STATE PLANT BOARD 
 
 March 1943 ET-206 
 
 United States Department of Agriculture 
 
 Agricultural Research Administration 
 
 Bureau of Entomology and Plant Quarantine 
 
 COUNTING PLATES FOR USE WITH LOW-POWER MAGNIFICATION 
 
 By F. H. Shirck 
 Division of Truck Crop Insect Investigations 
 
 A counting plate for use with binoculars may be made by solder- 
 ing No. 30 copper wire onto a tin plate in the shape of a spiral 
 (fig. 1), forming a groove 6 to 8 millimeters wide. The plate should 
 be painted some color that will contrast with the objects to be counted. 
 In using the plate, the matter to be examined (e.g., debris containing 
 insect eggs) is scattered over the plate in a thin layer. The count is 
 begun in the middle, the plate being revolved manually as the count pro- 
 ceeds so that the entire length of the groove passes through the field 
 of vision. This device is useful where considerable foreign matter is 
 present, such as sand or weed seeds, or where there is a mixture of 
 similar-appearing objects the differentiation of which requires the 
 use of low-power magnification. It insures that all the material will 
 be examined, and permits an accurate count without necessitating the 
 handling of each individual object counted. 
 
 The microscope field should have a diameter of 2 to 3 times the 
 width of the groove in the counting plate. The plate can then be easily 
 controlled without danger of missing a spiral as it is revolved. This 
 device has been used successfully with a magnification of 9 diameters. 
 
 The counting plate shown in figure ?. has been used in separating 
 wireworm eggs from soil and counting them under loupe binoculars. It 
 is made from the lid of a 1-pound coffee can. The central part of 
 the lid is cut out, leaving an edge about ^ inch wide. A disk of 
 40-mesh wire cloth is cut to the exact size needed to fit inside the 
 lid, and is soldered to the inside of the projecting edge. Masking 
 strips of |-inch adhesive tape are then affixed to the inner surface 
 of the screen, and spaces of about 1 millimeter are left between the 
 masking strips. Several coats of paint are then sprayed on, forming 
 lines on the screen in the spaces between the masking strips. When 
 the paint is thoroughly dry the strips of tape are removed. The 
 
bottom of the coffee can is cut out. leaving a cylinder to which the 
 disk is attached during the washing process. 
 
 In use, the counting plate is fitted tightly to the coffee-can 
 cylinder. The soil containing eggs is emptied into the device, and 
 water is sprayed on through a suitable spray nozzle until the eggs are 
 washed free of soil. If the soil has not been pre-screened it will 
 be advisable to have at hand another sieve of coarser mesh which will 
 allow the eggs to pass through. This may be made by cutting out the 
 top and bottom of a 1-quart can and soldering wire screen over one 
 end of the open cylinder. The screen used should be of a mesh just 
 large enough to let the eggs and fine dirt pass through but to retain 
 all matter larger than the eggs. The soil is flushed through the 
 first sieve into the one illustrated in figure 2. 
 
 After the washing is completed the counting plate is taken 
 off and set upon an absorbent towel for a moment to draw out the 
 water remaining in the meshes of the screen. The sample is scanned, 
 and any eggs which are on the lines are pushed into the spaces be- 
 tween (this may be done during the count if desired). The count 
 is then made, by following the spaces between the guide lines. The 
 eggs may be flushed from the plate with the spray and into a pail of 
 water, and kept in a cool place until needed. They will stand sub- 
 mergence for a day or two without injury. 
 
 Newly hatched wireworms may also be counted by aid of the 
 counting plate shown in figure 2, but in this case it is best to add 
 a little ether to the soil before attempting the count. Two or three 
 drops of ether to 3 ounces of soil will immobilize the young wire- 
 worms for the time required in counting. They survive this treatment 
 without visible injury. 
 
Figure 1. — ^^Fona of spiral used on covjnting 
 plate. Diameter, 7 to 8 cm o 
 
 Sif^ 
 
 ,^!^BOABe 
 
Digitized by tine Internet Archive 
 in 2013 
 
 littp://arcliive.org/details/countingplatesfoOOunit 
 
Figure 2. — Sieve made from l-poxaad coffee 
 OBJif used for counting wlreworm eggs. 
 
UNIVERSITY OF FLORIDA 
 
 lliliilllliililiill 
 
 3 1262 09240 8805