54 Diagnosing of Diseases 



by the leaf-eating insects. TItj only trouble is, that when 

 the damage has become a{)parent, 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 preventi\'e 

 measures should be taken as systematically as street clean- 

 ing, or as any other regular public function is performed. 



