-containing dust in order to obtain a definite, standard strength and also to get, if needed, a dust containing more nicotine than any ordinary tobacco dust. The plan in a general way, was to mix a definite amount of nicotine with an inert carrier or filler and reduce the product to a dry, uniform, extremely fine powder or dust. As a source of nicotine the commercial preparation called "Black Leaf 40" was the most available material. This is a concentrated liquid extract of waste tobacco, guaranteed to contain 40 per cent of nicotine in the form of nicotine sulphate. The carrier used was a fine grade of kaolin obtained from lone, California. 1 The first dust made in any considerable amount was mixed by hand in sheet-iron pans about six inches deep, each holding 100 pounds of material. The required amount of "Black Leaf 40" was diluted with enough water to make a stiff mud with the kaolin. This was thoroughly mixed, air-dried (requiring several days), and then pul- verized by some convenient means of breaking up the soft lumps. Material made in this way was used for determining the strength necessary to kill different kinds of aphids. Strengths ranging from 1 per cent to 10 per cent of "Black Leaf 40" by weight (.4 per cent to 4 per cent nicotine) appeared to be required for different species. Credit is due to Mr. F. A. Frazier of Berkeley for suggestions and assistance at this point in the work. Bulletin 336] NICOTINE DUST AS AN INSECTICIDE 263 At this time the name Nicodust was suggested by the writer and this term has come into quite general use to designate nicotine- containing dust mixtures. Nicosulphur is also a convenient name for the combination which it suggests.* The possibility of controlling the walnut aphis with nicodust led, at this point, to cooperative arrangements between the Agricultural Experiment Station of the University of California and the California Walnut Growers' Association. Work was commenced at Los Angeles, where several tons of dust was made during the summer of 1918. The method here consisted in placing the kaolin in the large pans pre- viously described and sprinkling over it the proper amount of ' ' Black Leaf 40,' (2 per cent for walnut aphis dust) without adding any water. This was mixed as thoroughly as possible with a garden rake, allowed to dry over night and then run through a "Hunter Lightning Sifter and Mixer," made by the J. H. Day Co., of Cin- cinnati, Ohio. In this machine the material is fed into the hopper or sifter, where it comes into contact with stiff, revolving brushes sweeping over the surface of a semi-cylindrical wire screen. This breaks up soft lumps and reduces the mixture to the fineness of the screen or finer. From the sifter the material drops into the mixing box, where a double-spiral agitator does the work of mixing in a very short time. By starting with pulverized kaolin and using no moisture except that contained in the ' ' Black Leaf 40, ' ' the lumps formed were very soft and easily broken by using a heavy wire, 45 mesh screen in the sifter. Equipment and facilities for this work were furnished by the Germain Seed and Plant Co. of Los Angeles and part of the expense was borne by the California Walnut Growers ' Association. Preparatory to the season of 1919 the work was located at Goleta, Santa Barbara County, California, where a number of growers had provided themselves with power dusters, and a mixing plant was set up in the packing house of the Santa Barbara County Walnut Growers' Association. This plant, after considerable experimenta- tion, was arranged as follows: On the second floor of the building was located a "Sifter and Mixer." Just over this was suspended a five-gallon iron tank for the "Black Leaf 40," with a %-inch pipe running down and opening into the end of the mixer. Connected with the outlet of this pipe was a small electrically driver blower fan. In mixing, the proper amount of pulverized kaolin was run into the mixer, using a ^-inch screen in the sifter simply to take out foreign matter. The right amount of "Black Leaf 40" was then * It should be clearly understood that the names Nicodust and Nicosulphur or Nicosulphurdust do not belong to any particular brand or company but may be used to describe any mixtures of the general nature indicated. 