INSECTS INJURIOUS TO CONIFERS. 



173 



to know tliat migration does occur, that the Spruce is the 

 primary host, and on it is found the sexual generation. No 

 males of C. Laricis — that is, no male forms on the Larch — have 

 as yet been detected, and, if the life-history given is correct, there 

 is little reason for supposing that they ever will be. 



If this alternation of host, or heteroecism," were merely an 

 interesting biological fact, it would have no place here, but all 

 such cases have an important practical side, whether they occur 

 among tapeworms, Wheat " rust," Hop-aphides, or Chermes ; and 

 the following questions immediately suggest themselves as a 

 result of the study of this life-cycle : — 



As the species alternates between Spruce and Larch, is it 

 specially injurious to either tree when the two are growing in 

 company, as in a Spruce-Larch mixture ? 



Can it support itself on, and injure Spruce when there are no 

 Larches in the neighbourhood ? 



Can it support itself on Larch when there are no Spruces in 

 the neighbourhood ? 



These questions cannot at present be fully answered, but it 

 is obvious that their solution can be furthered by anyone who has 

 large opportunities for studying the two trees growing together and 

 separately, and who is able to identify the " Spruce-galls " and 

 the " woolly-bugs" on the Larch-needles. No further entomo- 

 logical knowledge is necessary, and it is to be hoped that light 

 will be thrown on the subject by practical men ; for it is by the 

 accumulation of evidence that the real practical importance of 

 the question must be decided. 



In the papers already published more attention is paid to the 

 biological problems involved, but Blochmann says distinctly that 

 the first protective measure to be adopted is that of not growing 

 the two trees in company. In certain woods, where a mixture of 

 Spruce, Larch, and Pine was growing, he has observed the galls so 

 abundant on the Spruce that scarcely a twig was free. In others, 

 again, where the Spruce-plantations were almost entirely free 

 from Larches, there were large areas in which no galls could be 

 discovered, and he is strongly of opinion that the abundance of 

 Spruce- galls is correlated with the supply of Larches, on which 

 the laricis -foTm can live. At the same time he admits that 

 Spruces can be infected at a distance by the insects being carried 

 by wind. 



