INSECTS 229 



cleansing the trees, and opening up or destroying the lurking 

 places of the eggs or insects, which thus fall a prey to their 

 enemies. The trees at Woburn were formerly infested every year 

 to an exceptional extent by various caterpillars, but the virulence 

 of these attacks have notably diminished during the last ten 

 years, and, as summer spraying has not often been practised, this 

 can only be attributed to 'the winter washing of the trees, the 

 caustic emulsion (p. 161) having been applied once in about 

 every three years. The efficiency of such treatment was strikingly 

 illustrated in 1915 : owing to a miscalculation, sufficient of the 

 wash had not been obtained to spray the whole of the ground, 

 and an acre of it had to remain untreated ; in the following season 

 the trees throughout the farm were remarkably free from cater- 

 pillar, except on this one acre, and there the caterpillar attack 

 was severe. 



