LIBRARY STATE PLANT BOARD May 1950 ET-282 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine DIRECTIONS FOR APPLYING WINDBORNE AEROSOLS FOR INSECT CONTROL OUT OF DOORS By A. H. Yeomans Division of Control Investigations Studies have been made by us at the Agricultural Research Center, Beltsville, Md. , and all over the country, on the proper method of applying windborne aerosols for the control of insects on field crops and in orchards. Windborne aerosols are those applied as a cloud and carried by the wind across the area to be treated. The fundamentals of aerosol cloud behavior have also been studied in the laboratory by use of a wind tunnel. From this background of experience the following directions for applying aerosols for control of insects out of doors have been prepared. The insect control attained from windborne aerosols applied to field crops and orchards is due mostly to the deposit. Therefore, the con- siderations are directed to the factors that result in the best deposit. It is true, however, that if the aerosol is applied while the insect to be controlled is in flight, those individuals on the wing are killed as well as those that come in contact with the deposit. Machines for applying windborne aerosols should not be confused with machines that project sprays with a powerful air blast. The air blast machine can provide a more uniform deposit across a swath with the larger spray particles than can be obtained with the windborne aerosol machine. For satisfactory performance an aerosol cloud must be released under proper weather conditions, have uniform deposition in the selected swath width, be composed of particles of the proper size, be of the proper dosage and formulation, and be applied in the most economical manner. These requirements will be discussed. Weather Requirements Satisfactory movement of the aerosol cloud across the area is accomplished by making applications under the proper weather con- ditions. A light wind is needed, steady in direction, and moving at 1/2 to 8 m. p.h. Winds slightly stronger than 8 m.p.h. can be utilized -2 when the cloud is drifted uphill or when an orchard or other area with a high canopy is treated. A time of day should be selected when there is a surface inversion of temperature- -i. e. , when the air temperature at the ground level is cooler than at a height of 6 feet or more. Sur- face inversion keeps the aerosol cloud close to the ground and is most important when low -growing crops are treated and least important when trees having a canopy of foliage are treated. Good inversion usually occurs only at night from 1 hour after sunset imtil sunrise, but occa- sionally exists all day when the ground has been cooled by rain. In hilly terrain surface inversions usually occur only in the valleys. If it is necessary to make treatment in the daytime without the surface inversion, a wind of 5 to 10 m. p. h. is beneficial. Deposition The^ deposit is heaviest nearest the point of release and decreases as the distance increases, because the larger particles settle out first. Aerosol particles naoving with the wind deposit selectively on exposed vertical surfaces but settle uniformly on horizontal surfaces. The amount of deposit on the vertical surface depends on the size and shape of the object. Under similar conditions the deposit is much greater on objects of narrow width, such as pine needles, than on ones of greater width, such as maple leaves. The deposit on vertical surfaces becomes important when the aerosol is applied in winds stronger than 5 m. p. h. , and on small tender foliage it is sometimes heavy enough to cause injury. When a large proportion of the foliage is exposed vertically, as in a vineyard, the increased deposition is especially important. Experience has shown that under the best conditions only 25 to 50 percent of an aerosol containing particles of less than 50 microns mass median diameter deposits in swaths up to 2000 feet, the major portion drifting beyond the area under treatment. The deposits of aerosols of different particle size released in a 3-m.p,h. wind under good inversion conditions are given in table 1. These deposits settled on an open field; the percentage would have been higher if the aerosol had been released through dense foliage. Swath Width The swath width is chosen