CONNECTICUT VALLEY ONION INDUSTRY 49 



Dustintj for Control of Thrips 



In tlie Experiment Station study on thrips control, various types of com- 

 iiiercial nicotine dusts, as well as home prepared dusts, were tested. A 

 study was made of the comparative efficiency of different strengths of 

 nicotine in the dusts; and of the form in which the active principle was 

 present (free nicotine or nicotine sulfate). The influence of weather con- 

 ditions on the effectivness of the dusts was also studied, to determine the 

 best time to make the applications. Different methods of applying the 

 dusts were tried out, as well as the use of auxiliaries (hoods, curtains, etc.) 

 to confine the dust discharged, prevent drift and so counteract the inter- 

 ference caused by wind. 



Nicotine dusts, even those carrying a comparatively small percentage of 

 nicotine, readily kill all the insects reached and are effective early in the 

 season. By applying the nicotine dusts at intervals of about ten days, be- 

 ginning with the first appearance of the thrips, the insects are held down 

 sufficiently so that the plants can withstand a moderate infestation. The 

 cost of such a practice, however, is almost prohibitive. Later in the sea- 

 son, the nicotine dusts are not satisfactory, since they cannot penetrate 

 the tight crevices in the axils of the inner leaves, where a very large per- 

 centage of the thrips congregate. 



Study was also made of the recently introduced calcium cyanide dusts. 

 These possess excellent physical qualities, and the gas given off penetrates 

 the tiglit crevices in the axils of the inner leaves very successfully, over- 

 coming the thrips in practically every portion of the plant. The gas is 

 soon dissipated, however, and a very large percentage of the thrips re- 

 cover from its effects and resume normal activity. At present the cost of 

 these dusts is high. These difficulties do not appear to be insurmountable. 

 If the manufacturers make as rapid progress in the development of these 

 dusts in the near future as they have in the last two years, calcium cyanide 

 gives promise of becoming a very satisfactory control. 



Sprayinn for Control of Thrips 



In a study of the possibilities of sprays for the control of this pest, 

 tests were made of a long list of materials, including almost every type of 

 contact spray. Preliminary tests eliminated very many of these as in- 

 effective or impracticable from one cause or another, and the field soon 

 narrowed down to include the nicotine-soap combinations and certain oil 

 sprays. 



One of the greatest problems in thrips control is to reach and kill the 

 insects deep down in the axils of the leaves (chits), where they congregate 

 in large numbers. On account of the peculiar nature of the plant and the 

 smooth, waxy surface of the leaves, any spray to be successful must 

 possess excellent adhesive and spreading properties, as well as insecticidal 

 value. Oil sprays, while very effective against all thrips actually touched, 

 showed a very strong tendency to form into drops and roll off the plants 

 vithout thoroughly covering all the leaf surface, thus forming "islands" 

 where thrips were untouched by the spray and so escaped. Nicotine sprays 

 used alone do not have the necessary physical qualities. When they are com- 

 bined with soaps, however, they constitute a spray which possesses all of 

 these qualifications to a very high degree. 



Lack of machinery has stood in the way of successful use of sprays. 

 The ordinary types of horse-drawn sprayers cannot be used in onion fields. 

 The common no/.zles, delivering a cone spray, have not succeeded in fore- 



