282 THE PRACTICAL APPLICATION OF ECOLOGY 



with regard to the apphcation of acidic fertihsers are not in com- 

 plete accordance with opinions in Germany. 



It may be added that the beHef that the acidity of the cell-sap is an 

 important factor in relation to the attacks of sucking insects is by no 

 means new. Petri in 1911 claimed that it played an important role 

 with regard to the infection of vines by the Phylloxera, and Comes in 

 1914 held that it presented means of defence of various plants, more 

 particularly against Coccidse. 



The factor, or factors, responsible for immunity to woolly 

 aphis attack in Northern Spy and other varieties of apple are 

 discussed by Roach (1937). In the experiments described by this 

 author indications were obtained which point to the conclusion 

 that immunity is probably a biochemical problem and that it is 

 not due to mechanical or other properties rendering such varieties 

 more or less impenetrable to the stylets of the insect. The 

 woolly aphis was reared on artificial media prepared from the bark 

 of Northern Spy and of susceptible trees. It was found that the 

 development of the insects, when fed upon media of this kind, 

 corresponded to their development on living trees in the smaller 

 number and shorter life of the progeny associated with immune 

 extracts. The results seem to show that the cause of immunity 

 (or susceptibility) resides in some chemical substance insoluble in 

 alcohol or ether and only slightly soluble after prolonged boiling 

 in water. The behaviour of the insects on the media suggests 

 that, whatever the factors concerned may be, they are not 

 necessarily destroyed when the tissues containing them are 

 killed. 



2. The problem of the relative degrees of resistance of different 

 varieties of oats to attacks of the frit-fly (Oscinella frit) has, in 

 recent years, received considerable attention in England (vide 

 Cunliffe and Fryer, 1924). As to what is exactly implied by 

 " resistance " in this case little is definitely known, and it is 

 gauged by the relative freedom from infestation of different 

 varieties grown under similar conditions. Direct resistance 

 implies that the plant can resist entry by the larva of this insect, 

 or that it provides conditions so unfavourable as to cause the 

 death of the larva before material injury is sustained. On the 



