DISTRIBUTION OF INSECTS 29 



extent in searcli of their food plant, yet this insect required about 15 

 years to make its way from Colorado to New England. 



Practical application. In the case of canker worms, the white- 

 marked tussock moth and other species with wingless females, advan- 

 tage can be taken of their limited means of distribution to exclude them 

 from trees once cleaned. This possibility warrants considerable expense 

 in clearing them from a tree. The introduction of scale or other insects 

 on a farm may be prevented to a great extent by studying their means 

 of distribution and adopting proper methods to attain the desired results. 

 It would be much safer to buy trees that have been fumigated, or even 

 undergo the expense of fumigating purchased stock, rather than admit a 

 pest that can be excluded by reasonable care. There are some insects 

 which fly relatively short distances, for example the elm-leaf beetle; the 

 parent of the apple maggot is said to have this habit, and there are prob- 

 ably others, but we know altogether too little regarding how far each 

 species will fly. Those confined to relatively short aerial flights can be 

 kept in check in one orchard with comparative ease, even though neigh- 

 boring ones are badly aftected, but such is not the case when the females 

 habitually fly long distances before depositing eggs. It is only under 

 exceptional circumstances that the length of flight can be determined for 

 a species, but whenever an opportunity offers it should be seized. Study- 

 ing the spread of insects is most fascinating field work, something that 

 may be taken up by all and is also of great practical value, because an 

 insect can not be controlled in the best way till its limitations in this 

 respect are known. 



