Thrips resistance in the onion

does not appear. First page follows. Introduction The selection and breeding of plants resistant to parasites had its inception chiefly in the field of plant pathology, more specifically in the development of cereals resistant to rust. While the breeding for resistance to insects is still in its infancy, the possibilities in this field appear to be almost unlimited. In certain cases, among which may be mentioned the control of onion thrips, breeding for resistance seems to offer promise. In this paper are presented data which show that in the case of the onion certain varieties do possess a definite resistance to thrips, and the characters thought to be responsible for this resistance are described in some detail. Howitt,(22)5 McColloch,(30) Martin,(29) and others have given excellent general reviews of the development of resistant crop plants; here only the more important papers concerned with resistance to sucking insects are reviewed. The causes of resistance to insects have been grouped by Wardle and Buckle,(43) McColloch,(30) and Wardle(42) as physical, chemical, or physiological. The first category includes such characters as hairiness, thickness of epidermis, thickness of seed coat and rind, and habit of growth; the second, the presence of such compounds as acids, alkaloids, essential oils, and tannin together with the potash-phosphoric acid ratio; the third, such characters as vigor, seasonal adaptation, early maturity, ability to recover from injury, aand po-sitive or negative response to specific stimuli. In most instances, however, the characters, whether physical (morphological), chemical, or physiological, are probably genetic in nature and are therefore governed by the laws of inheritance. Resistance may result from one character, or from several combined; and the effectiveness of a character may vary with the soil condition and climate. Among the physical or morphological characters that seem to be intimately associated with host resistance is hairiness.