130 



FROGHOPPER BLIGHT OF SUGAR-CANE. 



E and F are unusual types in which the first brood is larger than 

 any of the others. E has occurred for two successive years (1918 and 

 1919) under my observation in the same area of Uba cane at Harmony 

 Hall Estate. F is illustrated by the captures at La Fortunee Estate in 

 1918 (see Fig. 5). 



The periodicity of the bad outbreaks, which appear to recur at 

 intervals of four or five year's {see )j. 42) is also at present without 

 explanation. Four or five years is approximately the agricultural cycle 

 for cane-gi'owing (planting to planting) and bad lands damaged in old 

 ratoons will be in the same unresistant condition after an interval of 

 four or five years. Although this will undoubtedly have its influence, it 

 does not explain all the facts, as for example the widespread nature ot 

 the blight in bad years, when its selective capacity is much lees 

 ajiparent, and good fields and bad fields, plants and ratoons are affected. 



Fig. 



Fig. 31 — Diagram to explain liability of various fields to blight in different 

 j'ears. For explanation see text. 



Fig. 31 presents a diagrammatic scheme of the way in which this 

 selection varies in different years. The total height of verti(;al columnt 

 represents the liability to damage of the various types of fields at 

 various periods of growth. The horizontal lines show the severity of 

 blight in that year, any column above this line indicating that the typ« 

 in question is damaged^ 



We see that in very good years only very bad fields are attacked as 

 old ratoons, in bad years all fields below the average are attacked even 

 in some cases as plants and only the best fields escape, while in very bad 

 years even good fields may be blighted. This scheme refers only to 

 attacks developing in the field and not to damage caused by afroghopper 

 invasion from neighbouring lands, which may obviously occur in the best 

 fields in the best years. 



CAUSE OF RECENT PREVALENCE OF BLIGHT. 



It will be seen from the history of the blight in Trinidad given in 

 Section III that it has apparently been more severe and of more frequent 

 occurrence in the last fifteen years than formerly, although serious 

 attacks did previously occur at longer intervals. 



