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 he 
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 larve, 40 nymphs 
and 80 adults were placed on plants, and results noted. 
The ventilators were not raised for 15 hours. At the end 
of this period, 32 larvee, 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 
