April, '21] parrott: dusts for sucking insects 211 



Nymphs of the four-Hned bug proved very much more resistant to 

 nicotine than the apple redbug or certain species of aphids. Prepara- 

 t'ons containing 2.0 percent, nicotine caused paralysis, from which none 

 recovered. As with the aphids, dense foliage produced by the new 

 growth made it very difficult to secure effective control by dusting. 



Principal Findings and Discussion of Results 



Dusting mixtures containing nicotine were toxic to aphids, redbug, 

 leafhoppers of the apple, the currant aphis and four-lined plant-bug. 



The degree of susceptibility to dusting preparations varied with .dif- 

 ferent species. Mixtures with 0.5 percent, nicotine were fatal to the 

 apple aphis and redbug and to the currant aphis. Preparations at this 

 strength also reduced n\-mphs of the four-lined plant-bug to a state of 

 inactivity, although the insects ultimately recovered with few fatalities. 

 Dusting with mixtures containing 2.0 percent, nicotine was fatal to the 

 n\Tnphs. Alixtures containing less than 2.0 percent, nicotine gave very 

 poor control of the potato aphis. The nymphs of the pear psylla dis- 

 played considerable resistance to preparations containing from 0.5 to 2.0 

 percent, nicotine. Mixtures with 0.5 percent, nicotine destroyed only 

 a small percentage of hibernating adults, and preparations with 1.0 

 percent, nicotine were less effective than spraying mixtures composed of 

 nicotine sulfate and soap in the usual proportions. 



Air currents, denseness of tree growth, low temperatures and exudates 

 of insects, such as wax and honeydew, exerted a greater adverse influence 

 on the efficiency of dusting preparations than on that of spraying mix- 

 tures. These results suggest that while certain principles of procedure 

 apply equally well to both systems of orchard treatment, the field tech- 

 nique required for effective dusting differs in important particulars from 

 standard spraying practices. In its present stage of development 

 dusting displays defects both as to methods of application and machinery 

 which will probably be overcome by the corrective processes of experience. 

 A study of present dusting methods shows also that a distinction should 

 be made between obvious limitations of dusting materials and machin- 

 €r>' and failures which arise from attempting to dust under conditons 

 that are unfavorable for effective work. In the foregoing experiments 

 operations which ]jroved promising when no breeze was stirring and when 

 the trees had open tops would not infrequently yield quite different 

 results when strong air currents prevailed and the tree growth was dense. 

 The inefficiency of dusting was m.ost marked with the pear psylla and 

 the woolly aphis, as the honey dew and waxy secretions of these insects 

 were repellent to the materials. It was also noted that during periods of 

 low temperature dusting was less effective than when high temperatures 

 prevailed. 



