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non-migrating species have not arisen because the intermediate host 

 larch was absent, but because they are the more successful species 

 even when spruce and larch are both present. The quest'on whether 

 the above cycles should be considered as those of separate species or 

 of biological races of one species can only be determined by further 

 research, both morphological and biological. Statistical research 

 will be important in determining this question. 



In investigating the relation of these genera to forestry, the methods 

 of infestation were found to be the same in both Chermes and Cnaj)Iia- 

 lodes. Conditions favouring infestation are proximity of the hosts 

 and favourable weather conditions. These explain the rapid spread 

 of the pest in forest nurseries, which are usually sheltered, wath the 

 trees crowded together. 



Spruce is infested from two som'ces : — (a) by sexuparae. The 

 parthenogenetic adaptations on larch are made at the expense of the 

 sexupara generation. Thus on the area under observation, although 

 colonici were very numerous on larch, the number of sexuparae at- 

 taining maturity, and still more those reaching spruce and laying 

 eggs, was small. Thus infestation from that source is not serious. 

 (6) By gallicolae non-migrantes. The majority of such gallicolae 

 settled on the tree on which they were born. This, together ^\ath the 

 high fertility of the two generations constituting that cycle, caused 

 the rapid increase on the host. Infestation from this source is therefore 

 serious. 



Larch is infested by gallicolae migrantes. This is the source of the 

 first infestation, but the species are thereafter principally continued 

 on this host by the parthenogenetic adaptations. 



Damage to spruce by Chermes. The species of this genus attack 

 strong growing spruce, hence the damage is primary. Under normal 

 conditions such damage is unimportant, but when allied with unsuitable 

 soil or atmospheric conditions, the work of this genus may play an 

 important part in killing the host. On the area under observation 

 the generations of the non-migrating species did much greater damage. 

 The species of Cnaphalodes attack only thin shaded branches, hence 

 the damage, as regards spruce, is secondary. Shaded spruce, however, 

 are quickly killed, as the galls terminate the twigs. Thus the damage 

 would be important where, in thinning a wood, the shaded spruce were 

 left for soil protection. As before, the non-migrating species was 

 foimd to be more dangerous. 



The damage to larch by the species of the two genera Chermes and 

 Cnaphalodes has undoubtedly been greatly increased by the planting 

 of that conifer in localities and under conditions very different from 

 those of its natural habitat, with a consequent weakening of the tree. 

 The species of both genera are usually present on the same tree. 

 Chermes is principally a bark-feeder as regards larch, hence the damage 

 is difficult to estimate. Frequently the bark is whitened by the 

 " wool " and cast skins of the colonici ; later the bark turns black. 

 The feeding of these numerous colonici, at a time when growth should 

 be at a maximum, must have a weakening effect on the host. The 

 punctures are small, but they are made at a time when girth is in- 

 creasing, so that they will be greatly increased in size and become a 

 possible means of infection by parasitic fungi. The damage resulting 

 from the feeding of the sexuparae is unimportant, as its duration 



