16 



MASS. EXPERIMENT STATION BULLETIN 326 



Table 9. — Relative Abundance of Red Spider Larvae on Plants 

 Fumigated with Various Dosages of Naphthalene at Different 

 Relative Humidity. 



Examined 3 days after fumigation. Temperature 80° F. 



Waltham, Massachusetts. 1931-32. 



I to 10 red spider larvae alive 



II to 25 red spider larvae alive 



KEY: 



26 to 50 red spider larvae alive 

 More than 50 red spider larvae alive 



These experiments and several years' experience have proved definitely that 

 a relative humidity of 75 to 80 percent is desirable for the most satisfactory 

 results, and every effort should be made to maintain approximately that con- 

 dition during fumigation with naphthalene. 



Unfavorable Conditions 



Few, if any, commercial greenhouses are so nearly air tight and provide so 

 ideal conditions for fumigating as the experimental fumigation chamber in 

 which most of the experiments reported herein were conducted. Leakage, 

 adsorption, air pockets, and failure to maintain satisfactory and constant 

 temperatures and relative humidities all tend to decrease the effectiveness of 

 the treatments. In the experiments reported in Table 10 all of these factors 

 contributed to the results, especially since the fumigations were made in the 

 daytime when favorable conditions are usually more difficult to maintain. The 

 percentage of dead spiders after one and two fumigations is lower than should 

 be expected, but this experiment provides a good example of the occasional 

 need for four successive fumigations to obtain effective control of the red spider. 



