52 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA. 



rather than from actual experience in the field. 1 The practice among 

 commercial fumigators has been to absolutely ignore these tables, 

 depending, instead, on their own judgment. The result is that 

 their scheduling differs markedly from that of the published 

 schedules. 



FACTORS WHICH AFFECT THE DOSAGE. 



Although citrus-fruit growing in southern California is restricted 

 to a limited area the climatic conditions are not uniform in all sec- 

 tions. The region adjacent to the coast generally is cooler and much 

 damper at night than in the interior valleys. This situation has led to 

 a diversity of opinion among fumigators as to the comparative 

 dosage for the two sections. Some hold that a heavier dosage is 

 required near the coast on the ground that the gas is absorbed by 

 the dampness; others, that the drier and lighter air in the interior 

 valleys allows a more rapid escape of gas through the tent, which 

 necessitates more cyanid than for the heavier air of the coast. Set- 

 ting aside these opinions and examining the situation as it actually 

 is, we find that the general dosage strength for a particular insect is 

 approximately the same throughout southern California regardless 

 of nearness to or remoteness from the ocean. Of course, there are a 

 few striking individual variations from this general statement, but 

 these variations are as noticeable in one place as in another. 



Personal experience in all sections has taught the writer that the 

 leakage of gas is for the most part noticeably greater in the dryer and 

 warmer interior sections than near the coast. Despite this condition 

 any given dosage appears to be as efficient in one place as in another. 

 This marked efficiency in the dryer and warmer sections regardless 

 of the greater leakage might possibly be due to the fact that the scale 

 insects are more susceptible to the gas in the higher temperatures 

 general there than in the cooler temperatures of the coast. It is 

 known among entomologists that insects are active at high tem- 

 peratures but become dormant at low temperatures and in the latter 

 condition are more difficult to destroy. Prof. Woodworth, of the 

 University of California, has informed the writer that laboratory 

 experiments performed by him have shown this condition to exist 

 among scale insects and that the temperature at which they become 

 dormant is relatively high. This interrelation of temperature and 

 activity has a very important bearing on the fumigation treatment 

 and demands much further experimentation in the field as well as in 

 the laboratory. 



The old conception that an increase of dosage was required near 

 the coast to offset the loss of gas from absorption by moisture is also 

 no longer tenable. Experience has shown that the results during 



1 Bui. 79, Bur. Ent. ? U. S. Dept. Agr., pp. 20-22, 1909. 



