350 ESSENTIALS OF BOTANY 
uses, and the younger trees should not be injured by let- 
ting those which are felled break them down as they fall. 
Many kinds of woodland may be kept for centuries in a 
condition of uninterrupted productiveness by thinning out 
only the largest trees. 
(2) Forest fires must be prevented, especially in conif- 
erous forests. This may be done by preventing fires from 
being kindled for any purpose in large tracts of woodland, 
and in some cases by cutting up woodlands, by means of 
roads, into wood lots of moderate size. In dry, hot weather 
constant inspection is needed to keep from spreading any 
fires which may occur. 
(83) Parasitic fungi and those which promptly seize upon 
newly felled wood should be destroyed by burning both 
them and the infected wood, taking suitable precautions to 
prevent the fire from spreading. 
(4) Destructive insects, such as the various wood-boring 
and leaf-eating species, should, if possible, be destroyed. 
In many cases the expense of killing the insect pests of 
forests would be too great to be undertaken on a large 
scale. But it is thought that sometimes this may be accom- 
plished by introducing into the region affected parasitic or 
other insects which will destroy the species injurious to 
trees! Attempts are now being made to get rid in this 
way of the gypsy moth, whose caterpillars strip both conif- 
erous and deciduous trees until the woods attacked by 
them look as if swept by fire. The importance of taking 
prompt action upon the appearance of a new insect-enemy 
of trees is well illustrated by the case of the gypsy moth 
1 A scale insect once very destructive in Californian orange orchards was 
successfully controlled by the introduction of an Australian lady-beetle which 
fed on the scale insects. 
