423 



made in 1917 failed to elucidate the cause of the outbreak, but revealed 

 the fact that in many instances the attack had failed owing to the 

 resistance offered by the trees, the larval galleries being only very 

 short. Another bark-beetle [Ips acuminatus, Gyll.) is one of the 

 commonest species in the north of Sweden, occurring in trees with thin 

 bark, and seems in this part of the country to replace Myelophilus minor, 

 which also prefers the thin-barked portions of pine trees. There is, 

 however, a great difference between the two species in so far that 

 Ips acummatus oviposits in branches measuring as little as |— f inches in 

 diameter, whereas Myelophilus does not do so in branches of less than 

 about 1^ inches. The practical consequence of this is that the increase 

 of the former species is favoured by the smaller branches being left 

 on the ground. 



The adult moths of Bupcdus piniarius L., are very susceptible to _ 

 rain, being often drowned in great numbers. An example of this 

 occurred at Tuna in 1916. In the beginning of July enormous numbers 

 of the moths were noticed. An investigation made at the end of 

 July, however, elicited the fact that hardly any eggs had been laid, 

 which was explained by the torrential rain on the 6th July, 

 when nearly 2| inches fell in a single shower. The number of eggs 

 laid on each needle is said generally to be about 7, larger mmabers up 

 to 30 having also been found. An investigation made at Sorby in 

 Sodermanland showed that on most needles only 2-4 eggs occurred, 

 7 or more being very seldom found, though it is impossible to say 

 whether this is the rule in Sweden, the number of eggs laid on 

 each needle being probably influenced by climatic conditions. The 

 larvae always begin feeding at the top of the trees, probably because 

 the eggs are chiefly deposited there, and therefore, at least during the 

 first year of an outbreak, it is easy to discern a very badly defohated 

 upper part of the crown, whereas in the lower parts there is hardly any 

 damage at all. As a rule an outbreak reaches its climax in the second 

 year and ceases during the third year, and it is the general opinion of 

 entomologists that parasites are the controlling factor. Even if this 

 may be to some extent an exaggeration, in so far that climatic condi- 

 tions may also play a part, a study of the parasites during an outbreak 

 is of very great interest both from a theoretical and a practical point 

 of view. The outbreaks in different parts of Sweden have therefore 

 during the last years been studied with special regard to parasites, 

 the time permitting only the investigation of those species that are 

 found in the pupae in the spring. In Sorby in 1917 nearly 100 pupae 

 were found per square metre, 18 per cent, of which were parasitised. 

 In 1918 only 8-8 pupae were found per square metre, 60 per cent, of 

 which were parasitised, which shows the important part played by 

 the parasites. Both flies and Ichnemnonids were reared from the 

 pupae. Besides Barichneumon locutor, Thunb., and Cratichneumon 

 nigritarius, Grav., which were reared from the pupae, Plectocryptus 

 arrogans, Grav., a species reared in north Germany from pupae of 

 Bupalus piniarius was taken in one of the forests where an outbreak 

 occurred. 



In order to study the effect of the injury on the trees several plots 

 were laid out, the trees on which were repeatedly examined. At 

 Sorby in the spring of 1918 no pine tree had succumbed to the attacks 

 of the two previous years, in 4 per cent, of the trees Myelophilus 



