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ET-178 August 1941

United States Department of Agriculture
Bureau of Entomology and Plant Quarantine

A DEVICE FOR ESTIMATING THRIPS POPULATION ON CITRUS FOLIAGE

By E. A. McGregor, Division of Fruit Insect Investigations

It is very difficult to make population counts of the citrus
thrips (Scirtothrips citri (Moult.)) because these insects are
small, active, and easily disturbed, and their color blends with
that of the foliage of the host. Because of this difficulty the
writer developed a number of years ago a "thrips meter," herein
described, for obtaining indices to thrips populations.

The thrips meter consists of four items of equipment (fig. 1),
each of which will be described separately. Item 1 is a light,
durable box, 8 by 6 by 2-1/2 inches in dimensions. A cigar box
weteetill these requirements. At one end of the inside of the. box
is placed a Spring catch and at the other end a ledge, to hold in
place items 2 and 3.

Item 2 is a sheet of black paper to which is applied a thin
film of sticky material, such as tanglefoot, and which fits into
the bottom of the box. For convenience in handling, the margin of
this sheet is left free of the sticky substance; this reduces its
effective adhesive area to about 23 square inches. Black paper is
used, as the pale insects are readily seen and counted on a dark
background. This item is referred to as the capture sheet.

For the preparation of the capture sheets the sticky material
is applied with a painter's brush. A supply of these sheets may be
prepared in advance and stuck face to face to await use. Care
should be taken to apply the sticky fluid as a continuous film of
proper thickness. IT is important that the exposeu, sticky area
of the sheet be always tne same, otherwise tne records obtained
would not be comqarable.

Item 3 is a frame or wood or thick carapoarda of the same
over-all dimensions as the sticky sneet. At one end it fits un-
der tne ledge in the box and at the other end under the spring catch.
The frame is put into place, or removed, by means or a knob fastened
to tne upper surrace of the frame on the end that fits under the
ledge, and is heid in place by the ledge and the spring catch. The
Opening of this frame corresponds in size to the effective sticky
area of the sheet and exactly registers with it.

Item 4 is a cover of wire screen soldered to a frame of
light angle material made of tinned sheet iron, fitting snugly over
the box. The mesh of the screen is large enough to permit the
thrips to fall through and at the same time prevent larger insects
and trash from passing through to the sticky sheet.

When the device is assembled for use, the arrangement is as
follows: The adhesive capture sheet lies directly on the inner
bottom surface of the box, the frame lies immediately over the cap-—
ture sheet, and the screen cover fits snugly over the box opening.

Both larvae and adults of the citrus thrips feed by pref-—
erence on the tender twigs or terminals of the citrus trees, and
these new growth terminals are used as the units of measurement for
the thrips counts. The twigs are beaten upon the screen with the
thrips meter held ina horizontal position (fig. 2), and the in—
sects fall through the screen and are retained by the sticky capture
sheet.

To facilitate the counting of the insects, the sticky sheet
is removed and cut into smaller pieces, usually six. Counting is
further aided by cutting a series of intersecting cross lines into
the surface of each small piece of the capture sheet. It is prefer—
able to make the count in the field with a hand lens before the in-—
sects have had time to become quiet or settle into the sticky
fluid, when counting becomes more difficult. If necessary, however,
the counting may be done later in the laboratory, with either a hand
lens or a binocular microscope.

Figure 1.—-Four component parts of the thrips meter:
1, Light box; 2, capture sheet provided with a film
of sticky material; 3, frame serving to hold the
paper in place and standardize the area of capture;
4, screen cover.

Figure 2.—Thrips meter in use in an orchard.

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