160 MASS. EXPERIMENT STATION BULLETIN 179. 



important when the plants have become so badly infested that webs 

 have been spun over the leaves, as the pickers passing from one house to 

 another carry infestation with them. . 



NATURAL ENEMIES. 



Red spiders out of doors have a very large number of enemies be- 

 longing to widely different groups, nine groups of predacious forms em- 

 bracing thirty-one species having been listed (McGregor, 1917) as attack- 

 ing the red spider. Under greenhouse conditions, however, red spiders are 

 exceptionally free from enemies. It appears that the red spider enemies 

 are unable to develop in the high temperatures which are necessary for 

 most greenhouse crops. In cucumber houses the wi'iter has repeatedly 

 examined infested leaves in the hope that some enemy would be found 

 able to withstand greenhouse conditions and prove useful in the control 

 of this mite, but these examinations have proved fruitless. On violets 

 which are gro%vn in a humid atmosphere and at a low temperature, a few 

 predaceous mites belonging to the order Acarina, family Gamasidcc, are 

 very beneficial. 



INTRODUCTION TO EXPERIMENTS. 



Before taking up the experiments conducted on the artificial control 

 of red spiders a few facts will be summarized in order that the failure of 

 some fumigants and sprays may be better understood. 



Cucumber plants grown out of doors are very delicate and susceptible 

 to injury of many kinds, while those grown in forcing houses are much 

 more so. Therefore the sprays and fumigants which can be used with 

 safety to the foliage are very few, while the red spiders are exceptionally 

 hard pests to combat. These two opposing factors have been found 

 extremely hard to satisfy. 



Many greenhouse men ask the following question, "Why is fumigation 

 not effective in controlling red spiders?" It has been known for many 

 years that these mites are very resistant to fumigation with our ordinary 

 poisonous gases, such as tobacco and hydrocyanic acid gas. To explain 

 this peculiarity we must contrast the respiratory systems, through which 

 all poisonous gases act, of mites and insects. The latter are efficiently 

 controlled, while only a very few of the former succumb to such treatment. 



In insects the respiratory system is composed of several large main 

 air tubes which repeatedly divide, forming very small tubes which ramify 

 into all parts of the body. This system of tracheal tubes opens to the 

 exterior by several small segmentally arranged openings called spiracles, 

 and through these the poisonous gas enters the air tubes, which conduct 

 it to every tissue in the body, and produces sudden death. 



Although the tracheal system of the red spider is better developed than 

 in most mites, it is far simpler than in the majority of insects, containing 

 a much smaller number of tubes. 



