82 INSECTS AND HUMAN WELFARE 



lifeless shade tree, but a forest tree killed by such insects as 

 do not affect the wood, is per se, at the time of its death as 

 valuable for timber as one felled for that particular purpose. 

 There are two factors which modify such conditions. In the 

 first place, such trees as are accessible and marketable are not 

 necessarily those that are affected by insects, and the real loss 

 depends upon what immediate use can be made of the timber. 

 Following severe epidemics or invasions of insect pests, it is 

 usually impossible to make prompt use of the killed trees and 

 they begin rapidly to decrease in value. A case in point is the 

 recent killing of spruce and fir by the spruce bud-worm, re- 

 ferred to on a previous page. The location of the areas af- 

 fected is such that the utilization of the dead trees before they 

 have become worthless is utterly impossible due to the elab- 

 orate preparations which have to be made for logging on an 

 extensive scale. This is, unfortunately, almost always true. 

 Secondly, the deterioration of the timber in killed trees is due 

 in great measure to the activities of many insects which will 

 not attack living trees, but which follow in the track of va- 

 rious primary destructive agencies, and reduce or utterly 

 ruin, the value of the trees as timber. There are many species 

 of insects of this type present in comparatively small numbers 

 in all forests, where they find a limited food supply in the 

 scattered trees that have recently succumbed to old age, 

 overcrowding, lightning, windstorms, etc. As these causes of 

 death remain tolerably constant from one year to another, 

 there is ordinarily no great variation in the insect population, 

 which unites with saprophytic fungi in hastening the disinte- 

 gration of the dead or moribund trees. 



Any sudden destruction of trees furnishes extraordinary 

 opportunities for an increase of these insects, and they conse- 

 quently appear in a secondary wave following outbreaks of 

 insects like the Dendroctonus bark-beetles mentioned on a 

 previous page, or in the wake of severe windstorms, brush 

 fires and the like. For example, the pine forests of the gulf 

 states are periodically swept by violent windstorms, embody- 

 ing the unspent energy of West Indian hurricanes that have 



