8 MASS. EXPERIMENT STATION BULLETIN 326 



(protonymphs) and the adult females. This may be partially explained in the 

 case of deutonymphs by the fact that many of them at the time of fumigation 

 are in the first or proto-quiescent molting period of suppressed activity, when 

 they are obviously less susceptible to toxic fumes. In the case of the adult 

 males, it is apparent that they are the most active of all stages and are able 

 to select most quickly any protective parts of the plant where the fumes 

 penetrate with difficulty. Red spider larvae are quite easily killed by naphtha- 

 lene fumes and those individuals which are so commonly present a few days 

 after treatment have hatched from eggs which survived the exposure. It has 

 also seemed very probable that many of the adult females which survive were 

 in the second molting period during fumigation. 



Table 3. — Percentage of the Different Active Stages of Common 

 Red Spider Killed by Naphthalene Fumigation. 



Dosage 3 ounces per 1,000 cubic feet. 

 Temperature 80°F. Relative Humidity 95 percent. Exposure 6 hours. 

 Waltham, Massachusetts. March 1930. 



One Fumigation Two Fumigations 



Total . . .454 70.28 333 98.81 



FACTORS INFLUENCING THE EFFECTIVENESS OF NAPHTHALENE 



FUMIGATION 



Crystal naphthalene volatilizes slowly at any temperature suitable for grow- 

 ing plants, but a sufficiently rapid volatilization for controlled fumigation 

 takes place only at temperatures near the melting point of the crystals ( 176.2 °F) , 

 and the experiments reported herein are based almost entirely on the volatiliz- 

 ation of melted naphthalene. In certain apparatus (p. 5) the naphthalene is 

 volatilized by blowing over the crystals air at normal greenhouse temperatures 

 or at moderately high temperatures, but these methods are effective only 

 when the exposure is two or three times longer than if the naphthalene is melted. 



Roark and Nelson (13) have calculated that the greatest amount of naphtha- 

 lene vapor which can exist in 1,000 cubic feet of air at 77° F. is 0.04 pound 

 (0.64 ounces or 18.14 grams) and that this constitutes saturation. The amount 

 necessary for saturation increases slightly as the temperature and vapor pres- 

 sure increase, becoming approximately 21 grams at 80° F. and 80 percent 

 relative humidity. 



Dosage 



The maintenance of a sufficient concentration of naphthalene vapor for 

 several hours is necessary for successful fumigation; and the correct amount 

 of naphthalene crystals to be vaporized in a given cubical unit in order to 



