126 FROGHOPPER BLIGHT OF SUGAR-CANE. 



Greater Attractiveness of Diseased Canes. 



Scattered throughout the writings on the bhght one finds continua 

 references to the j^ossibilit}- that the canes diseased or damaged by 

 other causes are more attractive to the fx'oghoppers than are healthy 

 canes ; and that when froghoppers are found in numbers in a blighted 

 field, it is possible that the infestation is the result and not the cause of 

 the trouble. 



Carmody (1909 p. Ill) commenting on an outbreak at Caroni in 

 1908 says " the virulence and concentration within a limited area of 

 the froghopper attack at Caroni were as surprising as the unexpected 

 abundance of the larvae of Castnia and strongly indicate the real cause 

 of the attack." 



In blighted canes the juice has been found to be acid. It is well 

 known that man.y insects are onlj- attracted to plants when some such 

 fermentation of the sap has taken place and Carmody (1. c.) suggests 

 that this occurs with the froghopper. 



Both CoUens and Hart suggest that froghoppers are more attracted 

 to canes already suffering horn root disease. 



I have often observed in the field the froghoppers more abundant on 

 fields or parts of fields where the canes are in poor condition or more 

 diseased, but a careful examination of each case has so far failed to 

 prove that the condition of the cane attracted the froghopper. In every 

 case it has been found more reasonable to explain the result by the 

 ascertained fact that the conditions of environment which produce 

 stunted and diseased canes are the same which allow the froghopper 

 to multiply to the greatest extent. This question is discussed below. 



DIRECT AND INDIRECT EFFECT OF ENVIRONMENT. 



A careful study of Section VIII brings to light a fact of great 

 significance. 



The evidence on the influence of coi^ditions on the blight indicates 

 that nearly all these act directly or indirectly through the amoxmt of 

 moist'ure present round the leaves and roots of the cane. 



With the rainfall this is directly* apparent, as also with the influence 

 of drainage. With the soil it is found that acid soils are more 

 frequently attacked, but this acidity is probably more an indication of 

 the physical condition that has brought it about : all the acid soils 

 were heavier and more impervious than the alkaline or neutral 

 undamaged soils. 



Agafn it is found that the date of cutting of the crop has an effect 

 on the numbers of the froghopj^er. After cutting the crop from a field, 

 however, that field is more exposed and hence dryer than while the 

 crop stands. It is also possible that the deposition of dew on standing 

 canes is much heavier than on fields in which the canes have been cut. 



The influence of the seepage of water from the pond in the field at 

 Hermitage already referred to must also be recollected. 



