APPEAEANCE OF LARV2E AND PUP^ WHEN DESTROYED. 



51 



turbing the insect or detaching the leaf from the tree. String tags 

 attached to leaves upon which are specimens classed as doubtful 

 will enable examinations of such specimens from time to time until 

 their condition is positively determined. A careful examination of 

 normal specimens and direct comparisons of these with those on 

 leaves of fumigated trees will assist in the ready identification of the 

 dead insects. 



DENSITY OF THE GAS AT VARIOUS HEIGHTS ABOVE THE GROUND. 



It is natural to presume that owing to the fact that hydrocyanic- 

 acid gas is lighter than air, its density during the process of fumiga- 

 tion is greater toward the top of the tree. In four of the nine obser- 

 vations on the comparative eft'ect of the gas at difi^erent heights 

 above ground the results of this variation in density are not evident. 

 In the other five observations the results are quite striking. In the 

 six experiments in which observations were made 10 feet or more 

 from the ground, the average percentage of insects killed up to 6 

 feet above the ground was 64, while from 10 to 18 feet above ground 

 the average percentage killed was 71. The data concerning the 

 effectiveness of the gas at various distances from the ground is sum- 

 marized in Table XL 



Table XL — Efficiency of gas as affected by height above ground. 



The results show that when examining for the results of fumigation, 

 the most significant effects are those within a few feet of the ground. 

 The observations concerning the results of the experiments upon 

 which the recommendations in this bulletin are based were made in 

 all cases within 7 feet of the ground, and included examinations of 

 insects on leaves closest to the ground in all cases. 



EFFECT OF FUMIGATION ON THE TREES. 



During the months of December, January, and February, until the 

 appearance of the new spring growth, fumigation for the white fly 

 with the dosage herein recommended will rarely occasion appreciable 