264 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION allowed to trickle slowly from the tank down through the pipe and the fan blew this in a fine spray into the kaolin while the mixer screw was turning. Each charge was allowed to mix about ten minutes, when the kaolin would be thoroughly impregnated with the nicotine solu- tion, but with many soft lumps. This mixer discharged into a per- pendicular chute leading down through the floor into a "Duplex Sifter" made by the same firm. This consists of a tight box enclosing a large, cylindrical frame covered with wire screen. When running this cvlinder revolves in one direction while brushes inside turn in it the opposite direction, sweeping over the inner surface of the screen, pushing the fine material through and breaking soft lumps. The fineness of the dust produced depends upon the fineness of this screen. The finer the screen the less dust can be produced in a given time. The speed at which material can be run through the screen is very greatly affected by its moisture content. If the dust is quite dry it will run through a very fine screen like water, while a moist, sticky mixture is slow and troublesome. Screens varying from 25 to 100 mesh were used in this outfit; when previously pulverized kaolin was used a heavy wire, 45 mesh screen gave a very satisfactory dust. Another mixer was placed below the "Duplex Sifter" to catch the material and mix it uniformly after breaking up the lumps. From this mixer the finished dust was drawn off into sacks or other containers. In the original outfit the upper mixer was equipped with a steam jacket and connected with a boiler, in order to dry the material while mixing. This proved troublesome and resulted in driving off a good deal of the nicotine. The idea was then hit upon of mixing an equal quantity of hydrated lime with the kaolin for the purpose of absorb- ing the excess moisture. The kaolin in use at the time was rather moist. With this mixture it was found possible to make workable dust up to 5 per cent "Black Leaf 40" without drying. Stronger mixtures were run through twice, using a 12 mesh screen in the "Duplex" the first time. Mr. Phil Marble, manager of the Santa Barbara County Walnut Growers' Association, deserves much credit for this plant. This outfit made 400 pounds of dust at a charge and with it four men were able to mix, sack and pile one ton per hour, or about six to seven tons per eight-hour day over a considerable period. About 400 tons of dust was made at Goleta during April, May and June, 1919, requiring the use of nearly ten tons of "Black Leaf 40." As a result of this quite successful season and the growing demand for nicotine dust for a variety of purposes, the directors of the Califor- nia Walnut Growers' Association formed a subsidiary company, the Bulletin 336] NICOTINE DUST AS AN INSECTICIDE 265 Walnut Growers' Spray Manufacturing Co., purchased a factory building in Los Angeles and moved the business to that city. The new mixing plant was similar in principle to the one at Goleta, but on a larger scale. Mixers holding a charge of 1000 pounds were used and various devices added for expediting the handling of the material. In this plant the ' ' Black Leaf 40 ' ' was simply poured slowly into the filler through two funnels while mixing instead of being blown in. There was also added to the outfit a Williams "Little Giant" mill so that, if desired, the filler could be ground before mixing, thus making it possible to use cruder material. A change was also made at this time in the material used for filler. In looking about for a cheaper substance than kaolin, attention was called to the waste lime from beet sugar mills, which is abundant in southern California, and may be had at practically the cost of handling. This material is composed of very finely divided calcium carbonate, with a varying amount of moisture and organic matter from the beet juice. As the available lime was quite wet, sometimes con- taining as high as 25 per cent moisture, and no facilities were avail- able for drying it, the plan was adopted of drying with quick-lime. A very good grade of lime was obtained from Colton, Cal., where it is burned in a rotary kiln. It was in small pieces, rarely over an inch in diameter. About 25 per cent of this lime was first mixed with the wet sugar beet lime by piling it in layers and shoveling over once or twice on the floor. This mixture became extremely hot and most of the quick-lime crumbled into a fairly fine condition. A large pile of this mixture was always kept on hand. For making nicotine dust the well-dried lime mixture was run through the Williams mill, con- veyed by air up into a large bin and thence discharged through an automatic feeder into the mixer. A large amount of material was turned out in 1920 with this outfit which often ran continuously night and day with three shifts of nine men each. This method was gradually changed and improved until, at the present time (July, 1921) the whole grinding and mixing