NAPHTHALENE AS A GREENHOUSE FUMIGANT 9 



make and maintain that concentration varies with the leakage, adsorption, 

 air drafts, temperature, and relative humidity. Therefore, any standard dose 

 which is determined under experimental conditions may necessarily be varied 

 when applied in actual practice in different greenhouses or under different 

 conditions in the same greenhouse. 



In a practically gas-tight fumigation chamber during a six-hour exposure 

 4 and 5 ounces of naphthalene per 1,000 cubic feet have been vaporized with- 

 out causing noticeable injury to resistant plants such as carnations. However, 

 the volatilization of such an excess above the computed saturation point 

 causes supersaturation which can exist only for a short time jwithout recrystal- 

 lization or other physical and chemical changes. In the experimental fumi- 

 gation chamber 2 and 3 ounces per 1,000 cubic feet have frequently killed 95 

 to 100 percent of the common red spider in one 6-hour fumigation and have 

 resulted in practically perfect control in two fumigations. With high relative 

 humidity, \ X A ounces per 1,000 cubic feet was equally effective in two and 

 three treatments, and occasionally in one treatment. A dosage of 1 ounce 

 per 1,000 cubic feet was not consistently effective in less than three successive 

 fumigations. 



The effectiveness of these dosages is somewhat decreased under 

 average greenhouse conditions, and the most satisfactory dosage under 

 normal conditions is 2 to 3 ounces per 1,000 cubic feet. 



Exposure 



The effectiveness of naphthalene fumigation is determined by the length 

 of time the red spiders are exposed to an atmosphere which is partially or 

 completely saturated with naphthalene vapor. Wilcoxon et al. (21) have 

 shown that satisfactory control of red spider resulted from an exposure of 

 14 to 16 hours to a 13 percent saturated atmosphere in one case, and an ap- 

 proximately one-half saturated atmosphere in another case, when unmelted 

 naphthalene was vaporized in special equipment. However, when the naph- 

 thalene crystals are melted and complete saturation of the atmosphere is 

 obtained, a 6-hour exposure is satisfactory. 



As shown in Table 4, the vaporization of H ounce of naphthalene each 

 hour failed to have an appreciable effect on red spider until after 3 hours 

 when % ounce of naphthalene had been vaporized. At this time the con- 

 centration approached saturation, and thereafter the kill increased each hour 

 until a mortality of 88 percent was obtained after 5 hours exposure. When 

 the spiders were first exposed 2 hours after fumigation began, at which time 

 Yz ounce of naphthalene per 1,000 cubic feet had been vaporized, a mortality 

 of 85 percent was reached after an exposure of 4 hours, 3 hours of which were 

 in a nearly saturated atmosphere. When red spiders were first exposed after 

 the vaporization of % ounce per 1,000 cubic feet in 3 hours had built up a 

 calculated saturated atmosphere, an 85 percent kill resulted after 3 hours 

 exposure. A mortality of 85 percent was also obtained in 3 hours when the 

 spiders were first exposed after 1 ounce of naphthalene had been vaporized in 

 4 hours, since in both cases the air was approximately saturated when the 

 spiders entered. However, a 2-hour exposure to saturated atmosphere in 

 which 1 ounce of naphthalene had been vaporized killed 68 percent of the red 

 spiders, but only 24 to 34 percent were killed by equal exposures to less 

 saturated atmosphere in which ^ or % ounce had been vaporized. 



