242 SCALE-INSECTS, BARK-LICE, MEALY-BUGS. 
Enemies of Plant-lice and Scale-insects. 
These two families of insects, notwithstanding that they can 
multiply so rapidly, do not have everything their own way. A 
large number of beneficial insects seem to make it their life-work 
to destroy them. Besides true parasites belonging to the order 
of wasps (Hymenoptera), some of which are illustrated in Figs. 
193, 194, 195, which feed inside their tiny hosts, there are numer- 
ous cannibal insects which destroy immense numbers. Chief 
among these cannibals are the very useful lady-bugs. Some of 
Fig. 196.—Small lady-bugs eating San Jose Scale. After Div. of Entomology, 
Dep. of Agriculture. 
which have been illustrated in Figs. 196, 197. These beetles, 
both in their larval and adult stage, feed almost exclusively upon 
plant-lice and scale-insects, and if these injurious beings are not 
too numerous they are soon decimated or reduced to such an ex- 
tent as to become harmless. If it is seen that lady-bugs are found 
in large numbers among the lice and scales upon the infested 
trees it is frequently best not to apply insecticides at all, otherwise 
we are apt to injure our friends 
