204 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



his observations are of great weight and authority. In what 

 manner the natural balance was destroyed cannot be ascertained 

 with any degree of certainty, but it is evident that the calamity 

 did not start from a general centre like a locust plague, but that 

 the development was entirely local. The incredible increase in 

 the number of nun-moths took place simultaneously in the various 

 countries, and all reports of sudden invasions are purely 

 imaginary. The nuns come out of their pupas on the first warm 

 day after a coldish period, and the sudden appearance of enormous 

 masses of them gave colour to the theory of invasion. The 

 fact that the first reports of invasions on a large scale coincided 

 with the first really warm day of the season, and that this 

 sudden rise in temperature happened simultaneously in the 

 various countries attacked, entirely disapproves the theory of 

 invasion. 



The natural duration of a nun plague is usually accepted 

 as three to four years, but from the time the first moths appear 

 till the end of the calamity is probably nearer eight years. 



The question naturally arises, whether it be possible to lessen 

 this period by active interference, or whether, on the contrary, it 

 might not act as a stimulus and prolong the plague. As a matter 

 of fact, neither result follows. The laisser faire policy in East 

 Prussia, and the energetic measures adopted in Saxony did not 

 prolong or shorten the duration of the plague by a single day. 

 They ended at the same time, but the losses in East Prussia were 

 enormous, while Saxony practically suffered none. Nature causes 

 the beginning of the plague and engineers its end. 



Tachines and Ichneumides, and other enemies undoubtedly 

 destroy countless moths, larva and pupse, but the most radical 

 destructive agent is the sleeping sickness. Some nuns escape the 

 disease the first year but fall certain victims in the second. 



During the previous plague, rings of non-drying caterpillar glue 

 were largely applied in Prussia, under the then existing belief that 

 this measure would put an end to the calamity ; but the efibrts, 

 made when thousands of eggs had been deposited on each tree, 

 and therefore too late to be of practical use, naturally proved futile, 

 and the future appHcation of such rings was condemned wholesale. 



The general use of glue rings was adopted in Saxony not as 

 a destructive but merely as a protective measure, and again they 

 were applied in the early stages of the calamity. They resulted 

 in preventing an excessive increase of the moth, till Nature, the 



