CHERMESID^ IN RELATION TO BRITISH FORESTRY. 1 53 



F. laricio (Poiret). These species, however, are rarely attacked 

 when P. silvestris is present in large numbers. 



(ii.) Pineus strobi. — This species has only been found on 

 Weymouth pine {P. strobus, L.), hence the other pines may be 

 considered immune. 



{d) Silver Firs. 



(i.) Drevfusia Niisslini. — The history of this species in Western 

 Europe is interesting, as showing how the Chermesidae can 

 adapt themselves rapidly to new hosts. At first Abies 

 nordmafmiana (Lmk.) was the principal host, and A. pectinata 

 (D.C.) was relatively immune. Our indigenous silver fir was 

 not able to keep this immunity, however, and is now severely 

 damaged by this Chermes. I carried out infection experiments 

 by Gallicolte of Drey. Niiss/int, from a gall on Picca orientalis. 

 The Gallicolre were placed on A. nordmanniana, A. firina, 

 and A. grandis. They laid eggs on the three silver firs, and 

 these eggs duly hatched. The larvae hibernated on the 

 A. nordmanniana, and developed the following spring ; but 

 the larv^ on the other two hosts died a few days after hatch- 

 ing out. It would, therefore, appear that A. firma and 

 A. grandis are relatively immune at present, but this requires 

 further confirmation. 



(ii.) Drevfusia picecc. — This species appears to attack all 

 silver firs, but the damage which frequently follows an attack 

 c^ A. nobilis makes it more conspicuous on this conifer. 



3. Chermesidse and Silvicultural Mixtures. 



After the discovery that most species of Chermesidae required 

 two hosts for their complete development, it was suggested that 

 these hosts should not be mixed together,);?.^, the use of spruce and 

 larch in the same mixture was deprecated. Recent research, how- 

 ever, has shown that, once infection has taken place, the species of 

 Chermesidte can carry on on either host without the other. For 

 instance the non-migrating species Chermes abietis (Kalt.), and 

 Cnaphalodes lapponicus{Cho\.),a.x& the more serious pests as regards 

 spruce, while the species of the genera Chermes, s. str., and 

 Cnaphaiodes, appear to be able to carry on indefinitely on larch. 

 Only the silver fir and pine parts of the life-cycles of Dreyfusia 

 Ndsslini and Pineus pini respectively are common in this 

 country, while Drey, picece and Pineus strobi are only found on 