process is carried on in a large ball mill, all the ingredients being put into the cylinder and then ground and mixed for the desired length of time. Much credit in connection with this plant is due to Mr. W. W. Thomas. The writer's connection with the Walnut Growers' Spray Manu- facturing Company ended January 1, 1921. Several other companies are now making nicotine dust, using various more or less successful but still largely experimental methods. One uses a ball mill ; another a Raymond mill, while still another sprays the "Black Leaf 40 "'in a very fine mist into the filler while 266 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION turning in a large drum so that no lumps are formed. All have had more or less trouble in obtaining the desired dryness, fineness and uniformity of the dust without loss of nicotine. WHAT ARE THE BEST MATERIALS FOR NICOTINE DUST As nicotine is a very expensive substance and comparatively large amounts are required in these mixtures, the main object to be kept in mind must be to produce the most powerful dust possible with the minimum amount of nicotine. The toxic strength of the dust is influenced both by the form and amount of nicotine used and the nature of the filler. How the dust acts. — So far as the writer's observations go, this dust acts entirely as a fumigant, the insects being poisoned by the nicotine vapor given off by the particles of dust.* Two alternatives are presented in utilizing a given amount of nicotine. 1. To make the dust of such a nature that the nicotine will be as volatile as pos- sible and the maximum amount be given off in the minimum of time. This means, of course, that the dust will lose its efficiency very soon after application. 2. To make the nicotine less volatile, so that the dust will act more slowly but for a longer time. All of our experience goes to show that the former of these methods is by far the better, namely, to make the dust of such a nature that the nicotine will be just as volatile and quick-acting as possible, even though its efficiency is thereby very short-lived. CONSIDERATION OF THE FILLER Several materials have been tested as to their adaptability for use as the filler or carrier. In general it may be said that no one substance has been found to be greatly superior or indispensable for this purpose, but almost any cheap, readily available material which is capable of being reduced to an impalpable, " smoky" dust, fairly bulky and free from undesirable effects, may be used. CHEMICAL NATURE OF THE FILLER Effect on nicotine. — When nicotine sulphate is mixed with kaolin or any other inert material the nicotine remains in the form of nicotine sulphate. When hydrated or quick-lime is used, however, a reaction takes place, forming sulphate of lime and free nicotine. The latter is more volatile than nicotine sulphate and for this reason dust made with quick-lime or hydrated lime is more powerful for the same amount of "Black Leaf 40" than that made with pure kaolin, sugar beet lime or any other inert filler. The mixing of "Black Leaf 40" with * See also Mclndoo. 2 Bulletin 336] NICOTINE DUST AS AN INSECTICIDE 267 lime appears to have a further effect than setting free the nicotine from nicotine sulphate, as a strong odor of ammonia is produced. This combined vapor of nicotine and ammonia is excessively power- ful and toxic, especially the most volatile portion which is given off when the reaction first takes place. As a rule a dust containing free nicotine is more effective than one with nicotine sulphate. Mention should further be made of an effect which takes place when ' ' Black Leaf 40, ' ' sulphur and hydrated lime are mixed together. In this case a decided reaction takes place ; the mixture becomes very hot, so that the sulphur melts or even takes fire. This heating is prevented by enclosing the mixture in an air-tight container or by exposing it to the open air until the reaction has become completed, although in the latter case some nicotine is lost. This nicotine-sulfur- lime mixture is a very powerful one and is sometimes worth using, especially when pests controllable by nicotine and others controllable by sulphur are both present. It is possible that a sulphide of nicotine is formed; if so, it seems even more volatile and toxic than nicotine itself. At present, however, this reaction is not well understood and considerable losses have occurred through the heating or spontaneous combustion of this particular combination. There has also been a good deal of uncertainty and irregularity in the final nicotine strength of such mixtures. Drying effect of the filler. — It is possible to use in the