140 i 1 ; \ Y 



spondence with the Secretary of the Board I,i nHtli-T <>f 

 should we expect the predaceous or parasitic forms to subdue their 

 hosts more effectually in America than they do in Europe, except in so 

 far as they were relieved, in the introduction into America, of whatever 

 enemies they possessed in their native home. 



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

 connection. One is, that while a plant-feeder's natural enemies are < pt 

 to cause its excessive abundance to be followed by a corresponding 

 decrease, yet this alternation of excessive abundance 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, immunity 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 inaccurate 

 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 observation will 

 determine the truth in such cases. The past year furnished a very 

 graphic illustration in point. Throughout Virginia and West Virginia, 

 where the spruce pines have for some years suffered so severely from 

 the destructive work of Dendroctonus 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. Hopkins, who has made the study of the subject a specialty. 



The clearest explanation of this sudden change is that the species 

 was practically killed out by the exceptionally severe cold of last 

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

 lowing so closely on the introduction by Mr. Hopkins of Glerus formi- 

 cariusj how easy it would have been to attribute the sudden decrease 

 to the work of the introduced Olerus had not the decrease been so 

 general and extensive as absolutely to preclude any such possibility. 

 In like manner a certain Scale Insect (Aspidiotus tenelricosus) had 

 become exceedingly destructive to the Soft Maples in the city of Wash- 

 ington last year, whereas the present year it is almost entirely killed 

 off, evidently by the same exceptional cold. Many of the affected 

 trees were painted with whitewash, with a view of destroying the 

 Aspidiotus, and the death of this last might have been attributed to 

 the treatment (and naturally 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 (Aspidiotus aurantii) is, in many orchards, losing its destructive. 

 ness through agencies other than its insect enemies, and in this case 

 the facts are particularly interesting because of the ease with which 



