178 Life-IIUtori/ of the Larch Chermes 



IV. ECONOMIC CONSIDERATIONS. 



A diagram is here given representing every stage in the life-cycle of 

 the Larch Chermes, and modified from that given by C. Borner in 1908, 

 in order to show more accurately the origin and development of the 

 Colonici generations. 



The dotted circle in the Spruce cycle denotes the Gallicola non- 

 migrans generation, and the Fundatrix generation arising from it, as 

 described by Borner. These generations may occur in Germany, but in 

 Great Britain we have no evidence that they exist, a fact which might 

 modify the most economic means of control considerably. 



There is no doubt that the brunt of damage is sustained by the 

 Larch in the case of young plants, and Mr Steven has shown that this 

 can be adequately controlled by fumigation. 



The Spruce does not appear to be attacked severely while young, 

 and, as applied to Larch, it would certainly seem that a legislative 

 measure is called for, providing for a compulsory fumigation of all 

 Larch trees before planting out in the forest, not only upon the principle 

 of starting with clean stock, but with a view to giving the individual 

 plants a chance to make a good growth from the beginning. 



With regard to older Larch trees, it is probable that a good control 

 on the Spruce, growing in the neighbourhood, might well obviate the 

 necessity of a costly and difficult control on Larch, for it now appears 

 that a large majority of the Sistens larvae perish during hibernation, 

 and there is no doubt that the Progrediens generations come to an end 

 with the fall of the leaves, thus accentuating the benefit to be derived 

 if the Gallicolae migrantes were prevented from their annual influx 

 from the Spruce. 



Except at unwarrantable cost, the galls on the Spruce could hardly 

 be destroyed, and the wintering Fundatrix larvae, with their diffuse 

 distribution over the Spruce buds, and their covering of wax-wool, 

 would present many difficulties if spraying were resorted to. The 

 Sexualis generation, however, is produced, by the Sexupara, on the 

 underside only of the older Spruce needles, during a short period of 

 the summer, and under the wings of the mother insect they remain 

 during their development, only partially covered by an otherwise 

 copious supply of wool, secreted by the parent. 



It is the Sexual generation, then, about which least is known, that 

 gives an opening for a method of control by spraying — a remedy rendered 

 economic by the fact that only the underside of the branches need be 



