164- PROTECTION AGAINST INSECTS. 



be done in cases of primary attack except to remove the parts 

 of the trees which have already been attacked, together with 

 the insects concerned ; species which make secondary attacks 

 can alone be caught by tree-traps. 



Species of insects which occur in forests, but confine their 

 ravages to grasses and herbage, are of no economic importance, 

 unless these products are of exceptional value. 



Finally species in themselves harmless, but which may be 

 mistaken for highly injurious insects, are termed by Eatzeburg 

 deceptive insects, e.g., Lithosia quadra, L., the larvae of which 

 appear on all conifers, as well as on oak, beech, etc., in con- 

 siderable numbers. It is quite harmless, feeding only on 

 lichens. It greatly resembles Liparis monacha, L., for which 

 it is often mistaken. Tenthredo cingulata, Fabr. ; the larvae 

 feeding on bracken, which they often completely devour, 

 occupy galleries of bark-beetles under the bark of pines and 

 make galleries of their own in pine bark, without injuring the 

 trees in any way (Fig. 61). The forester must -therefore 

 learn to distinguish harmless species from other injurious 

 kinds which they may resemble. 



2. Preventive Rules, 

 a. Sylvmdtural. 



Since the majority of injurious foreign insects, especially 

 bark-beetles, prefer to attack sickly, stunted or weakly forest- 

 plants, and may spread from these to their healthy neighbours, 

 the safest method for preventing insect attacks is to follow the 

 rules which experience has laid down in sylviculture and forest 

 utilisation for the formation, tending and harvesting of woods. 

 In general, the following rules should be observed : 



(i) Choices of suitable species of trees and proper systems of 

 regeneration. The species must be appropriate to the locality, 

 and the system must correspond to the nature of the species 

 grown. It is specially important to select strong healthy 

 plants for plantations, and to plant most carefully. 



(ii) Avoidance of extensive pure woods, especially in 

 coniferous forest. It is better to grow mixed woods, and to 

 mix broad leaved species with conifers. 