filler some materials which combine chemically with water, as well as simply absorbing it. Quick-lime, for instance, takes up a large amount of moisture in slaking or hydrating. This is a great advantage in making nicotine dust. The disadvantage of quick-lime is the heat which is produced in the reaction, causing the loss of some of the nicotine. Plaster (Plaster of Paris) combines with a considerable amount of water without producing any heat and has value in the filler on this account. Many other substances combine chemically with water and thus act as driers. Irritating or burning effect of the filler. — Nothing should be used in the filler which might possibly cause injury to animal or vegetable life. Hydrated lime is somewhat objectionable on account of its irritating effect on the skin. PHYSICAL NATURE OF THE FILLER Fineness. — The more finely divided the filler the better dust it will make. It will blow and stick better and thus the nicotine will be brought more closely into contact with the insects. Some substances break up easily into the desired condition, while others require pro- longed grinding. 268 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION Weight. — At first thought a light, bulky material would seem the best for dusting, but this is only partially true. To some extent at least a fairly heavy material, like sulphur or gypsum, is desirable because it can be blown into thick vegetation with more force and more in a solid mass like a water spray than a very light material. As a rule, a lighter filler is better for large trees like walnuts, especially with insects that are easily killed, while with small trees or vegetables and insects that must be hit directly with a considerable amount of dust, a heavier filler is better. Dryness. — It is very desirable that the filler be as dry as possible before adding the nicotine, as this will make it unnecessary to dry the mixture afterward. Absorptiveness. — It was at first thought advisable to use as a filler the most absorptive materials in order to take up the greatest possible amount of "Black Leaf 40" in a given amount of filler. Experience has indicated, however, that the nicotine is more freely given off from materials which do not absorb it readily and that fillers of the latter class therefore produce a quicker acting dust, which seems to be the best. MATERIALS WHICH HAVE BEEN TRIED Kaolin The material used came from a deposit at lone, California, owned by the Philadelphia Quartz Co., of Berkeley, Cal., where it occurs mixed with fine silica sand, from which it is separated with water. It is a white, soft, extremely fine substance, of rather light weight, quite absorptive of moisture and drying into soft, chalky, easily pul- verized lumps. It is free from chemical effects, and forms, in many respects, the best filler material which we have used. Hydrate© Lime This is the most generally available material which can be used. Its absorptive, blowing and sticking qualities are excellent. It reacts with nicotine sulphate, forming free nicotine. The chief objection to hydrated lime is the irritating effect which it has on the skin and eyes of the operator. Quick Lime Finely pulverized quick lime is a powerful drier, as already described on page 267. Lime Carbonate This is available in various forms, such as air-slaked lime, refuse sugar beet lime and various natural deposits of a chalky nature. When dried and finely pulverized such materials make a very satis- factory filler. They have no effect upon nicotine sulphate. Bulletin 336] NICOTINE DUST AS AN INSECTICIDE 269 Gypsum, Plaster Calcium sulphate or gypsum occurs abundantly in nature in various forms and from it a variety of materials called by the general term plaster are prepared by grinding and heating. All forms of gypsum and plaster are heavier and not so dusty as kaolin nor as some of the forms of lime mentioned above. They are also apt to be coarser unless reground. The various building plasters, from which a portion of the water has been removed by heating, act as driers and do not produce heat. They have the disadvantage of forming hard lumps in drying. None of the materials mentioned in this paragraph react with nicotine sulphate. Kieselguhr, Diatomaceous Earth This is an extremely light and absorptive material and is fre- quently suggested as an ideal filler for nicotine dust. In practice, however, we have had no success with it. It does not form a good dust, but tends to cling together in flakes and particles. Its great absorptiveness seems to be a disadvantage rather than an advantage, since the nicotine is not given up freely and quickly, as is most