THE OUTLOOK FOR THE FUTURE 103 



been effective in great measure in preventing the rapid spread 

 of insects, although in the nature of the case, they could never 

 be absolute without abolishing all commerce. In proportion 

 to the actual efficiency of the quarantine service, and the 

 difficulty of inspection, which naturally varies within very 

 wide limits, the spread of insects has been impeded. In many 

 cases the importation of certain plants and products has been 

 prohibited. This has been done on account of the fact that 

 the most general method of spread for agricultural, horti- 

 cultural and forest insects is upon the host plant, and one of 

 the more probable dangers is thus removed in so far as it is 

 possible to exclude the specific materials. Whether the 

 wholesale exclusion of food plants or their adequate inspec- 

 tion is to be preferred from a practical point, appears to be 

 rather a question of opinion. So far as the ultimate result is 

 concerned it matters little, for the immigration of undesirable 

 insects will take place only with greater difficulty and at a 

 slower rate, as human ingenuity is pitted against a seemingly 

 hopeless problem. 



What surprises the entomologist more than the introduc- 

 tion of so many insect pests into our own country, is the fact 

 that more have not established themselves. It is evident 

 that many insects do not become naturalized so easily as 

 might be expected, and that when once well established, some 

 of them spread very slowly. This is evidenced by the long 

 time required by the gipsy moth to become abundant at the 

 point of its introduction in New England and the subse- 

 quently deliberate extension of its range, even before adequate 

 control measures were inaugurated. Quite probably it now 

 occurs in many isolated places far outside its recognized 

 range, 1 but the spread of this species and that of the brown- 

 tail moth has been extremely slow in comparison with other 

 imported species like the cabbage butterfly and the Colorado 

 potato beetle. Such differences are partially explained by 

 the naturally migratory habits of certain species, but not 



1 Since this was written it has been found in two other states, New Jersey and 

 Pennsylvania . 



