172 MASS. EXPERIMENT STATION BULLETIN 179. 



Conclusions drawn from Commercial Spraying Experiments. 



Sprayings conducted on bright, sunny days with a rather high tem- 

 perature in the greenhouse resulted in slight injury to the edges of the 

 leaves, but if applications were made on cool, cloudy days this injury 

 did not occur. 



For a thoroughly efficient control at least three applications should be 

 given the cucumber plants at weekly intervals, as soon after they have 

 been set out in the greenhouses as possible. 



PREVENTION. 



The wTiter has been unable to conduct a thorough test in eliminating 

 red spiders from the whole range by cultural methods, because it was 

 found impossible to procure an establishment which would serve for this 

 purpose. In commercial greenhouses many factors enter into the red 

 spider problem which cannot be solved unless a suitable range is found 

 which will eliminate these confusing factors in order that some definite 

 knowledge may be gained by using preventive measures. However, 

 under greenhouse conditions, it is the writer's firm conviction that the 

 red spiders can be totally exterminated from commercial ranges by clean 

 culture, both within and outside the greenhouse. It is hoped that some 

 experimental work may be conducted on this important control measure 

 in the near future. , 



CONTROL MEASURES. 



The general biology and development of experimental and commercial 

 control measures have already been discussed, but only in a general way. 

 Under this heading the methods used for the prevention and repression 

 of red spiders will be taken up more in detail. Having established the 

 efficiency of the repressive measures, only the preparation and application 

 of spray materials will be considered. 



Preventive Measures. 



The solution of the red spider control problem in cucumber greenhouses 

 should be accomplished through preventive efforts rather than by re- 

 pression, if it is to be done most economically. The commercial grower 

 should do everything possible to eliminate these pests, both within and 

 outside his greenhouses. 



In the majority of cases cucumber plants are infested either in the 

 plant house or soon after they have been set out in the greenhouse. The 

 origin of this infestation may be weeds which have harbored mites through- 

 out the winter inside the greenhouse, or weeds and grasses immediately 

 surrounding the house at the base of which the mites winter over and 

 migrate into the greenhouse early in the spring. The first is very im- 



