No. 6. DEPARTMENT OF AGRICULTURE. 797 



Because it has been observed that the little mite of an insect, 

 known to gardeners and florists and all those who grow plants and 

 flowers under glass as the "red spider," appears to increase and mul- 

 tiply more rapidly in the most dry and hottest part of the green- 

 house, the cooelusiou has been reached and the rash statement 

 made that moisture in the atmosphere will effectually banish him. 

 Nothing could be farther from the truth. When the only method 

 of heating greenhouses known was the flue system, which was 

 in vogue hundreds of years ago probably, and which, owing to its 

 primitive methods of distributing the heat, that end or part of the 

 greenhouse where the furnace was located was the hottest and dryest, 

 and that is where the spider flourished and was doing the most 

 damage; and because at the end farthest from the fire there were less 

 of the insects, the writers of those days were led to make the asser- 

 tion that a moisture-laden atmosphere was death to the Red Spider, 

 and many writers since have perpetuated the same old story. Much 

 that passes for scientific knowledge too often is not the result of 

 observation and investigation, but is merely compilation. 



Professor Cranefield is a keen investigator, and the followiog re- 

 sults of his recent investigations are taken from the columns of 

 the American Florist: 



"Is it necessary to maintain a moist atmosphere in the greenhouse 

 in order to keep down the red spider? Will not the sjjider thrive as 

 well in a moist air as in a drier air? According to the evidence of 

 experienced florists the spider is most destructive to Carnations 

 when the temperature of the houses is maintained at 50 degrees or 

 above at night and 70 degrees to To degrees in the daytime, whether 

 the air be moist or dry. When the houses are maintained at 38 de- 

 grees to 45 degrees at night and 10 degrees higher in the daytime the 

 spider is rarely troublesome, regardless of the moisture in the air. 

 Roses are affected more or less according to the temperature main- 

 tained, the higher the temperature the more abundant the spider, re- 

 gardless of the moisture in the air. Palm and Fern houses are 

 usually free from spider, not on account of the moisture in the air, 

 but because of the almost daily syringing the plants receive, thus de- 

 stroying the spider by force, and secondarily, of course, the fact that 

 these species of plants are less susceptible to attack by the spider. 



•'During the protracted heat and drougth of the past summer in 

 southern Wisconsin many species of trees and flowering plants, 

 notably Plums and Asters, were severely injured, in some cases killed 

 outright .by the red spider. This condition was no doubt largely, if 

 not wholly, due to the lack of rain, the lack of the impact of rain 

 drops on the leaves and consequent drowning of the insects rather 

 than the lack of moisture in the air. Evidence in support of this 



