GREENHOUSE RED SPIDER. 161 



The number and location of the spiracles in red spiders have not been 

 determined because of their minuteness, but they are probably two in 

 number and are situated in the vicinity of the head region. Therefore, 

 although the red spider can be killed by fumigation with hydrocyanic 

 acid gas, it is impossible to do so without severely damaging plant life, 

 due to the concentration of the poisonous gas required. 



An infested plant has at all times every developmental stage of the 

 red spider on its leaves, but in artificial control methods we need to con- 

 sider only three general stages. 



1. Egg Stage. — At the present time no spray is known which will 

 affect this stage without severely injuring the plant. 



2. Quiescent Stage. — As explained under the life history, the young 

 larva? on hatching feed for a day, and then settle down on the leaf in a 

 premolting or quiescent state during which time no nourishment is taken. 

 These quiescent mites form a new chitinous layer beneath the old external 

 skin covering of the preceding stage. Thus during this period a red 

 spider has two chitinous layers covering the body instead of the normal 

 one, and because of this it has been found very difficult to kill by contact 

 sprays. By reference to the life history it will be seen that each female 

 mite passes through three of these quiescent periods before reaching the 

 adult state. If red spiders in this stage of development are not killed by 

 the spray material recommended for control, it will be almost impossible 

 to eradicate this pest unless sprayings are conducted daily. 



As soon as the spray applied to an infested plant has evaporated, the 

 mites will be found inactive, and many workers have concluded that all 

 mites above the egg stage have been killed. However, if the leaves were 

 kept under careful observation it would be seen that many of the mites 

 quiescent at the time of application later molt and establish themselves. 

 This point has been overlooked by former workers on the control of red 

 spiders, but is a very important one. 



3. Feeding Stages. — A large number of spray materials efficiently 

 control mites in the active feeding stages, but because of their inefficient 

 control of the quiescent stages have been discarded. 



EXPERIMENTS CONDUCTED IN THE LABORATORY. 



Fumigation Experiments. 



Several fumigation experiments were conducted in the hope that some 

 gas might be found effective for red spiders without being injurious to 

 cucumber plants. 



(a) Sulfur Dioxide (SO2). 



In many commercial forcing houses ' sulfur is burned between crops, 

 in order to rid the house of all insects, fungous diseases and mites. To 

 prove whether this was an efficient method, the following experiments 

 were performed. 



