INSECTS ETC. INJURIOUS TO VARIOUS CROPS IO65 



ber in the different districts was below that of 191 1. The reduction was 

 at first put down to exceptional circumstances. 



In order to secure exact data, the experiments were continued in the 

 spring of 1913, on the same number of plots as in 1912. The number of 

 larvae had again diminished. 



The results obtained in the autvimn of 1911. in March and April 1912, 

 and in March 1913 for all the stands showing 200 or more larvae on the area 

 shaded by one tree were compared with each other. 



It was found that taking as a basis the number of larvae found in 

 1911 = 100, this number had fallen off 19.5% for the spring of 1912 and 47.5% 

 for the spring of 191 3. 



From this it is evident that the number of larvae decreased in the course 

 of the year observed by the Writer. 



Control measures were only undertaken in 1913 ; therefore the reduc- 

 tion was due to natural factors. Lang has shown that in the forests of 

 Upper Ffanconia, the larvae of Raphidia ophiopsis can attack the eggs and 

 young larvae of Lvda. Yet the reduction in the number of Lyda could not 

 be put down to this enemy, as it was only rarely found in the excrement 

 of the insect. Traumatic causes, such as injury to the larvae, were also not 

 responsible for the reduction in number. Nor were there more parasites, 

 as it was observed in 1912-1913 that a large number of larvae shvit up for 

 several weeks contained only a small number of ichneumonidae. The Writ- 

 er however, at the end of 1914, found ichneumonidae in some parts of 

 the forest in no less numbers than the Lyda flying in June. The outbreak 

 of war however prevented him from ascertaining whether these ichneumon- 

 idae were connected with the appearance of Lyda. Quite possible there 

 is a connection, as Baer claims to have observed that the ichneumons [Poly- 

 cinetis aethiops) attack the almost full-grown larvae of Lyda. 



The damage caused by Lyda to the epicea population of the forest 

 of Roggenburg was very great, but not sufficient to bring about the death 

 of the trees. The tops which were bare the spring of 191 2 regained their 

 greenness, and in the following year the damage had to a great extent disap- 

 peared. Only the branches in the lower part and interior of the tree 

 top, where the sunlight is verj^ feeble, retained their bare appearance. Other 

 circumstances, such as the presence of a large number of Bostrichids, were 

 not observed. It was likewise not possible to determine to what extent 

 the growth of the trees had suffered through the influence of the insect. The 

 increase in size depends primarily on the length of time during which the 

 insect carried out its destructive work. It is generally supposed that the 

 larva destroys the needles during June, July and the beginning of August 

 and afterwards shelters in the ground. It is pointed out that this observa- 

 tion is only partly correct. There are some larvae which remain longer on 

 the tree than others. In one case, the Writer observed that certain larvae 

 continued to destroy the needles right up to December. 



For effective control of Lyda it is essential to know in good time when 

 they will fly. On the occasion of the flight in Upper Franconia it was ob- 

 served that the larva of Lyda undergoes no external change before con- 



