148 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



The following is a diagram illustrating the life-history of 

 Dreyfusia piceis, according to Marchal : — 



Fig. 4. — Diagram of the Life-History of Dreyfusia picew, a^ter Marchal. 



III.— RELATION TO FORESTRY. 



From a variety of reasons, conifers have become more and 

 more the principal forest trees in temperate climates. As this 

 tendency is likely to increase in the immediate future, it is 

 becoming increasingly necessary that the diseases of coniferous 

 trees should be understood. The Chermesidae are all enemies 

 of coniferous trees, and these our most important conifers, 

 namely, spruce, larch, pine, silver fir, and, in America, Douglas 

 fir. The damage done by these pests is largely dependent on 

 the vigour of the hosts. Thus, when the environment of the 

 conifers is ideal, the damage done by the Chemies species is 

 not important, but the biological adaptations of the Chermesidae 

 are such that the slightest diminution in the health of the trees 

 means that these pests become serious enemies. This varying 

 damage, depending on the vigour of the hosts, explains, in part 

 at least, the different estimates of the damage done by the 

 Chermesidae. In the following pages I have attempted to 

 estimate the damage done to the various conifers by these 

 pests under average conditions. As this is a very difficult 

 matter, I shall be very pleased if foresters will assist by sending 

 me estimates of the damage done by the Chermesidae under 

 the various conditions in this country. I have also attempted 

 to estimate the relative immunity of the various exotic conifers 

 to the European species of Chermesidae. As the data for this 

 are not too numerous, I shall also be pleased to have the 

 assistance of members of the Society in this important question. 

 On the other hand, we must guard against the introduction of 

 non-European species of Chermes, e.g., there is a species — 

 Gillettea cooleyi — infesting Sitka spruce and Douglas fir. Galls 

 of the European species of Chermes are common on Sitka spruce 



