NAPHTHALENE AS A GREENHOUSE FUMIGANT 13 



In the average greenhouse, circulation during fumigation is obtained only 

 by the movement of warm and cold air and by the buoyancy of the fumes. 

 Observations have indicated that diffusion is greater in large compartments 

 than in small ones. In greenhouses containing 50,000 or more cubic feet, 

 satisfactory results are obtained by using one stove in each 7,000 cubic feet or 

 fraction thereof. In smaller compartments it is advisable to place a stove in 

 each 5,000 cubic feet. 



The vapors from fuming naphthalene tend to diffuse with a mushroom effect 

 and frequently they follow the pitch of the roof upward before rolling downward 

 to envelop the plants. It is, therefore, advisable to place stoves at about the 

 same height as the top of the growing plants in order to avoid an excessive 

 concentration of vapor near the stoves, and also to place them in two rows, 

 each row being about one-third of the distance from the sides toward the center. 



Light 



Exposure to naphthalene vapor during sunlight and photosynthetic action 

 is much less injurious to plants than a similar exposure to such fumigants as 

 hydrocyanic acid gas and volatile nicotine. The most tolerant species of 

 plants growing under normal conditions have consistently shown no detri- 

 mental effects from naphthalene fumigation in sunlight, and many of the less 

 tolerant species have been safely fumigated in bright light when other condi- 

 tions were favorable. When unfavorable conditions exist, however, fumigation 

 in sunlight may aggravate any injury which results from the treatment. In 

 the spring when greenhouse plants are soft, following forced growth in continued 

 cloudy weather, they are less resistant than at other times and should not be 

 fumigated in bright sun. 



It is often advisable to take advantage of sun heat for maintaining the 

 necessary temperature, and in cool weather day-time fumigations of carnations 

 and other tolerant plants are satisfactory. In hot weather when excessive 

 temperatures result from closing the ventilators during the day, fumigations 

 should be started in late afternoon and continued into the night until the 

 necessary period of exposure is completed. The less resistant species of plants 

 should be fumigated on cloudy days or at night. When a fumigation is started 

 on a cloudy day and the sun shines before the exposure is completed, it is 

 advisable to discontinue the treatment of susceptible plants; but the fumigation 

 of tolerant plants may continue in the sun until the temperature rises to 95° F. 



Temperature 



Recrystallization of naphthalene fumes which occurs at 72° F. or lower may 

 be injurious to plants, and temperatures of 95° F. or higher if maintained for 

 several hours are unfavorable to plant growth. Therefore, the temperature 

 limits for fumigation with naphthalene are established at 72° to 95° F. 



Theoretically, the capacity of air to hold naphthalene vapor increases as 

 the temperature increases (13), and the greater amount of naphthalene in the 

 air at a higher temperature should kill insects and spiders more easily and 

 quickly. This theory is substantiated by Table 6, which shows that 26.48 

 and 45.30 percent more common red spiders were killed by fumigations at 90° 

 than at 70° F., and that the treatment was noticeably more effective at 80° 

 than at 70°. 



