THE ENEMIES OF ENGLISH WOODLANDS 275 



repeated attacks no doubt weaken them to a dangerous 

 extent, although whether weakening of already stunted trees 

 can be called a serious iajury is open to question. 



As preventive measures which may be adopted when 

 forming a young plantation, may be mentioned the mixing 

 of oak with other species, such as beech, hornbeam, and so 

 on ; but probably the greatest aid to the forester lies in the 

 fact that the sessile-flowered oak is rarely, if ever, attacked, 

 and on dry soils it is undoubtedly the proper species to 

 plant, as much on sylvicultural grounds as to prevent insect 

 attack. Eeliable evidence points to the fact that this variety 

 makes much more growth and more healthy growth than 

 the pedunculate variety on dry sandy or gravelly soils, where 

 the results of attack by the moth are most serious. Beyond 

 such measures as these little else can be done, for we have 

 not yet got to that stage in entomological bacteriology when 

 a few phials of the germs of some deadly disease can be 

 squirted over the caterpillars, and set an epidemic to work 

 which will destroy them in the course of a few weeks. 



One reassuring feature of the attack is the irregularity 

 of its occurrence. For one, two, or three years it may be 

 widespread, and then for several years following practically 

 disappear. This may be accounted for by the existence of 

 some epidemic disease, as suggested above, which affects the 

 caterpillars, parasites which destroy the eggs, or by the 

 absence of oak-leaves on which the caterpillars can pupate, 

 and which compels them to choose less suitable sites, and 

 indirectly aids in their destruction. The idea prevails to 

 a great extent that cold, dry weather has something to do 

 with bad attacks. It may aggravate them, no doubt, but 

 the real cause must lie in the number of eggs laid in the 

 previous season, and their successful hatching out into cater- 

 pillars ; and this leads to the inference that bad attacks are 

 preceded by ordinary ones until the climax is reached, and 

 some cataclysm occurs which reduces their numbers to a 

 minimum, and they practically disappear for a time. 

 Amongst birds, rooks are fond of these caterpillars and 

 get rid of a good many, and the same may be said of 

 starlings and other insectivorous birds. 



