42 



FROGHOPPER BLIGHT OF SUGAR CANE. 



DISCUSSION. 



From an examination of the above records the following points of 

 interest will be seen. 



(1.) The dates of the broods vary a little each 3'ear and are related to 

 the date of the commencement of the dry season {see fig. 25 and p. 102.) 



(2.) The relative size of the broods varies from place to place 

 and from year to year, 



(3.) The second brood is usually the most injurious, while the third 

 brood is often unexpectedly small. 



. (4.) There appears to be a recmn.-ence of periods of severe damage at 

 intervals of four or five years. The damage in the years 1906-1919 is 

 shown diagramatically in Fig. 7, and this regular periodity is well 

 marked. "WTiether it is accidental or significant cannot be deteiTnined 



Fig. T 



Fig. 7 — Diagram of intensity of ^Blight during the years 1906-1919 show"ing 

 the periodic nature of its occurrence, 

 without a longer series of records. Similar valuations over long periods 

 are known in other pests, but the}' are usually put down to the gradual 

 increase of insect enemies. In this case this explanation does not seem 

 to hold as each dry season most 'of the enemies are reduced and each 

 wet season they increase rapidly in successive broods so that they are 

 usually sufficient to control the third brood of froghoppers. 



