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 2 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 1 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.-Fori -' spIral used on counting
plate. Diameter, 7 to 8 cm.
Figure 2.-Sieve made from 1-pound coffee
can, used for counting wireworm eggs.
UNIVERSITY OF FLORIDA
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