Date of Award:

5-1946

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Entomology

Committee Chair(s)

George F. Knowlton

Committee

George F. Knowlton

Committee

C. J. Sorenson

Committee

W. E. Peay

Abstract

The purpose of this study has been to determine which species of thrips infest tomato plants and fruits in northern Utah, and to find out which other plants occurring in this area also serve as hosts for tomato-infesting thrips.

Because of the importance of tomato-infesting thrips to the canning industry, it was decided that more complete knowledge of the species of thrips which are found on tomatoes should be obtained. Because most thrips generally have been difficult to control, it was believed that a general knowledge of the plants on which these thrips occur would enable tomato growers to eliminate many such plants from tomato fields and lands adjacent to them, and that such cultivation would tend to decrease thrips populations on tomato fruits and in tomato fields.

In 1941, Dr. G. F. Knowlton, R. S. Roberts and others looked into the problem of thrips occurring in canned tomato products in northern Utah. These workers found that thrips sometimes occur in cavities at the blossom end of the tomato fruit (data unpublished). Since this time, study has been devoted to this problem by entomologists of the United States Department of Agriculture and the Utah Agricultural Experiment Station under the direction of Mr. W. E. Peay, Entomologist, U. S. D. A. Bureau of Entomology and Plant Quarantine.

L. B. Reed in 1943(6) wrote, "The thrips were found to enter the green fruit from the blossom end through small openings to cavities that normally occur in the central core of a small percentage of sound fruits. Sometimes the openings are closed as the tomatoes mature, and the thrips are imprisoned."

The primary interest in thrips on tomatoes in northern Utah is due to their appearance in canned tomatoes and tomato products. The United States Pure Food and Drug Commission has set a tolerance for the number of insect fragments which may occur in canned products shipped in interstate commerce. In former years the main concern of tomato processors was the occurrence of fragments and frass from the tomato fruitworm, Heliothis armigera (Hbn.), in the canned tomato products. During recent years the efficiency of methods for determining insect fragment counts has been increased. This improved sampling procedure has made it necessary for the canners to raise their standards to a very high level. More effective control methods have been developed for tomato fruitworm control, and the number of insect fragments due to this pest has been reduced. However, thrips still constitute a problem in the progress of the canners to further raise quality standards of tomato products.

It has been generally believed that thrips reduce tomato yields by increasing blossom-drop. Severe thrips feeding results in a devitalization of the plant. A silvering of the foliage of tomatoes results from the feeding process which destroys many cells in the outer layers of the plant, reducing the efficiency of the leaves. When such damage is extensive, a reduction in yield and fruit quality also occurs. The market value of tomato fruits for table use is sometimes reduced by off-shaped fruit. Such injury sometimes is the result of thrips injury occurring when the fruits are small. The extent and exact nature of thrips injury to tomato fruit blossoms is not fully understood.

Thrips populations on tomatoes seldom have been considered severe enough to warrant the expense of chemical control. However, during seasons of abundance they may constitute a serious problem to the food processor. The life cycle of thrips also makes their control by the usual chemical means difficult. Thrips pass a "pupal" stage in the ground where they are well protected from the action of insecticides. Their habit of feeding deep in the flowers of the host plants also makes them difficult to kill with contact insecticides which has been the most common method of control up to the present time. Chemicals, generally applied for thrips control on tomatoes up to the time this investigation was well under way, seldom have given results justifying costs of application.

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