14 MASS. EXPERIMENT STATION BULLETIN 326 



Table 6. — Relation of Temperature to the Effectiveness of 

 Naphthalene Fumigation on the Common Red Spider. 



Dosage 1 or 1 ' 2 ounces per 1,000 cubic feet. 

 Relative Humidity 75 percent. Exposure 6 hours. 



Waltham, Massachusetts. 1934-35. 



Dosage 1 Ounce Dosage 1}^ Ounces 



Temperature 



Number Mortality Number Mortality 



of Spiders Percent of Spiders Percent 



70° F. . . 278 52.51 291 61.85 



80° F. . . 286 83.91 278 88.84 



90° F. . 137 97.81 120 88.33 



Many other experimental fumigations have indicated that variations in 

 temperature between 75° and 85° F. have no significant influence on the 

 effectiveness of the fumigation, and that the permissible range of temperature 

 during fumigation is limited from a practical standpoint by the e°'ect on the 

 plants rather than by the effect on the toxicity of the fumigant. 



Cne or more accurate thermometers placed at strategic locations in the 

 greenhouse should be read frequently. 



Air pockets in corners and around certain benches are frequently cooler 

 than the average air temperature, and the temperature in such places should 

 be watched carefully. When the temperature rises above 95° F. and can be 

 lowered only by ventilation, the fumigation should be discontinued. 



During commercial fumigations a comparatively constant temperature 

 between 75° and 85° F. is favorable to effective results, and provides a satis- 

 factory leeway for correcting unexpected temperature changes. 



Relative Humidity 



Relative humidity or the amount of moisture in the air is usually compara- 

 tively high in the greenhouse and can be quite easily regulated by careful 

 watering of the soil. When pipe heat is used, however, it is difficult to maintain 

 a constant high humidity, and under such conditions it is frequently necessary 

 to wet the walks and pipes every two or three hours. 



Hygrometers or psychrometers for determining the relative humidity should 

 be placed at representative positions in the greenhouse and consulted frequently. 

 A satisfactory type of hygrometer consists of a dry thermometer, a wet ther- 

 mometer, and a tube of water for supplying the bulb of the wet thermometer 

 with moisture. When both the dry and the wet thermometer read the same, 

 the relative humidity is 100 percent. By interpreting the readings of the two 

 thermometers on a simple chart which accompanies such equipment, the 

 relative humidity is easily determined. 



The effect of humidity on the red spider is well shown in Table 7. In this 

 e<periment where no fumigant was used, two and three exposures of 6 hours 

 each showed that the mortality of the spiders increased with each increase in 

 the relative humidity, and that this mortality was from 24 to 36 percent greater 

 at 90 percent relative humidity than at 60 percent. 



