Influence of Particle Size on Application of Insecticide Sprays

Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Beltsville, Md. Particle size is shown to have an effect on the distributi...
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Influence of Particle Size on Application of Insecticide Sprays A. H. YEOMANS

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 13, 2018 | https://pubs.acs.org Publication Date: June 17, 1952 | doi: 10.1021/ba-1951-0007.ch007

Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Beltsville, Md.

Particle size is shown to have an effect on the distribution and deposit of the insecticide and on foliage injury. A brief description is given of methods of determining par­ ticle size. Examples are given of using aerosols and mist blowers in buildings and in the field, and airplanes in the field to illustrate the importance of particle size.

P a r t i c l e size of insecticidal sprays has a bearing on the a b i l i t y of airborne sprays to be transported and deposited, on foliage i n j u r y , and on the efficiency of residues i n k i l l i n g insects. E a c h problem should be carefully analyzed for the particle-size re­ quirements. I n studying these problems the author applied the fundamental information available, such as Stokes' law for the settling rate of particles a n d Sell's work (5) on the efficiency of deposit of particles i n a moving air stream. However, this funda­ mental work is based on ideal conditions never found i n practical problems, so that the job was to correlate the fundamental information w i t h actual conditions. A standard method of determining the size h a d to be developed (5). N o n e of the various methods used to atomize liquids give homogeneous particle sizes; there­ fore a representative value, such as mass median diameter, was used to compare particle size of insecticidal sprays. If the spray must be described more completely, this is done b y breaking down the particle size range into several divisions and report­ ing the accumulative percentage of the mass of the spray according to each division. Various methods of taking samples of spray were tested. Impinging the spray on a coated microscope slide a n d then measuring the deposited particles under a microscope was found to be the best method for computing the sizes accurately. One method of impingement is to wave a slide through the spray. A n o t h e r is to air blast the spray against the slide, a n d , for very fine spray having a mass median diameter of less t h a n 10 microns, to use settling or electrical precipitation. F o r non­ volatile o i l sprays the slides are coated w i t h a n oleophobic material t h a t gives a u n i ­ form spread a n d maintains the round shape of the droplet. M e t h o d s of determining the spread of the droplet as i t rests on the slide have also been developed (δ). F o r volatile solutions the slides are coated w i t h magnesium oxide or carbon soot, and the particle leaves a crater on contact. T h e thickness of this type of coating establishes the amount of deviation of the crater size from the actual particle size. A l t h o u g h most oils have similar surface tension a n d density, they differ greatly i n viscosity. T h e viscosity, the output of l i q u i d , the efficiency of the application of energy to the l i q u i d , a n d the degree of evaporation after the particle is formed are i m p o r t a n t factors to utilize i n producing the desired particle size. I n heat vaporization aerosols the distillation range of the solution is also i m p o r t a n t . Three methods of a p p l y i n g atomized liquids were tested—by aerosols, b y mist blowers, and b y airplane. Sprays were released b y the first two methods i n buildings and b y a l l three methods i n the field. 56

AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.

YEOMANS—INFLUENCE OF PARTICLE SIZE ON APPLICATION OF INSECTICIDE SPRAYS

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 13, 2018 | https://pubs.acs.org Publication Date: June 17, 1952 | doi: 10.1021/ba-1951-0007.ch007

