HOW TO COMBAT THE ATTACK OF INJURIOUS FOREST INSECTS. 7. 
too large quantities of viscid resinous substances ; neither are trees 
infested which have been dead for a year or so, doubtless owing to 
the tissues being no longer suitable for the nourishment of the 
insects and their young. Why pines, and especially the Scots pine, 
are utilised by the insect for breeding purposes in preference to 
other trees can only be explained by the assumption that this 
particular insect has become gradually adapted for feeding in trees 
of this particular genus, just as the turnip fly only attacks crucifers, 
or Scolytus Ratzeburgi only birches. 
Suppose, now, that we had taken our incredulous friend into a 
wood in April or May, and, having found a Scots pine which had 
been blown down or felled during the previous winter, were to 
show him the characteristic galleries of the pine beetle underneath 
the bark, he would most likely remain unconvinced of our having 
made out a strong case against the insect. He might say, and so 
far it would not be easy to contradict him, that surely the boring 
of even a large number of small passages underneath the bark of 
trees already dead, could do no great amount of harm, more 
especially as even the surface of the wood was hardly touched. 
If we were to cease our attempts at conversion at this point, we 
would be forced to acknowledge defeat, but the better course to 
pursue would be to pause and consider what would happen if the 
insects were to be placed in circumstances which would enable 
them to increase in numbers at a rapid rate. Now, in the absence 
of exceptional disturbing causes, practically the only limit to the 
increase of this beetle, and of many other forest insects, is inter- 
posed by the want of proper material in which to oviposit ; that is 
to say, if a sufficient number of old Scots pines in a suitable state 
be provided, conditions are offered which enable the insect to 
increase in numbers to almost any extent. The most common 
cause of the production of a large quantity of breeding material 
is a severe gale, or a succession of severe gales such as we experi- 
enced in Scotland some years ago. At that time, as will be remem- 
bered, whole woods were levelled with the ground over wide areas 
of country, and for some years afterwards the timber could neither 
be cut up nor marketed. That, then, was an opportunity for //. 
piniperda, as well as for all bark beetles, to increase at a prodigious 
rate, and one which the results show they were not slow to avail 
themselves of. Let us look for a moment at the rate of increase of 
which such an insect as the pine beetle is capable when placed in 
the possession of an almost unlimited supply of breeding material. 
