INSECTS INJURIOUS TO VEGETATION 41 



into it by the scale insect. Two principal methods have been 

 devised to destroy scale insects. One method is to spray the 

 infested plants, during the winter when they are dormant, with 

 a strong solution of lime-sulphur mixture or kerosene oil; the 

 other is to cover the fruit trees one by one with a sort of tent, 

 and generate beneath this hydrocyanic acid gas, which quickly 

 kills the scale insects. Among other scale insects that are in- 

 jurious to fruit trees may be mentioned the oyster-shell scale 

 and scurfy scale. 



FIG. 23. A stable made by ants for plant lice or aphids. (From Cornell 



Leaflet.) 



PLANT LICE (Figs. 18, C, and 23). Closely related to the 

 scale insects is another group of sucking insects, the aphids or 

 plant lice. The woolly apple aphis is a very destructive pest in 

 young apple orchards. It works mostly upon the roots and thus 

 often escapes notice until the trees are badly injured. The com- 

 mon apple aphis of Europe also attacks young apple trees, caus- 

 ing the leaves to curl up and drop off. Associated with the 



