INSECT ENEMIES AND THEIR CONTROL 125 



millimeter in length. The mite is provided with suc- 

 torial mouth parts, requiring for its extermination contact 

 insecticides rather than stomach poisons. 



Numerous measures are recommended for the control 

 of spider mites in greenhouses. The destruction of all 

 weeds in the greenhouse during the summer season is a 

 valuable precaution. Weeds near the houses may also be 

 a source of infestation. Plants which are purchased or 

 transferred to other houses should be carefully examined, 

 and sprayed if found to be infested. Infested individual 

 plants may be found from time to time. Such plants 

 should receive prompt attention, to prevent the distribu- 

 tion of the pests. Plants should be promptly removed 

 from the houses after crops have been harvested, so as 

 to prevent further breeding of the mites. Rotation of 

 crops is always helpful in controlling the ravages of the 

 red spider. Fumigation with tobacco and ordinary 

 strengths of hydrocyanic gas is not effective, because the 

 mite, not being a true insect, does not possess spiracles or 

 breathing spores, hence killing by suffocation is ex- 

 tremely difficult. An experiment was made by Ewing, 

 in which he used approximately one ounce of potassium 

 cyanide to 1,000 cubic feet of space ; 50 larvae, 40 nymphs 

 and 30 adults were placed on plants, and results noted. 

 The ventilators were not raised for 15 hours. At the end 

 of this period, 32 larvae, 25 nymphs and 27 adults were 

 found to be alive, thus proving the inefficiency of this 

 gas in killing spider mites. 



Various sprays are used successfully in combating this 

 enemy of greenhouse crops. Water has long been known 

 as an enemy of the red spider, though the use of water 

 alone does not always prove fully effective. There are 

 abundant evidences that the force of the spray, whether 

 of water or some other solution, is an important factor in 

 destroying mites. A fine spray applied with force knocks 

 the mites from the leaves, thus injuring them so that few 



