CROP NUTRITION 271 



downwards, reaches the ear ; such leaves may, therefore, be 

 termed " critical leaves." A leaf arising from a shoot half-way 

 up the ear is to be regarded as a " half critical leaf," since the 

 larva would only be able, at most, to attack the base of the 

 ear (Fig. 74). 



The permanent barley plots, on the Hoos Field at Rothamsted, 

 have yielded significant data indicating that the summer infesta- 

 tion by gout-fly shows a wide range of intensity, according to the 

 manurial treatment adopted. Frew's observations indicate that 

 manurial treatment affects infestation by altering the early growth 

 of stem and ear, and, consequently, by altering the number of 

 " critical leaves " present at the time the insect is egg-laying, and 

 during the period of incubation of eggs already deposited. Field 

 counts showed that the number of critical leaves per shoot was 

 lowest on plots least affected by the insect, and vice versa. Two 

 years' observations, conducted on the twenty-eight plots of the 

 Hoos Field at Rothamsted, involved the counting of over 60,000 

 separate barley shoots, and the noting of the numbers infested by 

 gout-fly in each case. It became evident, as the result, that plots 

 receiving superphosphate, farmyard manure or complete mineral 

 fertilisers,! were remarkably constant in showing the lowest 

 percentage of infestation. Untreated (control) plots were heavily 

 infested, while those treated with nitrogenous fertilisers indicated 

 that the latter, in some cases, exercise a beneficial effect, but that 

 heavy dressings do not conduce to reduction of infestation, and 

 may increase it through retarding the growth of the ear. The 

 application of nitrogenous fertilisers, in conjunction with either 

 superphosphate or complete minerals, however, proved markedly 

 beneficial in each case. 



3. A further example of the influence of differential nutrition 

 of crops, in relation to insect damage, is afforded by the 

 experiments of Jepson and Gadd (1926) in Ceylon with reference 

 to the shot-hole borer (Xyleborus fornicatus) of the tea plant. This 

 insect injures about 80 to 90 per cent, of all tea bushes growing 

 below an elevation of approximately 4,000 feet. The insect does 



1 These comprise sulphate of potash, superphosphate of lime, and sodium 

 and magnesium silicates. 