desirable. Talc Finely pulverized talcum may have some value as a filler. It is heavy and more expensive than the materials thus far discussed. When thoroughly pulverized it makes a very good dust and sticks well. It does not react with nicotine sulphate. Sulphur Sulphur is frequently added to nicotine dust on account of its value as an insecticide or fungicide, rather than simply as a mechan- ical carrier. In the latter capacity it is heavy, nonabsorptive and rather expensive. The experience has been quite general, however, that when mixed with sulphur a given amount of nicotine is more effective against insects than with any of the other fillers mentioned. Why this is so is not entirely certain. We have already mentioned the fact that when "Black Leaf 40," sulphur and lime are combined a reaction takes place which may result in a more toxic form of nicotine. Even when no lime is used, however, the nicotine seems to have an increased effect with sulphur. This may be due to the fact that the sulphur, being a poor absorbent, gives off the nicotine more rapidly than do some of the other materials. If this is the case a cheaper material having the same quality may be found. Only the very finest grades of sulphur should be used. Sulphur cannot be used on certain crops in California in hot weather without danger 270 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION of burning. Cantaloupes, strawberries and walnuts are particularly sensitive. There is also danger, in some cases, of a so-called sulphur poisoning, especially on apricots and sometimes on apples. This shows as a yellowing of the foliage and stunting of the fruit. Tobacco Dust When well ground this is a very absorptive material and makes a good dust. It may be fortified with nicotine by adding ' 'Black Leaf 40," but is commonly too expensive to be considered simply as a carrier. Tobacco dust acts more slowly than nicotine dust containing the same amount of nicotine. CONSIDERATION OF NICOTINE Pure nicotine is an oily liquid and one of the most deadly poisons known. Chemically it is a weak base, uniting with acids to form salts. It is therefore possible to obtain a great many different compounds of nicotine and it is reasonable to suppose that some of these may be more toxic to insects than others. In our rather brief experiments, however, we have found nothing which seems to be decidedly superior to nicotine itself. This has been tested by using the commercial "Nicofume" as a source of nicotine, combining this in the theoretically proper proportions with the following acids : acetic, boric, carbolic, cresylic, formic, hydrochloric, hydrocyanic, hydrofluoric, oleic, salicylic, stearic and tartaric. Of these compounds, none has shown positive superiority over nicotine itself and only one has shown any promise whatever. This is the oleate. Nicotine oleate, made as described by Moore 3 and made into a dust with kaolin, is quite toxic and yet the most lasting of any of the compounds tried. The writer has found such a dust of particular value as an ant powder, especially against the Argentine ant, which is extremely susceptible to nicotine. Nico- tine oleate does not seem to be broken down by lime as is the case with the sulphate. USE OF POISONS OTHER THAN NICOTINE The question is frequently raised whether some other substance, cheaper or more powerful than nicotine, may not be found. We can only say that this has not yet been «done. Experiments have been made with dusts containing the following as the active ingredient: Sodium cyanide, chloride of lime, kerosene, gasoline, benzine, various crude phenols and cresols, sulphur chloride, carbon tetrachloride, pyrethrum and other miscellaneous materials. Pyrethrum is power- ful against some insects but too expensive; none of the other materials tried showed any promise as compared with nicotine. Bulletin 336] NICOTINE DUST AS AN INSECTICIDE 271 ADDITION OF OTHER SUBSTANCES TO NICOTINE While nothing* has been found to replace nicotine in the dust there are some substances which seem to increase its effect somewhat. The addition of a small quantity of kerosene, for instance, seems to give nicodust a distinctly increased effect, although a plain kerosene dust without any nicotine has little or no effect upon aphids. About 5 per cent of kerosene may be added to a fairly dry dust without destroying its dusting qualities. Increased efficiency is also obtained by adding "Dry Lime Sulphur" or any similar preparation. No injury to vege- tation has thus far been found from the use of at least 10 per cent of this material. A very powerful dust is obtained by using all three, "Black Leaf 40," kerosene