October, '21] morrill: nicotine dusts 397 



out of 40 adults recovered within two hours in case of the vine dusted 

 with the 2.23 dust whereas with the 1.55% dust only one adult out of 75 

 under observation recovered during a period of 7 hours. 



No conclusions should be drawn from the foregoing observations 

 except to the efifect that either the nicotine content does not necessarily 

 indicate the relative toxicity of nicotine dusts of otherwise practically 

 identical chemical composition or that the methods described for secur- 

 ing the results are of very questionable dependability. 



As alread}^ noted the standard method of checking results of dust 

 applications consists in observing the insects on the ground under the 

 treated plant. On June IS, Mr. A. J. Flebut of the U. vS. Bureau of Ento- 

 mologA", and the writer undertook to determine the difference in the 

 percentage of recovers- of adult leaf-hoppers left on the ground under 

 dusted vines, on screens and in buckets. Of 109 adults under observa- 

 tion 56 were in buckets and none of these recovered, 40 were on fine 

 wire screens and of these 28 recovered while 13 were on the 

 ground where they fell from the vines and of these 11 recovered. Of 21 

 n\-m.phs under observ^ation in the same tests, all on the screens, 7 recover- 

 ed. These figures do not necessarily indicate the value of dusting for 

 the leaf-hoppers since many of the insects, both n\Tnphs and adults, 

 drop onto the upper surfaces of leaves where they remain in contact with 

 dust and apparently are under as unfavorable conditions as the adults 

 which were under observation in the bucket. Furthermore many or 

 most of the n^Tuphs which recover on the ground probably fail to get 

 back on to the plants. This probably explains the reason why satisfac- 

 tory' results were reported by many vineyard owners who used the dust- 

 ing method early in the season. In the tests above mentioned dusts 

 were used in which 7>2% and 10% of Blackleaf 40 were used in manu- 

 facture, the analysis showing 2.4 and 3.8% of nicotine respectively; the 

 former with 90% hydrated lime and 10% sulphur as a carrier and the 

 latter with a carrier composed of approximately 75% sul]jhur and 25% 

 lime. 



From the writers observation it appears that the use of ordinary nico- 

 tine dusts against the grape leaf-hopper is of little value against the adults. 



Against the nymphs the value appears to consist in stupefying a 

 large percentage causing them to drop. Early in the season before the 

 vines have runners lying on the ground or in the case of trellised vines, 

 this probably is equivalent to killing a large percentage of the insects. 



In certain exDcriments in which a second application of dust was made 

 to vines from one to 24 hours after the first, it was estimated that the 

 first application caused 100 per cent of the nymphs to drojj in the first 



