LIBRARY 
 
 ;tate plant board 
 
 ET-178 August 1941 
 
 United States Department of Agriculture 
 Bureau of Entomology and Plant Quarantine 
 
 A DEVICE FOR ESTIMATING TKRIPS 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 ( Scirtot h rips 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 
 will fill 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 v/hich 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 oreoaration 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 exposea, sticky area 
 of the sheet be aiways tne same, otherwise tne records obtained 
 would not be comqarable. 
 
 Item 3 is a frame or wood or tnick caraooara of tne same 
 over-all dimensions as the sticky sneet. At one ena it fits un- 
 der the ledge in the box and at the other end under the spring catch. 
 The frame is put into place, or removed, by means oi a Knob fastened 
 to tne upper suriace or the frame on the end that fits under tne 
 ledge, and is held 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. 
 
- 2 - 
 
 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 in a 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 sheei provided with a film 
 of sticky material; 3> frame serving to hold the 
 paper in place and standardize the area of capture; 
 X, screen cover. 
 
 Figure 2. — Thrips meter in use in an orchard, 
 
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