CROP NUTRITION 269 



those regularly affected by the insect are less than 60 per cent, 

 saturated. There seems to be no correlation between gross lime- 

 content and susceptibility to insect damage, and there is stated to 

 be little doubt that the soils of areas liable to severe froghopper 

 attacks are lacking in combined lime (exchangeable calcium), and 

 therefore usually give a definitely acid reaction. The reason why 

 damage prevails under these conditions has not so far been 

 elucidated. Both Williams (1921) and Withycombe (1926) have 

 advanced the theory that the severest injury is largely dependent 

 upon the water-relations of the crop. P'ield observations, 

 subsequently carried out, show that differences between the acidic 

 and alkaline areas are reflected in the moisture contents of the 

 leaves of the cane-plants growing on these areas. In the 

 unfavourable areas the transpiration rate of the canes is increased, 

 which in itself leads to rapid drying-out during the afternoon and 

 evening, which is the time when the froghoppers feed. It has 

 been further noted that a field survey of canes, growing in soils 

 known to favour froghopper damage, yielded evidence that 

 marked amounts of iron and aluminium accumulate in their 

 nodal tissues. Toxic aluminium, for example, often begins to 

 appear when the degree of lime saturation falls below 60 per cent., 

 and this may markedly increase the plants' susceptibility to 

 insect attack. 



The foregoing empirical observations serve to indicate how 

 closely susceptibility to froghopper damage is correlated with 

 factors relating to the soil and to plant nutrition ; for further 

 information the reader should consult the Froghopper Committee's 

 publications (1925 onwards). 



2. The extensive observations conducted by Frew (1924), at 

 the Rothamsted Experimental Station, have a direct bearing on 

 the influence of the nutrition of barley upon attacks by the gout-fly 

 (Chlorops tamiopus). The flies commence to appear in May, and 

 deposit their eggs on the leaves of young spring-sown barley. 

 They are usually laid singly, on the upper side of a leaf, and it is 

 rarely that a shoot bears more than one egg. Upon eclosion the 

 larva bores its way downwards into the centre of the shoot, and 

 eats a groove along one side of the developing ear and to the 