and "Dry Lime Sulphur," mixed with a filler. This mixture, like that with sulphur, is likely to heat under some conditions but not when placed in a tight container. HOW MAY NICOTINE DUST BE IMPROVED AND CHEAPENED? In using "Black Leaf 40" as a wet spray a strength of 1 part to 1000 of water (.1 per cent) is considered a strong mixture. To kill the same insects with the dust a 10 per cent "Black Leaf 40" dust is often required. In other words, in particles of dust and water of the same weight, the former contains 100 times as much nicotine as the latter. Can not this amount be reduced? If it is true that nico- tine in a liquid spray acts upon insects as a fumigant, just as it appears to do in the dust (see Mclndoo 2 ), then it must be that the insect is subjected to more nicotine vapor from the wet spray than from the dust, even though the latter actually contains a hundred times as much. If this is the essential difference it is probably due to the more intimate contact of the wet spray, the fact that more water than dust sticks to the insect and that the nicotine is set free more quickly and completely from a quickly evaporating film of water than from solid dust particles. (It is true, however, that the amount of spray applied per acre or plant is much greater than that of dust, thus equalizing somewhat the amount of nicotine used.) Contact of the dust with the body of the insect depends upon its fineness, its quality of adhesiveness (some materials sticking better than others even with particles of the same size), the amount of dust used per tree or acre, and the nature of the body surface of the insect itself. The nature of the dust particles can be controlled to a con- siderable extent by the choice and preparation of the filler, but the covering of each individual insect with a heavy coating of dust can- not always be insured even with a heavy application of a very fine and adhesive material, since most of the problems generally handed 262 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION over to dusting are those where for some reason the insects are hard to reach, as in the case of tall trees, curled leaves, or the under side of leaves, close to the ground where a direct hit is impossible. Since dust preparations have been on the market, it has been very notice- able that the preference of the public is for the stronger mixtures, even at a much higher price, on account of the fewer applications and smaller quantities required. These stronger mixtures, like that con- taining 10 per cent "Black Leaf 40," are more effective because it is inevitable that many of the insects receive few or no dust particles and it is therefore necessary that the whole atmosphere about them be charged as heavily as possible with nicotine vapor. This has been accomplished thus far by using, in difficult cases, an extravagant amount of nicotine in the dust. It would then appear that the greatest possibility of reducing the amount of nicotine lies along the line of making it as quickly volatile as possible. TEMPERATURE The success of nicotine dusting is greatly affected by tempera- ture ; the hotter the weather the greater the effect produced by a given strength of dust, showing again that the effect is one of fumigation and depends upon a rapid giving off of nicotine vapor. This is more affected by temperature than by any of the other factors which have been discussed. POSSIBILITY OF REDUCING COST OF MANUFACTURE The present prices of nicotine dust may be reduced somewhat by economies in the process of manufacturing and distribution, but it is evident that so long as it is necessary to use any such quantity as 10 per cent, or even 5 per cent, of a high-priced finished product like "Black Leaf 40," subject this to manufacturing and distribution costs and pay transportation charges on mixtures containing 95 per cent or more of inert material, the consumer's price of the finished mixtures must always be high. SUGGESTION OF A NEW TYPE OF DUSTING MACHINE In conjunction with Mr. W. W. Thomas the writer has developed somewhat the idea of a combined dusting and mixing machine, the plan being to place the raw materials in the machine in the field and do the mixing in the hopper of the dusting machine, just as is done in the tank of a liquid sprayer. Such a machine, if feasible, would have the following advantages : The grower could buy materials like ' ' Black Leaf 40, ' ' hydrated lime and sulphur almost as cheaply as the Bulletin 336] NICOTINE DUST AS AN INSECTICIDE 273 present dust manufacturers and save the expense of mixing, con- tainers, "overhead,' 1 several profits, and much of the freight. He