54 Diagnosing of Diseases 
by the leaf-eating insects. The only trouble is, that when 
the damage has become apparent, it is either too late, or 
very difficult, for that season at least, to combat them. 
Since in these cases preventive measures alone are effica- 
cious, it is desirable to secure, in due time, evidence of the 
probable appearance of large numbers of these pests, or 
else, as a matter of precaution, to apply preventive measures 
(spraying poisons) even without that evidence. 
In the German forest administrations, trial collections 
of the hibernating insects are made every spring to deter- 
mine whether necessity for preventive measures exists. 
Cocoons, caterpillars, and beetles are collected from under 
the bark of stems and from under the moss and rubbish at 
the foot of the trees, and an estimate of their probable 
number is made. 
In parks, similar precautions can be practised to advan- 
tage, and the collection, instead of being a trial one, may be 
one directly destructive of the pest. 
Observation of other signs, which, however, require the 
eye of the practised entomologist or dendrologist, leads to a 
recognition of the impending danger. 
The number of insects observed in previous seasons may 
also be used as an indication of what to expect in the follow- 
ing year, since insect pests are apt to grow from year to 
year until a climax is reached and their progress checked 
by natural enemies or by an untoward season. 
In city streets, where only certain species of trees are 
planted and the same species lines street after street, the 
development of insects depending on that species is naturally 
favored: a city of elms is also a city of elm-beetles. Hence, 
one may expect here a yearly incursion, and preventive 
measures should be taken as systematically as street clean- 
ing, or as any other regular public function is performed. 
