71 



was found to be more prevalent in Maryland near the District line 

 than on the grounds of the Department of Agriculture. In the latter 

 place there was practically no infection worth mentioning. 



A very large proportion of cabbage loopers was affected by the 

 minute parasite Copidosoma truncatella, perhaps 15 per cent during 

 September, but none in earlier and less in later months. 



Observing that the larvae were most extensively affected by the 

 Copidosoma parasite in a region badly infected with rot, a number 

 of healthy larvae were placed on potted cabbage affected with both 

 the bacterial and brown rots, while others were kept as a check lot 

 on fresh cabbage, this experiment being made to ascertain if the rots 

 were in any way responsible for the diseased condition of larvae. 

 Somewhat to the writer's surprise it could not be seen that the 

 larvae placed with the diseased plants were affected in any manner 

 more than those kept under the same conditions with perfectly healthy 

 plants. 



The melon plant-louse, Aphis gossypii Glov. , affords a striking 

 example of the combined effects of weather and natural enemies in the 

 control of an insect. Of all common plant-lice this species appears to 

 be most susceptible to climatic variations. During moist or humid 

 weather, particularly in the early portion of the summer, this species 

 is capable of propagating in the greatest numbers, but during pro- 

 tracted heated and dry spells, such as happened in the season of 1900, 

 its natural enemies, which are legion, are able to keep it almost com- 

 pletely under control. During the year it was not reported at this 

 office as doing any damage save in one locality in Nebraska, a State in 

 which it does as much if not more damage than an}^ other in our 

 county. In the year 1899 this species was very destructive in the 

 States of Florida, Texas, Maryland, Virginia, Pennsylvania and 

 Georgia, and District of Columbia, while the previous year it did dam- 

 age over much the same territory, as well as in Kansas and Arkansas, 

 injury being particularly pronounced in Texas. 



SOME GENERALIZATIONS. 



As a result of study of the subject of the effects of weather upon 

 different species of injurious insects which occur in the neighborhood 

 of the District of Columbia during the past year in connection with 

 observations that were made the previous year the writer has deduced 

 certain conclusions. One of these, not expressed in the earlier article 

 on this subject, is that there is a tendency on the part of introduced 

 forms to develop one or more generations in their adopted habitat 

 than native northern species produce, a habit which conduces very 

 largely to their destruction, resulting in a corresponding decrease in 

 their numbers. 



