Albert Howard 385 



trolling attacks of green-fly is to spray the trees with some wash in which 

 soft soap is one of the chief constituents. In practice, however, this 

 procedure is of limited value as the affected trees soon become re-infected 

 and the process has to be repeated, an important matter now that labour 

 is becoming so expensive. It is generally considered that the only cause 

 of green-fly attacks is the insect itself. If this is so, there seems no 

 reason why the pest should not spread rapidly in any garden after its 

 first appearance in the spring. This was not the case in the Quetta fruit 

 experiment station, where this pest has been under observation for some 

 years. Frequently trees remained quite free from green-fly in the 

 immediate neighbourhood of others badly affected, and this has hap- 

 pened year after year. It would appear, therefore, that something else 

 besides the green-fly is necessary for successful infection to take place. 



Some light has been thrown on the conditions necessary for green-fly 

 attacks as the result of a number of irrigation experiments at Quetta. 

 Following American experience on certain soils, an attempt was made 

 to store up water in the subsoil during the winter and spring for use 

 during the subsequent hot weather, when water is very scarce. The 

 experiments were successful as far as the saving of summer watering 

 was concerned but the system had to be given up on account of the rapid 

 increase in green-fly attacks which accompanied the winter irrigation. 

 After the summer, only one watering is now given in October so as to 

 ensure a sufficient supply of moisture in the soil to prevent the freezing 

 of the roots by early frosts before the winter rains begin. The cessation 

 of winter watering has at once been followed by the recovery of the trees 

 from green-fly attack. 



Among the experiments which have been conducted on this subject, 

 the following may be quoted: 



1. Four heavy winter irrigations were applied to three plots of 

 peaches and one of nectarines during the winter of 1915-16. In all cases, 

 the trees were very badly attacked by green-fly during April, 1916, and 

 the attack was much more severe than in the neighbouring gardens. 

 Further watering was then stopped and the soil round the trees was 

 broken up down to the upper roots. In this way aeration was restored, 

 and after about a month the new growth produced was free from Aphides. 

 The trees then presented a remarkable appearance. The first-formed 

 leaves on each twig showed extensive damage by Aphides, the late 

 formed leaves on the same branch were normal and perfectly healthy. 



2. The above four plots were treated in quite a different manner 

 during the winter of 1916-17. After the summer, only one watering 



