396 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 14 



insects were much easier to locate and keep under observation. When 

 the insects dropped onto papers spread on the ground even a slight breeze 

 had a tendency to blow the insects and dust together in depressions giv- 

 ing far from normal conditions. One would naturally suppose that by 

 carefully picking up adults which had fallen onto canvas and dropping 

 these into glass jars the insects would be under conditions as favorable for 

 recovery'- as though they were lying on the ground under the plant. This 

 however, does not appear to be the case. When collected in a bucket 

 placed under the dusted plant and the dust sifted from the mass of insects 

 a very noticeable difference in the percentage of recovery was noted in 

 tests with different lots of dust. It has not been determined whether this 

 is entirely due to a difference in toxicity or is partly due to other factors, 

 such as air movement affecting the amount of dust adhering to the insects. 

 In order to determine whether field results from different lots of dust 

 corresponded with the nicotine content two sample dusts which apparent- 

 ly were of low toxicity were analysed^ for comparison with two other 

 lots of similar dusts of apparent!}^ high toxicity. The analyses and field 

 results are shown in the following table : 



Lot no. 1 



Percentage adult leaf hoppers recovered 14.3 

 Nicotine shown by analysis of sample 1.95 



Field tests of lots 2, 3 and 4 were made on June 16 in the same vine- 

 yard near Fresno within a period of 1 hour, using exactly the same meth- 

 ods. All conditions were supposedly practically identical. The test of 

 lot 1 was made in another vineyard on the preceding day. No difference 

 was noted in temperature or wind conditions which could account for the 

 difference in killing effects as compared with the other three samples. 

 In these tests the insects dropped into a bucket and were separated from 

 the dust by means of a sieve, then kept in closed Mason jars for obser- 

 vations. 



In another field test using a nicotine dust which analysed 2.23% nico- 

 tine the field tests showed results strikingly inferior to those with a dust 

 of exactly the same composition but with only 1.55% nicotine. These 

 tests were made near Holtville in the Imperial Valley, one application 

 immediately following the other and all conditions supposedly practically 

 identical. The insects were collected on canvas spread under the vines, 

 picked up carefulty with forceps and kept in vials for observation. Eleven 



^Analyses made through cooperation of Prof. Gray, Chief Div. of Chemistry, 

 Cal. Dept of Agric. 



