24 ENTOMOLOGICAL SOCIETY 



question of the relative importance of parasites to host and of 

 hyperparasites to primary enemies, that the latter far over- 

 shadowed the former. While the proportion of parasitized 

 caterpillars varied but little from year to year, approximating 

 each season from 15 to 20 per cent, at times as many as 90 

 per cent of the parasites were destroyed by secondary enemies. 

 In contrast to this, out of several hundred breeding notes upon 

 the parasites of wood-boring Coleoptera, there is but a single 

 well-authenticated instance of hyperparasitism. 



In view of this, it might at first glance appear as though 

 many instances would show an influence of the parasites upon 

 the abundance of the hosts greater than in the case of Mala- 

 cosoma or Hemerocampa. The investigations so far made do 

 not appear to confirm this theory to any great extent. Just 

 why it does not hold, is rather difficult to explain satisfactorily 

 in some instances, though in others apparent explanations are 

 not difficult to find. One of these is the dependence of a 

 great many species upon a suitable alternation or sequence of 

 hosts. Take for example a parasite upon a barkbeetle having 

 a well-defined life history with but a single annual generation 

 such as Bracon scolytivonis Riley upon Hylesinus aculeatns 

 Say in ash, which is able to make the attack upon its host 

 only at a certain stage in the development of the latter. As 

 it rapidly passes through its own transformations and emer- 

 ges many months before this particular host is again ready 

 to serve in that capacity, it becomes a necessity for the parasite 

 to seek another that will be suitable. This is far from being 

 the simple matter that it is with a great many of the parasites 

 of the Lepidoptera, as it is not only necessary to find another 

 host more or less closely related to the first, and with similar 

 habits, but the nature of its surroundings must be approxi- 

 mately the same. A tree with thicker or harder bark may 

 ofifer insuperable obstacles to oviposition. 



It is only in rare instances that all the individuals of a 

 brood of wood or bark borers are equally exposed to the 

 attack of any one parasite, and a proportion will almost al- 

 ways escape. This condition has been at times strikingly illus- 

 trated by a study of the proportion of parasitized individuals 

 of Dendroctonus frontalis Zimm. in different trees and differ- 

 ent portions of the same tree. In the tops, or in small trees 

 where the bark was thin, braconid parasites were once found 

 to have attacked something like 45 per cent of the larvse. 

 while lower down on the same tree it was almost entirely 

 absent. It is easy to see how conditions like these, which are. 

 of course, intensified when different hosts in different species 

 of trees are involved, have great influence upon the fortunes 



