684 ^^^ POPULAR SCIENCE MONTHLY. 



mies, a great number of which are known, a proper study of them 

 and the introduction of the most effective could result in no pos- 

 sible harm and might be productive of lasting good. 



There are two other laws which it is worth while to consider in 

 this connection. One is, that while a plant-feeder's natural ene- 

 mies are apt to cause its excessive abundance to be followed by a 

 corresponding decrease, yet this alternation of excessive abun- 

 dance and excessive scarcity will often be produced irrespective 

 of such natural checks. An injurious insect which has been on 

 the destructive march for a period of years will often come to a 

 sudden halt, and a period of relative and sometimes complete im- 

 munity from injury will follow. This may result from climatic 

 conditions, but more often it is a consequence of disease, debility, 

 and want of proper nutrition, which are necessary corollaries of 

 undue multiplication. Frequently, therefore, it may be inaccu- 

 rate and misleading to attribute the disappearance of a particular 

 injurious species to some parasitic or predaceous species which 

 has been let loose upon it, and nothing but the most accurate ob- 

 servation will determine the truth in such cases. The past year 

 furnished a very graphic illustration in point. Throughout Vir- 

 ginia and West Virginia, where the spruce pines have for some 

 years suffered so severely from the destructive work of Dendroc- 

 tonus frontalis, not a single living specimen of the beetle has been 

 found during the present year. This has been observed by every 

 one who has investigated the subject, and particularly by several 

 correspondents who have written to me : by Mr. E. A. Schwarz, 

 who was commissioned to investigate the facts, and by Mr. Hop- 

 kins, who has made the study of the sul)ject a specialty. The 

 clearest explanation of this sudden change is, that the species was 

 practically killed out by the exceptionally severe cold of last win- 

 ter, since such was the case with several other insects. Now, fol- 

 lowing so closely on the introduction by Mr. Hopkins of Clerus 

 formicarius, how easy it would have been to attribute the sudden 

 decrease to the work of the introduced clerus, had not the de- 

 crease been so general and extensive as absolutely to preclude any 

 such possibility! In like manner a certain scale-insect {Aspidio- 

 tus tenehricosus) had become exceedingly destructive to the soft 

 maples in the city of Washington last year, whereas the present 

 year it is almost entirely killed off, evidently by the same excep- 

 tional cold. Many of the affected trees were painted with white- 

 wash, with a view of destroying the aspidiotus, and the death of 

 this last might have been attributed to the treatment (and natu- 

 rally would be by those employing it) were it not that the same 

 result was equally noticeable on the trees not treated. Reports 

 from southern California would indicate that the red scale {As- 

 pidiotus aurantii) is in many orchards losing its destructiveness 