Spray Particle Size in Indoor Treatments Tests with Aerosol Sprays. One of the chief means of a p p l y i n g concentrated o i l sprays is i n aerosol form. A n aerosol generally remains suspended i n the air for some time a n d is carried b y normal w i n d or a i r currents. Aerosols are probably best adapted to interior applications b u t have been used w i t h some success outdoors. T h e y m a y be produced b y liquefied-gas formulations released through capillary nozzles, b y steam a n d air atomization, b y centrifugal disks a n d rotors, b y extremely high pressure, and b y heat vaporization. M a n y factors influence the selection of the proper particle size when concentrated oil sprays are being used. I n large closed warehouses, aerosol treatments can be used to protect stored products from exposed crawling a n d flying insects. Aerosols having various particle sizes were tested a n d the results compared b y t a k i n g residue samples throughout the warehouse. Insect m o r t a l i t y studies also served as a basis of comparison. A s a result, a particle size between 1 a n d 5 microns mass median diameter was selected. These small particles are produced b y thermal aerosol generators that can be operated outside the warehouse a n d the fine particles i n t r o duced through a n open door. T h e aerosol is carried first to the ceiling, b u t b y the time the treatment is complete i t is well distributed throughout the interior b y convection currents. T h e warehouse door is then closed a n d overnight the particles penetrate into most of the cracks a n d crevices a n d settle o n the top of the exposed horizontal surfaces. T h e small particles i n this test gave the best penetration a n d insect control, a n d also were the easiest to apply. F o r control of flying insects w i t h aerosols, research showed that there is a n o p t i m u m particle size needed to obtain m a x i m u m k i l l . T h e size was determined through extensive research i n w i n d tunnels {2) a n d P e e t - G r a d y chambers i n which the most nearly homogeneous sprays possible were used. F o r adult mosquitoes the o p t i m u m particle size was found to be between 10 a n d 15 microns mass median d i ameter a n d for adult houseflies between 15 a n d 20 microns. I n treating some closed interiors w i t h aerosols, i t m a y be necessary to l i m i t the time of application. I n this case the particle size must be large enough to settle out i n the time available. A 10- to 15-minute exposure time is the m i n i m u m for satisfactory results. A n aerosol spray h a v i n g a mass median diameter of 15 to 30 microns is sufficient for the short-exposure applications, b u t w i l l not penetrate so completely as the smaller particle size. Furthermore, aerosols of this size must be released from more t h a n one point if the radius of the area is more t h a n 15 feet. W h e n heat from thermal generators causes excess breakdown of the insecticide, equipment t h a t p r o duces larger particle sizes must be used. T h e relationship between particle size and foliage i n j u r y was shown i n work w i t h liquefied gas aerosol formulations of hexaethyl tetraphosphate for greenhouse a p p l i cations (1). Particles larger t h a n 20 microns diameter injured some varieties of foliage. Increasing pressure a n d using smaller nozzles lowered the mass median diameter b u t d i d not eliminate the m a x i m u m sized particles. T h e problem was finally solved w i t h a low-concentration formulation, i n which the particles larger than 20 microns i n diameter could be eliminated. Tests with Mist Blowers. W o r k carried out i n large open warehouses, such as those used for storing tobacco hogsheads (4), showed that the particle size must be about 50 microns mass median diameter to prevent too much loss through the large wall openings. T h e spray was blown over the tops of the hogsheads w i t h a specially built mist blower i n order to get proper d i s t r i b u t i o n .

Spray Particle Size in Outdoor Treatments Tests with Aerosol Sprays. Insecticide aerosols m a y be applied outdoors for controlling insects o n crops. T h e aerosol is released from a line source and is carried b y the w i n d across the area to be treated. A number of tests were made w i t h aerosols of various particle sizes under different weather conditions (6). F o r satisfactory AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.

