NAPHTHALENE AS A GREENHOUSE FUMIGANT 29 



each stove into six equal parts and adding one part each hour, adjusting the 

 wick so that each hourly quota is vaporized in one hour. In large greenhouses 

 one stove unit should be used in each 7,000 cubic feet of space, but in small 

 compartments a stove unit for each 5,000 cubic feet is advisable. 



Resistant species of plants can be safely fumigated in the sunlight. In cool 

 weather sun heat can be used to aid in maintaining the necessary temperature. 

 In warm weather high temperature can be avoided by starting the fumigation 

 in the late afternoon and continuing into the evening. The less resistant species 

 of plants should be fumigated only on cloudy days or at night. 



Although the effectiveness of naphthalene fumigation increases as the tem- 

 perature increases, the recrystallization of the fumes at about 72° F., and the 

 undesirable effect on the plants at 95° F. or higher, limit the temperature range 

 between these points. In commercial fumigations a nearly constant tempera- 

 ture between 75° and 85° F. is practical and effective. 



High relative humidities are unfavorable to red spider development and in 

 a series of experiments in which no fumigant was used 59 percent of the red 

 spiders were dead after three 6-hour exposures at 90 percent relative humidity 

 compared to a 35 percent mortality after a similar exposure at 60 percent rela- 

 tive humidity. 



In experimental fumigations the treatment was significantly more effective 

 at 80 and 90 percent relative humidity than at 60 and 70 percent relative 

 humidity, especially when 1 ounce and l)i ounces of naphthalene per 1,000 

 cubic feet were used. The greater effectiveness of naphthalene fumigation at 

 the higher humidities was also evident from the consistently greater number 

 of red spider larvae which hatched from eggs fumigated at the lower humidities. 

 Relative humidities of 75 to 80 percent are most satisfactory during naphthalene 

 fumigation. 



Because of the promising results from the first experiments with naphthalene, 

 several combinations of materials have been used to enlarge the scope of the 

 treatment and to simplify the operation. Locally a mixture of 85 parts of 

 naphthalene and 15 parts of paradichlorobenzene was effective but not sig- 

 nificantly superior under experimental conditions. Mixtures of naphthalene 

 and nicotine were compatible and the residues from sulfur and Bordeaux mix- 

 ture were not harmful on fumigated plants. Other investigators used solutions 

 of naphthalene in sulfur and in motor oil very effectively in special apparatus. 

 Two commercial liquid fumigants containing chemicals similar to naphthalene 

 and used in the same way appear to give very satisfactory control of the red 

 spider, and can be used safely at temperatures lower than are safe with naph- 

 thalene alone. 



Greenhouse thrips and onion thrips are readily killed by naphthalene fumi- 

 gation. The most common species of aphids and the adults of the greenhouse 

 white fly are also killed but not as effectively as with nicotine fumes or hydro- 

 cyanic acid gas. Mealybugs are only partially controlled. Cyclamen mite is 

 quite resistant to naphthalene fumigation, but the broad mite, a closely related 

 species, is well controlled. Bulb mites and soil inhabiting pests are killed 

 when exposed but because of their habits are seldom controlled. Leaf roller 

 and leaf tyer moths are killed by strong fumigations but the larvae are more 

 resistant. 