first by the locations in the crop through which the machine can be taken with the least damage from the wheels. The minimum swath width is limited by the particle size requirements. A narrow swath requires large particles to settle out in the swath. Large particles sometimes cause foliage injury when oil solutions are used. The maximum swath width is limited by the dosage requirements. A wide swath requires a heavy output from the machine. Too heavy an -3- output, even though the particle size is small, will cause foliage injury due to the heavy deposit near the source. The smaller particle sizes are less efficient in depositing within a limited area. For this reason it is best to select a swath as narrow as possible without causing too much damage from the wheels of the machine or foliage injury from the large particles. Some recommended swath widths for use of DDT on various insects are given in table 2. Particle Size It is important to select aerosols of the proper particle size. The proper particle size depends on the swath width the aerosol is expected to cover, the wind velocity, and the amount of foliage pene- tration required. After the swath width has been chosen, the particle size that will give 25 to 50 percent deposition of the aerosol at different velocities may be obtained from table 3. These values were computed for an aerosol cloud released at an average height of 10 feet and under good inversion conditions. If penetration of dense foliage is required, the particle size indicated should be reduced by one-half. For example, 3 pounds of DDT in 3 gallons of solution released with a particle size of 40 ntiicrons across a 100-foot front in a 3-m.p.h. wind will leave a deposit of about 1 pound per acre across a 300-foot swath. If 1 pound of DDT in 1 gallon of solution is released across a 100-foot front and the particle size is raised to 70 microns, the deposit will be 1 pound per acre across a 100-foot swath with the sajne wind. When the proper particle size has been selected, the aerosol machine should be set to produce this particle size according to the directions of the manufacturer. Sometimes temporary control of flying insects is desired. The optimum particle size for this type of treatment depends upon the kind of insect, and not upon the deposit. For adult yellow-fever mosquitoes the optimum particle size has been found to be about 15 microns, and for house flies about 22 microns. Dosage The dosage depends on the deposit per acre of insecticide required to control the insect. It is measured by the annount applied per 100 feet of front, and varies with the swath width. Some recommended dosages of DDT against various insects are given in table 2. These values are based on the premise that 25 percent of the insecticide deposits in the first swath. Application to successive swaths results inoverdrift, which increases the deposit by 10 to 20 percent in each swath. In a large field the wastage due to overdrift is thus reduced to that from the last few swaths. For swaths less than 300 feet wide the dosage can be reduced by about 20 percent, and for swaths 300 to 500 feet wide by about 10 percent in each successive swath until the 50- percent point is reached. This reduced dosage should then be repeated to the end of the plot. The amount of insecticide required per 100 feet can be released by two methods. The total amount required on a front can be measured, then applied by moving the aerosol generator back and forth across this front until the entire amoimt has been exhausted. The second method is to calibrate the output of the generator and then calculate the proper speed to move across the front to give the desired dosage. As an example, of second method only, if it is desired to release 2 gallons of solution per 100 feet from a generator with an output of 40 gallons per hour, the output per minute would be 2/3 gallon. Since 1 m.p.h. is equivalent to 88 feet per minute, the speed of movement would be 100 X 2 =3.4 m.p.h. 88 2/3 Forn:iulations To prevent rapid evaporation it is desirable that at lease one-fourth of the aerosol solution be a nonvolatile liquid. Best results have been obtained with a very concentrated solution. A much used formula is 5 to 7.5 pounds of DDT dissolved in 2 gallons of benzene or xylene plus 3 gallons of SAE lOW motor oil