86 INSECTS AND HUMAN WELFARE 



then destroy more healthy trees, especially small ones, which 

 cannot withstand the onslaughts made by swarms of bark- 

 beetles. The elm bark-beetle has not yet spread far beyond 

 the point of its introduction, but like the leopard moth will 

 unquestionably extend its range by slow steps until it occurs 

 generally throughout the United States. 



From the preceding account it can be readily seen how the 

 elm leaf-beetle, the leopard moth and the elm bark-beetle 

 naturally follow one another and how they affect the condi- 

 tions of the elms growing where they are prevalent. At the 

 present time the only part of the country where all three 

 occur is included withing a few townships in eastern Massa- 

 chusetts, but there the havoc they have wrought is painfully 

 evident. In Cambridge, Boston and many surrounding 

 towns that once boasted of many magnificent elms, only 

 pitiful remnants remain. 



A condition of this sort is of course an extraordinary in- 

 stance, as all three insects are introduced species freed from 

 many of the handicaps that hamper their multiplication in 

 their native land, but it illustrates most beautifully in an 

 exaggerated form what is constantly taking place in our 

 forests. Agricultural crops do not lay themselves open to 

 attack by insects in this way, as they are replaced by a new 

 generation each succeeding year, and consequently their re- 

 lations to insects are in this respect at least, less complex. 



Lastly, there is a type of injury by forest insects which 

 closely parallels that mentioned in connection with agricul- 

 tural products in storage. Stored lumber, and even that 

 which has been fashioned into tools or furniture is frequently 

 damaged by insects which are capable of developing in ex- 

 tremely dry wood. These are, in the main, certain beetles 

 characteristic of arid or desert regions that find a close ap- 

 proach to their natural environment in stored forest products. 

 Their originally very limited distribution has been greatly 

 extended, and they are becoming more or less cosmopolitan 

 in range, establishing themselves as opportunity offers in 

 places far from their native homes. 



