REMARKS ON INJURIOUS AND BENEFICIAL INSECTS ir 



is commonly the case when the tree is 

 encouraged to grow pure in dense masses. 



Trees grow in different ways, and have 

 in some cases acquired characters more or 

 less resistant to insect attacks. For in- 

 stance, the thick bark of some of the conifers 

 offers impediments to the burrowing pro- 

 pensities of certain Scolytidae, although 

 others are not deterred by its presence. 

 Polygraphias major, a scolytid beetle which 

 attacks the blue pine {Pinm excelsa), only 

 infests the branches and upper portions of 

 the main stem. It is never found burrow- 

 ing into the thick bark of the tree. Simi- 

 larly, although it also infests the deodar 

 {Cedrus deodar a), it only attacks young sap- 

 lings and seedlings, and is much less happy 

 in this tree, finding the bark and wood too 

 hard for its purpose. Again, the thick rough 

 bark of the Pinus longifolia serves as a 

 deterrent to several bark-beetles which are 

 confined to the smaller branches and twigs 

 of the tree. 



The conifers when attacked or wounded 

 have the power of exuding masses of resin 

 w^hich are obnoxious to most mature forms 

 of insect life, although the grubs of the 

 same species can exist and flourish in this 

 resin. Numbers of the smaller Buprestidae, 

 Cerambycidae, and most species of the Scoly- 

 tidae which infest Coniferae, are drowned in 

 the outflow of resin put out in response to 

 the tunnelling operations of these insects. 

 The species alluded to tunnel down into the 

 bast (or wood) to prepare galleries in which 

 to lay their eggs. Healthy trees respond 

 to the burrowing of the insect b}- an out- 

 flow of resin which fills the tunnel and 

 drowns the insects. It is only when a tree 

 is so sickly that it has lost the power to 

 answer to such attacks by an outflow of 

 resin in sufficient quantity that the beetles 

 gain the upper hand and kill it. 



Again, the thick bark of a tree or the 

 thick shell or coat of the fruit or seed 



Fig. 7. — Young deodar root girdled 

 by root-feeding caterpillar. 



