272 PARASITES OF GIPSY AND BROWN-TAIL MOTHS. 



Pteromalus began to issue coincidently with the beginning of the new 

 year, and they were at once supplied with a quantity of nests of the 

 brown-tail moth collected in the open and containing living cater- 

 pillars. In most cases the females almost instantly entered these 

 nests and oviposited upon the still dormant caterpillars (fig. 66) with 

 the result that in three and four weeks large numbers of a second 

 generation began to issue. This successful outcome to what was 

 considered to be, until that time, an experiment of rather doubtful 

 utility, was very encouraging, since it was at once evident that any 

 desired number of Pteromalus might easily be reared in captivity. 

 Accordingly the work of rearing it on a large scale was begun, with 

 the result that by the end of March American nests which contained 

 the progeny of some 100,000 individuals were available for coloni- 

 zation. 



Meanwhile large numbers of nests of the brown-tail moth — several 

 thousand, in fact — had been collected in the neighborhood of the 

 colonies which had been planted in 1906 and 1907 and no Pteromalus 

 issued from them. It was evident that the colonization experiments 

 of the spring of 1907 were no more successful than those of the spring 

 before, and it was no longer possible to consider the bad results as due 

 to the unusual mortality of the brown-tail moth in the vicinity of the 

 colonies. It was necessary to seek some other explanation for this 

 apparent failure to establish the one parasite which had been imported 

 and colonized in wholly satisfactory numbers, and it was thought that 

 this might be found in the circumstances under which the parasites 

 were reared and liberated. 



In 1906 and 1907 the adults had been liberated in the field some two 

 or three weeks sooner than they would normally have issued as adults 

 on account of their development having been hastened by the storing 

 of the nests of the brown-tail moth at an artificially high temperature 

 during the time that they were in transit from Europe. This, it was 

 believed, might be responsible for the fact that the species had failed 

 to establish itself and it was planned to do things very differently in 

 the spring of 1908. 



In accordance with these plans the nests containing the brood (as 

 well as quantities of healthy caterpillars) were placed in large tube 

 cages, which were fitted with a " tanglefooted" shield within, intended 

 to prevent the emergence of the caterpillars without hindering the 

 egress of the winged parasites, and four colonies, each of which was 

 estimated to consist at the very least of 50,000 of the parasite larvae, 

 were located in four widely separated localities in eastern Massachu- 

 setts. The cages were simply taken into the field and left, so that the 

 parasites were free from the moment of their emergence. 



Considerable trouble was experienced at first on account of the 

 "tanglefooted" shields failing to do all that was expected of them 



