162 PROTECTION AGAINST INSECTS. 



etc., has no subsequent fatal effect on the plants. Such are 

 tortrices and leaf-miners, and many gall-insects. 



In a time of exceptional multiplication, a slightly injurious 

 insect may become decidedly injurious, or a decidedly injurious 

 insect, highly injurious. A single species of insect may also 

 be injurious in a different degree to different species of trees : 

 it may prefer one to another, or one tree may recover more 

 easily than another from its attacks. Liparis monacha, L., <'.//., 

 is far more injurious to conifers than to broadleaved trees, 

 and to the spruce than to the Scots pine. 



b. Kind of Damage. 



Insects may be classed according to the kind of damage they 

 do, as commercially or physiologically injurious. The former 

 class renders useless, or greatly reduces the commercial value of 

 the part of the tree they attack, as when wood has been bored 

 by Sirex, Cerambyx, Anobium, or Lyctus. 



Physiological injury on the contrary is that which interferes 

 with the vitality of plants, checking the growth, or even killing 

 them outright, as when the cambium of a tree is eaten by 

 Tomicus typograpJius, L., or the needles by Gasteropacha pini, 

 L., or by Liparis monacha, L. 



Insects coming under the second category are therefore more 

 hurtful than those which merely destroy wood, although the 

 burrows of the latter are sufficiently conspicuous. Moreover, 

 most wood-borers live in dead wood. It is, however, possible 

 to pay too little attention to commercially injurious insects. 



The degree of physiological injury depends on the species 

 of insect, its mode of attack, numbers, and also on the species, 

 degree of healthiness and age of the tree, the season of attack, 

 and on other local circumstances. Mention has been already 

 made of the greater susceptibility of conifers ; the spruce 

 suffering most of all, then the Scots pine, silver-fir and larch, 

 the latter bridging the way towards broadleaved trees. Young 

 trees, especially one to three years old plants, suffer more than 

 old ones; and injury in the spring is more harmful than that 

 done in summer or autumn. Cambium eaters do more damage 

 than wood-eaters; leaf-eaters more than flower or fruit-eaters. 



The healthier and stronger the attacked plant, the better it 



