PINACE^ 273 



taken to limit larch plantations to suitable soils and situations, 

 much could be done to keep some of the worst diseases in check. 

 Conditions peculiarly favourable to the spread of disease are 

 brought about by planting on low-lying, wet ground ; on land 

 at both high and low elevations that is cold and wet ; on land 

 with an impenetrable subsoil ; and by overcrowding. Larch often 

 succeed better when planted on land with a north exposure than 

 in positions facing south or west. 



Chermes lands, Hartig (Larch Aphis), is one of the most 

 conspicuous insect pests of larch. It is found on bark, young 

 shoots, and leaves, but is most abundant on the latter, its presence 

 being easily detected by a white protective exudation. The 

 " queen-mother " generation hibernates on the trees and becomes 

 active about the end of March ; egg-laying begins as soon as 

 the leaves appear. The larvae spread rapidly, and by the middle 

 of summer a tree may be covered by the perfect insects. The 

 larvae cause great damage by sucking the sap of leaves and young 

 shoots, thus preventing proper development. The punctures 

 of this insect are also a fertile source of infection for the larch 

 canker fungus (see p. 274). C. laricis is usually most prevalent 

 during hot and dry summers. Isolated trees and nursery stock 

 may be cleaned by spraying with a paraflfin wash several times 

 during summer. Nursery stock can also be cleaned by fumigation 

 with hydrocyanic gas before dispersal, but it is not possible to 

 clean large plantations by this method. Larix occidentalis 

 growing with common larch usually escapes injury from this pest. 



Nematus erichsoni, Hartig (Large Larch Sawfly). This insect 

 came into prominent notice during the early years of the present 

 century, when it caused a good deal of loss in several parts of 

 England and in southern Scotland. It was first noticed in Cum- 

 berland and quickly spread to other places. The injury is caused 

 by the larvae. Mature insects appear in June and eggs are laid 

 in clusters about the base of the leaves. On hatching, the larvae 

 begin to feed on the foliage and continue until the middle of 

 August. They are so voracious that whole plantations may be 

 denuded of leaves. The fully fed caterpillars leave the trees in 

 August and encase themselves in cocoons amongst moss or 

 leaves at the base. Pupation takes place in late spring. When 

 once it becomes well estabhshed it is difficult to eradicate this pest, 

 and a careful look-out should be kept for the first sign of its 

 presence, which may be recognized by the browning of the young 

 shoots on the higher branches. By collecting and destroying 

 larvae and cocoons its spread may be checked. Trees easy of 

 access should be sprayed with an arsenical wash, made by mixing 

 1 lb. of Paris green with 150 gallons of water. 



Coleophora laricella, Hubner, is a small moth which causes 

 considerable harm to the leaves of larch. Mature moths are to 



