256 PARASITES OF GIPSY AND BROWN -TAIL MOTHS. 



THE EGG PARASITES OF THE BROWN-TAIL MOTH. 

 THE GENUS TRICHOGRAMMA. 



The parasites belonging to the genus Trichogramma, of which sev- 

 eral have been reared from eggs of the brown-tail moth, are the most 

 minute of any which have been handled at the laboratory, and are 

 among the smallest of insects. The egg of the brown-tail moth is in 

 form of a flattened spheroid, approximately as large in its greatest 

 diameter as the printed period which ends this sentence. Normally 

 two or three individuals of the parasite pass through all of their 

 transformations from egg to adult upon the substance of a single 

 host egg, and in exceptional instances as many as 10 perfect adults 

 are known to have issued from one egg. This is the more remarkable 

 when it is remembered that the female Trichogramma is sexually 

 mature at the time of issuance, and ready to 

 deposit a large number of eggs for a new 

 generation. 



The mother parasite exhibits little discretion 

 in the selection of host eggs for attack (fig. 57), 

 and if any dependence is to be placed upon 

 observations which have been made in the 

 laboratory, she is quite as likely to select eggs 

 which contain caterpillars nearly ready to 

 hatch as those which are freshly deposited. 

 The feeding habits of her young are such as to 

 permit a considerable latitude in this respect, 

 but there is a certain limit, and after the em- 



FlG. 57 .—Trichogramma sp. in . , ' 



actofovipositioninaneggof bryological development has passed beyond a 

 the brown-taii moth. Great- cer tain point in the host egg, the attack by the 



ly enlarged. (Original.) r . >, ' • 1 if*. 



parasite is unsuccessful. It is much better 

 that the host egg be dead than that it contain a living embryo in the 

 later stages of its development. 



The life cycle, from egg to adult, varies very considerably in 

 length in accordance with the prevailing temperature. In the summer 

 it may be completed in as short a period as nine days, while in the 

 fall three weeks or more may be required. If the temperature falls 

 below certain limits the young parasites will hibernate or attempt 

 to hibernate, and thereafter their development may be delayed for 

 several weeks, or even months, even though they are exposed to 

 continuous high temperature during this period. 



After about one-third of the time requisite for the completion of 

 the life cycle has elapsed, the eggs begin to turn dark, and finally 

 become shining, lustrous black (fig. 58). This change is brought 

 about by the preparation of the larvae for pupation. 



