158 PROTECTION AGAINST INSECTS. 



In accordance with their choice of nutriment, insects may 

 be classed as mono-, poly- or pantophagous. Monophagous 

 insects only attack certain plants or at most a group of plants, 

 such as broadleaved, or coniferous trees. Polyphagous insects 

 attack trees of both kinds, whilst pantophagous attack herbage 

 as well, and are least numerous of all.* 



Observations are not yet complete regarding the monophagy, 

 or polyphagy of certain insects. The hitherto accepted 

 monophagy of some insects has often been upset. Tom i en ft 

 typographic, L., and T. amitinus, Eichh., formerly considered 

 exclusive to spruce, have also been found on Scots pine and 

 larch. Myelophilus piniperda, L., has been found on spruce, 

 as well as on Scots pine. In this respect, the observation of 

 beetles is more difficult than of Lepidoptera, that live in the 

 open. 



Even amongst carnivorous insects, monophagous and poly- 

 phagous species are to be found. Many parasitic insects, for 

 instance, attack only a single species of moth, and in one stage 

 only of its growth, in the egg, larval, or pupal state. 



Conifers suffer much more than broadleaved species from 

 insect-attacks. They afford nourishment to a greater number 

 of injurious kinds, and do not recover from damage so readily, 

 as they cannot replace injured members so easily as broad- 

 leaved trees. The Scots pine and the spruce are attacked by 

 the greatest number of species of insects, and pure woods of 

 these trees suffer most severely. Amongst broadleaved trees, 

 oak, beech, poplars and willows suffer most, the birch and 

 alder less, and less still hornbeam, maple, ash. Least of all 

 robinia, mulberry, walnut, plane, sweet- and horse-chestnut. 



Suppressed, weakly, and injured or diseased trees generally 

 suffer more from insects than healthy trees ; at any rate this 

 holds good for mature trees with thick bark. Hence injurious 

 insects increase and become more dangerous when trees have 

 suffered from various climatic or physiological injuries, or 

 those caused by abrasions, bad pruning, etc. 



Species of insects which live on dying, dead or rotten wood 



* This definition is that of Hess. As a rule, entomologists would hardly 

 call a species that feeds indifferently on Picea, /'/////*. and Larix monophagous, 

 but would reserve the term for those insects whose diet is limited to a single 

 species or genus. 