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results a surface inversion i n the air is required to keep the aerosol close to the ground and a light w i n d of steady direction is necessary. Particle size should be selected according to swath w i d t h and w i n d velocity. For a 3 mile-per-hour w i n d the particle size should v a r y from a mass median diameter of 70 microns for a 100-foot swath to 18 microns for a 1500-foot swath. The swath w i d t h should not be so narrow t h a t too large a particle size is necessary, for penetration is then poor and foliage is injured near the source. Deposit from this type of treatment was studied b y insect counts across infested fields before and after treatment, b y chemical analysis for insecticidal residue on foliage, b y microscopic examination of droplets deposited on coated glass slides, and b y dye samples recovered across the treated area. T h i s type of sampling showed t h a t even under the best conditions only about 2 5 % of the insecticide can be recovered on the first swath, b u t t h a t up to 5 0 % can be recovered if there are m a n y overlapping swaths. T h e deposit was also found to taper off from the source, the tapering being more pronounced as the particle size increased. T h i s v a r i a t i o n has been explained b y the larger range of particle sizes contained i n the larger mass median diameter sprays and, therefore, the greater differences i n the settling rates of the particles. M o s t of the particles settle on low-growing crops, but if m u c h of the foliage is exposed vertically and the w i n d velocity is greater t h a n 5 miles per hour a substantial deposit is obtained on the exposed vertical surfaces. W h e n dye is added to the solution, small vertical surfaces can be seen to have the heaviest deposit. T h i s finding agrees w i t h wind-tunnel data (7) and with Sell's work (5). S m a l l surfaces are usually the most tender and are sometimes injured when application is made i n w i n d y weather. Moreover, applications are not usually recommended i n wind velocities greater t h a n 8 miles per hour, because much of the insecticide is lost. Tests with M i s t Blowers. M i s t blowers are well adapted to l a y i n g down a uniform deposit across a low-growing crop. Several of these machines have an air output of about 10,000 cubic feet per minute. T h e relation between the velocity of the air and that of the liquid is one of the principal elements i n controlling particle size. T h e size is adjusted m a i n l y b y the method of introducing the l i q u i d , because the air velocity is usually maintained at the m a x i m u m . Sprays having particle sizes between 50 and 150 microns mass median diameter are the most useful. M i s t blowers can apply larger particle-sized sprays t h a n the aerosol machines, because the air nozzle can be pointed upward at an angle of about 45° and the large particles can be blown some distance from the machine. S w a t h widths up to about 500 feet can be treated i n strong winds w i t h a mist blower. F r o m a machine h a v i n g a n air output of 8000 cubic feet per minute w i t h the air nozzle pointed up 45° i n an 8-mile-per-hour wind and a spray h a v i n g a particle size of 65 microns mass median diameter, a relatively uniform deposit can be made across a 150-foot swath. A b o u t 5 0 % of the i n secticide is deposited i n a swath of this size, and the recovery approaches close to 8 0 % when several swaths overlap. Particle size should be varied according to w i n d velocity and swath w i d t h . W i t h mist blowers the best practice is to use a particle size between 50 and 65 microns mass median diameter, and adjust the swath w i d t h to suit the w i n d . A w i n d between 5 and 10 miles per hour is most suitable for this type of application. I n treating t a l l trees w i t h the mist blowers, selection of the proper particle size is of great importance. I t has been difficult to secure sufficient deposit on the tops of such trees without overdosing the lower branches and wasting material. T h i s was especially true i n certain prefoliage treatments where a heavy deposit was required. I t was found t h a t the most efficient particle size was t h a t predicted b y Sell's data (5). T h e size depends on the diameter of the branches and the air velocity available at their elevation, i n addition to the velocity required to overcome the force of g r a v i t y on the particles. F o r mist blowers h a v i n g an air volume of about 10,000 cubic feet per minute, the o p t i m u m particle size for deposit at the top of a tree was found to be about 100 microns mass median diameter. Tests with Airplane Sprays. I n airplane spraying the selection of the proper particle size is difficult. Weather conditions, the type of plane and spray apparatus, AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.

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the habits of the insect, and foliage injury must be considered. The planes are flown into the wind as close to the ground as safety will permit. Calm, subsiding air, with a surface inversion—sometimes found in the early morning—is the ideal weather. The particle size should be between the large particles that fall rapidly and the small ones that give a wide, uniform swath. For control of forest defoliators, such as gypsy or tussock moth, a C-47 with a boom sprayer produced satisfactory results with a spray of 275 microns mass median diameter. For mosquito control where penetration below heavy foliage is required, particles of 100 to 125 microns mass median diameter are recommended. A study of the effect on insects of varying the particle size of deposits on different surfaces indicated that the residues from sprays of fine particle size are most effective soon after application, but that residues of the large particles will weather best and be least absorbed in the material. Large particles, therefore, will give more effective residue over a longer period than the fine particles. There is still much to be done in developing equipment and formulations, as well as in studying the best particle size to get the most out of concentrated-oil sprays. An occasional poor application, and especially one that injures foliage, will materially retard the widespread use of concentrated-oil sprays.

Literature Cited F u l t o n , R. Α., et al., Agr. Chemicals, 4, 35-8, 67 (1949). L a t t a , R a n d a l l , et al., J. Wash. Acad. Sci., 37 (11), 397-407 (1947). Sell, W., Forschungsarb. ver. deut. Ing. ( V . D . I . ) , Verlag 347 (1931). Tenhet, J. N., U. S. D e p t . A g r . , A g r . Research A d m i n . , B u r . E n t o m o l . P l a n t Quarantine, E C - 9 (1949). (5) Yeomans, A. H., Ibid., ET-267 (1949).

(1) (2) (3) (4)

(6) Ibid., ET-282 (1950).

(7) Yeomans, A. H., et al., J. Econ. Entomol., 42, 591 (1949). RECEIVED

July 20,

1950.

AGRICULTURAL APPLICATIONS OF PETROLEUM PRODUCTS Advances in Chemistry; American Chemical Society: Washington, DC, 1952.