75 



ON A NEW METHOD OF ASCERTAINING THE PARASITES OF THE 

 RESPECTIVE HOST-INSECTS IN A MIXED INFESTATION. 



By Ivar Tragardh. 



Swedish Institute of Experimental Forestry, Entomological Department, 

 Experimentalfdltet, Sweden. 



When studying the injurious insects of a plant it is, as a rule, easy when several 

 insects occur together to isolate them and breed them separately in order to ascertain 

 their parasites. In some instances, however, this method is unworkable, and this 

 applies particularly to the often very complicated fauna of the cones of the coniferous 

 trees. 



During my recent investigations into the insects injurious to the spruce-cones of 

 Sweden * this fact was very distressing. The researches were based on a material 

 of some 14,000 cones collected from about 70 different localities distributed all over 

 the country, and I had the hope of being able in this way to find at least in some 

 localities cones attacked only by one species and thus to secure reliable data as to 

 the parasites of the different species. 



This did not, however, prove to be the case, all cones as a rule containing at least 

 two or three different injurious species, which rendered it impossible to draw any 

 conclusions as to which were the hosts of the different parasites that emerged. 

 Moreover, it was, at least as regards the seed-eating species, out of the question to 

 isolate the different forms, since the seeds could not be separated according to the 

 species which attacked them. 



It became therefore urgent to find another, indirect method of solving this problem, 

 and the system employed will now be briefly described. 



The cones were kept in breeding-cages of the so-called American type, and the 

 insects emerging from them were collected daily or every second day. Diagrams 

 were then made illustrating what percentage of each species emerged during each day. 



I took as my basis the supposition that a certain relation existed between the 

 time of emergence of the host and its parasite ; the latter being adapted both morpho- 

 logically and biologically to its host and appearing invariably at the moment most 

 suitable for its successful propagation. This relation might, of course, be different 

 according to the instar of the host which was attacked by the parasite. 



It is true that the insects were not subjected to natural conditions as regards 

 temperature, light and humidity in the breeding-cages, their development being 

 doubtless much accelerated. But owing to the deplorable lack of an insectarium 

 in the Institute it was impossible to make suitable arrangements for the breeding- 

 cases, which had to be kept in the laboratory subjected to a temperature of about 

 +17° C. 



* Investigations into the insects injurious to spruce and vine cones. — Report of the 

 Swedish Institute of Experimental Forestry, vol. 13-14, pp. 1141-1204, 44 figs. — 

 Stockholm, 1913. 



