• 
 
 ET-165 November 1940 
 
 United States Department of Agriculture 
 Bureau of Entomology and Plant Quarantine 
 
 A MEASURED DROP METHOD OF APPLYING LIQUID INSECTICIDES 
 
 By E. R. McGovran. G. L. Phillips, and E. L. Mayer, 
 Division of Control Investigations 
 
 The method of applying measured drops of liquid insecticides 
 to individual insects has frequently been used. Nelson et al.l/ 
 describe a method using micropipettes that, when filled, hold a 
 test dose for a single housefly. Smith2/ uses a cactus spine to 
 apply small quantities of oil to the spiracles of scale insects. 
 Woodbury and Barnhart3/ describe a method utilizing a micropipette 
 that holds sufficient insecticide to treat a number of bedbugs. 
 The dose for the individual bug is delivered by air pressure. 
 
 The apparatus described here is diagrammed in figure 1. 
 A metal hypodermic needle (F) is attached by DeKhotinsky cement 
 to a 0.2-ml. pipette (C), which is subdivided into one-thousandths 
 of a ml. The upper end of the pipette is attached to glass tubing, 
 which leads through a stopcock (B) to a reservoir (A) for rinsing 
 and to a microinjector (G) that will deliver small drops of liquid. 4/ 
 When the hypodermic needle is to be attached to the tip of the 
 pipette, clogging is prevented by inserting a fine wire through 
 the needle and into the pipette and leaving it there while the 
 melted cement is being applied. 
 
 1/ Nelson. F. C, H. E. Buc, N. A. Sankowsky, and M. A. 
 Jernakoff. A new method for evaluating the relative toxicity of a 
 liquid insecticide. Soap 10 (10): 85, 87, 89, 91, 105, 107. Oct. 
 1934. 
 
 2/Smith, Ralph H. Microtechnique method of testing oil in- 
 secticides on scale insects. Jour. Econ. Ent. 31 (5): 632-633. 
 Oct. 1938. 
 
 3/Woodbury, E. N., and C . S. Barnhart . Tentative methods for 
 evaluating liquid household insecticides against the German cock- 
 roach and the bedbug. Soap 15 (9): 93, 95, 97. 99. 101, 103. 105. 
 107, 113. Sept. 1939. 
 
 4/The authors are indebted to C . S. Wilson, of the Bureau of 
 entomology and Plant Quarantine, for suggesting the use of this type 
 of microinjector. 
 
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 Figure 2 is a photograph of the apparatus in operation'.' 
 The glass tube is joined to the microinjector with beeswax, Joints 
 in the glass tubing and where the pipette joins the glass tubing 
 are made of rubber tubing covered over and sealed with beeswax. 
 The glass is brought together as close as possible inside the rubber 
 tubing. This method makes a tight seal and also allows slight 
 flexibility, which DeKhotinsky cement does not permit. The entire 
 syscem must be entirely free of leaks to operate satisfactorily. 
 The entire apparatus is securely fastened to a wooden base by wood 
 screws, as shown in figure 2. 
 
 The whole apparatus is completely filled with distilled 
 water before any insecticide is drawn into the pipette. If ap- 
 preciable amounts of air are left in the microinjector or the 
 tube connecting it with the pipette, the delivery of insecticide 
 at the point of the needle will not be at a uniform rate. When 
 the microinjector crank is turned to force the insecticide out of 
 the needle, any considerable volume of air (1 or more ml.) will 
 be compressed by the surface tension of the liquid across the very 
 small opening in the tip of the needle. When this pressure becomes 
 great enough to stretch this surface, a drop is formed on the tip 
 of the needle. However, the slightly compressed air in the ap- 
 paratus expands as much as possible at this time, and this expansion 
 often results in a larger drop of insecticide forming on the tip of 
 the needle than is desired. Also, owing to further expansion of the 
 air. additional material may come out of the pipette when the drop 
 is removed. For this reason distilled water, or some other rela- 
 tively noncompressible material that will not appreciably affect 
 the toxicity of the insecticide, should be used in the microinjector 
 and the tube connecting it with the pipette. 
 
 Operation of Apparatus 
 
 After the apparatus below the stopcock has been filled with 
 distilled water the microinjector plunger is screwed completely 
 in. Turning the crank shown in figures 1 and 2 in a clockwise 
 direction moves it to the left until it rests against the body of 
 the injector. Any water on the tip of the needle is wiped off. 
 The microinjector plunger is then withdrawn slightly by making 
 two turns of the crank in a counterclockwise direction. This 
 allows a small amount of air to enter the needle. The tip of the 
 needle is then submerged in the insecticide, and the plunger -is 
 withdrawn by rotating the crank until the desired quantity of 
 insecticide is drawn into the pipette. The air drawn into the 
 needle forms a bubble about 3 mm. long (D in fig. 1) in the pi- 
 pette between the water and the insecticide (E) . The level of the 
 insecticide is then read on the pipette, The scale on the board 
 is used only for approximate readings. The microinjector crank 
 is then turned in a clockwise direction until the desired quantity 
 of insecticide has been delivered on the tip of the needle. The 
 insect is then brought into contact with the drop of insecticide. 
 
- 3 - 
 
 If the insecticide has the right properties, it will immediately 
 spread over the integument of the insect. Volumes of liquid of 
 one-half of one-thousandth of a ml. or larger can be measured and 
 applied to the insects with a pipette of this size. 
 
 To rinse the apparatus with distilled water, open the stop- 
 cock (B in fig. 1) and allow the water to flow through the pipette 
 until it is thoroughly rinsed. 
 
 The apparatus does not need any parts especially made for 
 it. and consequently it could be modified to fit the needs of the 
 user. A pipette of smaller bore would allow accurate readings of 
 smaller quantities. A glass tip on the pipette could be sub- 
 stituted for the metal one, to allow the use of cleaning fluid 
 for removing insecticidal residues which cannot be removed by 
 noncorrosive solvents. 
 
 Active insects may be held in the hand as shown in figure 2, 
 placed in small cages, chilled to immobilize them, or anesthetized 
 while the insecticide is being applied. Many slo.v-moving insects, 
 such as some lepidopterous larvae, may be treated during normal 
 activity. 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation 
 
 http://archive.org/details/dropmeOOunit 
 
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 DIAGRAM OF MEASURED DROP APPARATUS 
 
 A- Distilled water reservoir 
 
 B- Stopcock for rinsing micropipette 
 
 C- Graduated micropipette 
 
 D-Air column between water and insecticide 
 
 E- Insecticide 
 
 F- Hypodermic needle 
 
 G-Microinjector 
 
 Figure 1. — Diagram of apparatus for applying measured drops of iicuia 
 insecticides to insects. 
 
Figure 2.— Photograph showing operation of apparatus for applying 
 measured drops of insecticide to the integument of insects. 
 
UNIVERSITY OF FLORIDA 
 
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