could vary the strength of the mixture according to his needs. Little or no nicotine would be lost in the process of mixing and handling. The first full strength of the nicotine and ammonia when set free from "Black Leaf 40 ' : by lime would be utilized. By adding a small amount of quick-lime, or other material, heat could be developed in the mixture and the dust discharged in such a hot condition that the nicotine would be very volatile and active, and a smaller amount of "Black Leaf 40" would therefore be required. This is perhaps the greatest advantage of all in the use of this method. The idea of the machine which we have had in mind contemplates a hopper containing a mixing screw into which finely pulverized and liquid materials would be placed in the proper proportions. Hydrated lime would be the most generally available filler material, with ' ' Black Leaf 40" as a source of nicotine. Pulverized quick lime or "Dry Lime Sulphur" could be added if necessary for drying, or for pro- ducing heat. From the hopper, after brief mixing, the material would be drawn through the fan (using an exhauster) rather than blowing it out ahead of the fan as in the present machines. The action of the fan would complete the mixing and break up the nicotine lumps. This would, of course, be a power machine ; for hand use, ready-made mixtures would still be required. USES OF NICOTINE DUST Without attempting extensively to discuss at this time the use of nicotine dust in the control of specific insects, the following species may be mentioned as having been experimented upon with at least promising results: Walnut Aphis (Chromaphis juglandicola Kalt.), Rose Aphis (Macrosiphiim rosea Linn.), Pea Aphis (Macrosiphum Pisi Kalt.), Melon Aphis {Aphis gossypii Glover), Black Citrus Aphis (Toxoptera aurantiae Koch), Bean Aphis (Aphis rumicis Linn.), Cab- bage Aphis (Aphis brassicae Linn.), Rosy Apple Aphis (Aphis mali- foliae Fitch), Citrus Thrips (Scirtothrips citri Moulton), Onion Thrips (Thrips t abaci Lindeman), Pear Thrips (Taeniothrips pyri Daniel), Beet Hopper (Eutettix tenella Baker, Vine Hopper (Ery- throneura comes Say), Squash Bug (Anasa tristis De Geer), Cucum- ber Beetle (Diabrotica spp.), Cabbage Worm (Pontia rapae Lmn.)j Tent Caterpillars (Malacosoma spp.), Brown Day Moth (Pseudohazis eglanterina Boisd.), Fall Webworm (Hyphantria cunea Drury), Chal- cedon Butterfly (Lemonias chalcedon D. & H.), Thistle Butterfly (Vanessa cardui Linn.), West Coast Lady Bug (Vanessa caryae 274 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION Hubn.), False Chinch Bug (Ny silts ericae Schilling and the small variety mimitus Uhler). These insects show a great difference in their susceptibilty to nico- tine, the range being from a 1 per cent "Black Leaf 40" dust, which gives very good control of the walnut aphis, to the 10 per cent mixture which is required satisfactorily to control some of the more resistant species. Essig 3, 4 and Campbell 5 have been the principal experi- menters along this line. COMBINATIONS Nicotine dust may be mixed with sulphur, lead arsenate, dry, pul- verized Bordeaux Mixture, and, in fact, with any dry fungicide or insecticide for the control of more than one pest at one application. ACKNOWLEDGMENTS The results described in this publication have not been obtained without the assistance of many helpful suggestions and various forms of aid from others. Particular credit is due to the following : Mr. F. A. Frazier, of Berkeley, representative of the Niagara Sprayer Company of Middletown, N. Y. ; Mr. Carlyle Thorpe, manager of the California Walnut Growers' Association, Mr. W. W. Thomas, formerly of the Walnut Growers ' Spray Manufacturing Co. ; Mr. Phil Marble, manager of the Santa Barbara County Walnut Growers' Association, Mr. J. S. Davis, President of the California Sprayer Company, of Los Angeles, and Mr. E. S. Kellogg, County Horticultural Commis- sioner of Santa Barbara. LITERATURE i Smith, Ralph E. 1918. Dust Spraying Proves Sure Killer of Walnut Aphis. In Diamond Brand News (Cal. Walnut Growers' Assoc, Los Angeles, Cal.), vol. 1, pp. 4-5, illus. 2 McIndoo, N. E. 1916. Effects of Nicotine as an Insecticide. In Jour. Agr. Research, vol. 7, no. 3, pp. 89-122, pis. 1-3. 3 Moore, Wm. 1918. A New Fungicide of Economic Importance. In Journal of Economic Entomology, vol. 11, p. 341. 4 Essig, E. O. 1920. The Pear Thrips. Cal. Agr. Exp. Sta. Circ. 223, 9 pp., illus. 5 Essig, E. O., and Horne, W. T. 1921. Plant Disease and Pest Control. Cal. Agr. Exp. Sta, Circ. 227. e Campbell, Roy E. 1921. Nicotine Sulphate in a Dust Carrier against Truck Crop Insects. U. S. Dept. Agr. Circ. 154, 15 pp., illus.