or an agricultural oil. An agricultural oil is used where tender foliage is present. The amount of DDT that can be dissolved in the solvent depends on the temperature. Benzene is preferable to xylene. Benzene hexachloride, dinitro-orthocresol, pyrethrum, rotenone, toxaphene, chlordane, hexaethyl tetraphosphate, tetraethyl pyrophosphate, and nicotine have been similarly formulated. The last three insecticides are particularly noxious in aerosol form; an operator should therefore wear a proper gas mask when releasing them. Method of Applying Aerosol The initial impetus of the aerosol cloud, as it is emitted from the nozzle of the machine, should only place the cloud in the wind, and not deposit it on the foliage. The initial impetus is usually expended within 15 feet or less. It is preferable to point the nozzle low and back of the line of travel. The nozzle should never be pointed at foliage within the range of the initial impetus, because heavy deposit might cause burning. For treating low-growing crops the aerosol should be directed below the top of the surface inversion. When aerosols are applied in towns, the nozzle should be pointed over the tops of parked cars. Table 1. --Effect of particle size on the percent of total insecticide depositing on 100 -foot strips across a field. Distance fr Dm (feet) Mass median diameter of particles release front 75 microns 40 microns 25 microns 0-100 25.8 7.6 1.4 100-200 16.6 7.5 1.4 200-300 11.1 6.5 1.4 300-400 5.5 4.7 1.4 400-500 2.8 2.8 1.4 500-600 1.4 1.8 1.4 600-700 0.9 1.7 1.4 700-800 0.5 1.7 1.0 800-900 0.5 1.7 0.7 -6 Table 2. --Recommended swath width and dosage of DDT for control of insects in the field with an aerosol under good conditions. /Numbers in parenthesis refer to Literature Cited7 Insect Stage of insect Recommended swath width Dosage of DDT Approximate deposit of DDT Larva Feet 100 Pounds per 100 feet Pounds per acre Cabbage looper^ 1 Armyworml' Larva 250 2/5 Leafhopperli Adult 500 1/2 1/10 Spotted cucumber beeilei/ Adult •100 1 Japanese beetle (6) Adult 100 1 Lygus bugl' Adult and nymph 250 2/5 False chinch bugl' Adult and nymph 250 2/5 Tarnished plant bug (8^) Adult and nymph 250 2/5 Pentatomid (£) Adult 250 2/5 Fall armywormi/ Larva 250 2/5 Mosquito (JL^) Adult • 500 1/2 1/10 Mosquito (2) (3) Larva 100 1 Sand fly (4) Adult 150 2/3 House fly 2/ Adult 150 2/3 Horn flyi/ Adult 150 2/3 Black fly (5) Adult 150 2/3 Greenhouse whitefly^ Potato flea beetle-^ Adult 150 2/3 Adult 150 2/3 Chinch bugl^ Adult and nymph 250 2/5 Gypsy moth (7) Adult and nymph 250 2/5 1/ Tests with these insects were made by H. A. Jaynes in 1945. 2/ Tests with these insects were made by the author. 7- Table 3. --Optimum particle size, in m.icrons mass median diameter, of aerosols for application at different swath widths Swath width (feet) Wind velocity in miles per hour ' 3 5 111 9 11 13 15 50 55 98 -- -- -- -- -- 100 40 70 92 -- -- -- -- — 200 29 50 65 77 90 98 -- - • 300 24 40 53 63 70 79 88 92 400 21 35 46 55 62 68 73 79 500 18 33 41 49 55 61 66 70 600 17 29 37 45 50 55 60 65 700 15 27 35 41 47 51 57 60 800 14 25 33 39 43 48 53 57 900 13 24 31 36 40 47 51 55 1000 13 22 29 35 39 42 48 51 1200 12 21 27 32 36 39 43 46 1500 10 18 24 29 32 . 35 39 40 1/ Application at higher wind velocity is not recommended. sta:^^ -&£'BoMa> UNIVERSITY OF FLORIDA ° 3 1262 09242 9124 Literatiire Cited (1) Brescia, Frank. 1946. Salt marsh and anopheline mosquito control by ground dispersal of DDT aerosols. Jour. Econ. Ent. 39: 698-715. (2) and Wilson, Irwin B. 1947. Larvicidal treatment of large areas by ground dispersal of DDT aerosols. Jour. Econ. Ent. 40: 309-313. (3) 1947. Treatment of native villages with the aerosol generator. Jour. Econ. Ent. 40: 313-316. (4) 1947 Aerosol generator as used for sand fly control. Jour. Econ. Ent. 40: 316-319. (5) Glasgow, R. D., and Collins, D. L. 1946. The thermal aerosol fog generator for large scale application of DDT and other insecticides. Jour. Econ. Ent. 39: 227-234. (6) Langford, George S. , and Vincent, Rufus H. 1948. Fogging with DDT for Japanese beetle control. Jour. Econ. Ent. 41: 249-251. (7) Latta, Randall. 1946. Field experiments with heat generated aerosols. Jour. Econ. Ent. 39: 614-619. (8) Snapp, Oliver I. 1948. Control of sucking bugs that cause deformed peaches, • Jour. Econ. Ent. 41: 555-557.