144 ON THE DISEASE OF THE LAKCH. 



and near cold, damp situations. Pressure on the branches from 

 late spring snows, especially those storms late in April, after 

 the tree is in leaf, are, along with the cold and frosts, the princi- 

 pal cause of this disease. Trees planted singly are seldom affected 

 with cancer. The branches of such trees are more robust and 

 better able to resist pressure, and this shows the necessity of early 

 and judicious thinning, so as to admit light and air sufficient to 

 encourage a strong and healthy growth. From the above state- 

 ment it is not to be inferred that the larch is too tender for our 

 climate, the contrary is the fact ; it will stand any amount of 

 cold and frost, provided they be seasonable. But this is not the 

 case in our variable climate. This disease reduces the value of 

 the timber very much. Large timber sometimes has to be sold for 

 fence and pit wood at less than half price, as the blemish renders 

 it unfit for boat or ship building. With our present limited know- 

 ledge of the diseases of trees, it is my opinion that we cannot do 

 anything to cure them, but we can in a great measure prevent 

 them by planting in suitable soils, situations, and exposure, early 

 and judicious thinning, and subsequent good management in the 

 latter. 



Insect or Blight. — Although found on strong healthy trees, they 

 seldom occur to any great extent, and only appear like a disease 

 on plants and trees whose vigour is impaired by some other cause. 

 The sap of such trees is of a saccharine nature, and favours the 

 attack of insects. 



Coccus laricis, or mealy bug, is the principal depredator. Both 

 the insect and its eggs are microscopic, and can be observed to 

 advantage only by a powerful lens. An ordinary magnifier shows 

 the insect to be of an oblong shape, all closely corded across the 

 body, and of a bluish purple colour. Its eggs are oval in form, 

 and of a pale grey colour, approaching to green. They are encased 

 in a white downy-looking substance, and under this covering 

 the insect may be found. Plants or trees affected with the C. 

 laricis are easily detected. The bark of the stem and branches 

 presents a dirty black appearance in spring, and in summer the 

 foliage appears of a grey whitish hue at a distance, but on close 

 examination it is covered with small white woolly flakes, adhering 

 tenaciously to the leaves and bark. The excrements of the insect 

 make the branches feel gummy to the touch, and taste like the 

 honey-dew of the oak or beech. As already stated, the C. laricis, 

 although found on healthy plants and trees, mainly attacks only 



