328 PRINCIPLES OF ECONOMIC ENTOMOLOGY 



3. Tropical insects that are limited to tropical plants for 

 food, will have little importance in temperate regions except 

 in greenhouses. 



]\Iost of the scale insects are tropical and in this climate 

 are on the borderline of their destructive region, hut on 

 account of their general food habits they constitute a general 

 pest. 



The question of the control of the distribution of the 

 destructive species is yet a new problem and one over which 

 there is considerable dispute; any measure intended to exclude 

 a foreign species must be adapted to a particular species, as 

 a general law will not suffice. Even the general exclusion of 

 the foofl plant will not alone be effective in keeping out the 

 pest, as it may have other means of dispersal. 



Inspection and enforcement of restrictions at the port of 

 entry are being quite generally adopted. Inspection is most 

 effective when applied to plants. Sometimes it is necessary to 

 introduce a foreign species to prey on another foreign insect 

 that is already here, as was the case with cottony cushion 

 scale in California. 



