4 BULLETIN 847, IT. S. DEPARTMENT OF AGRICULTURE. 



parently a constant preference for apple over the other species of 

 host trees present in this instance. . 



Table I. — Relative numbers of roundheaded apple-tree borers found in different 

 species of host trees at French Creefo, W. Ya. 



Trees examined. 



Number 



of 

 borers. 



Number. 



Variety. 



194 



Seedling apple 



85 



11 



Pear 







823 



Wild crab 



9 



405 



Hawthorn (Crataegus) 



1 



50 



Service 









Total 





1,483 



95 









It will be noted from Table I that in this case the 50 service trees 

 (PL I) examined contained no borers. This is far from the rule as 

 regards the service, for in woods where that tree and mountain ash. 

 abound tracts are often found where practically every tree is in- 

 fested. (PL I, B.) Other areas near by of equal size are quite likely 

 to occur where no borers can be found, although the host trees may 

 be just as abundant as where infestation is general. This arises 

 from the fact that under natural conditions families or communities 

 become established and reproduce through many generations within 

 restricted areas. It is probable that adult males fly readily from one 

 breeding center to another, preventing thereby an excess of inter- 

 breeding, but the females clo not normally tend to go far from the 

 tree in which they developed, provided other host trees are near. 

 This tendency for infestation to be confined to limited groups of 

 trees is often noted in cultivated orchards. Isolated trees are some- 

 times attacked, however, and there is no question that the female is 

 capable of flying to a considerable distance when impelled by a 

 scarcity of trees in which to oviposit. 



DISTRIBUTION AS AFFECTED BY NATIVE HOST PLANTS. 



As has been pointed out, the principal native host trees of the 

 rounclheaded apple-tree borer are the service, mountain ash, wild 

 crab apple, and hawthorn. Of these wild hosts service and mountain 

 ash seem to be preferred to the others. It is an interesting fact that 

 the service tree (PL I, A) occurs over practically the same region 

 of North America as does the insect in question. 



These wild food plants undoubtedly play an important part in 

 the local and general distribution of the borer. Infestations commonly 

 attributed by orchardists to certain soil conditions, to newly cleared 

 land, or to the hilly contour of the land are in reality usually due to 



