HOST RELATIONS OF COMPSILUEA CONCINNATA 11 



LIPARIDAE 



Among the host insects readily accessible to Compsilura in the 

 spring are species belonging to the family Liparidae. Two of these 

 in particular, the brown-tail moth (Euproctis chrysorrhoea L.) and 

 the satin moth (Stilpnotia salicis L.), are species hibernating as 

 larvae and periodic in their abundance. Both of these insects seem 

 to be firmly established in Massachusetts. Although chrysorrhoea 

 has several introduced tachinid parasites of importance, it would 

 appear that salicis has none of consequence except Compsilura. 

 Without doubt these species are primary hosts. 



The remaining species, the gipsy moth (Porthetria dispar L.), 

 the white-marked tussock moth (Henierocqmpa leucostigma S. and 

 A.), and the rusty tussock moth (Notolophus antiqua L.), hibernate 

 in the egg stage. As Compsilura does not habitually attack larvae 

 that have not reached the third stage, its attack upon these species 

 is somewhat later than upon those liparids which hibernate as larvae. 

 In abundance, P. dispar exceeds all of the others by far, and for this 

 reason would naturally be the most advantageous host for Compsi- 

 lura. Because of the great number of this host, Compsilura need 

 waste no time hunting for a more suitable one, but attacks this 

 species with a vengeance. With such an abundance of favored host 

 material, it follows that Compsilura could reproduce at best and 

 increase in such proportions that there would be at the close of the 

 gipsy-moth season the maximum number of individuals in the field. 

 This is most fortunate, for from now on there is no general infesta- 

 tion of insects to replace the gipsy moth and Compsilura must seek 

 out its host. 5 At times, naturally, there might be in certain localities 

 an ample supply of favored ones, but more often it would be a case 

 of finding a solitary or at least a less abundant species. On the other 

 hand, it may be that because of the fact that there are so many 

 Compsilura in the field the hibernating hosts in and adjacent to a 

 heavily infested area are eventually reduced to a minimum, thereby 

 causing a shortage of winter hosts. Since the amount of parasitism 

 of the gipsy moth by Compsilura is governed largely by the abun- 

 dance of the first generation, less parasitism should be expected in a 

 heavily infested area. An apparent corroboration of this reasoning 

 appears in the parasite records of the gipsy moth laboratory, which 

 show that the high percentages of parasitism are invariably from 

 the lightly infested areas. In the opinion of the writers, this con- 

 clusion is not justified by the facts, the records merely representing 

 a percentage method of reckoning. It seems certain that there is an 

 equal distribution of Compsilura over the entire infested area (fluc- 

 tuations from year to year in certain localities, due to various causes, 

 excepted) and that if the number of parasites were based upon the 



B A factor of considerable importance pertaining directly to this subject is that of the 

 dispersion of Compsilura. For several reasons the collections of native larvae examined 

 by the writers shed but little light on this point. Culver (2, p. ?') places the spread of^_ 

 Compsilura at approximately 25 miles per year, basing his claim upon scouting and larval 

 collections, rather an unsatisfactory way to obtain notes on the dispersion, but neverthe- 

 less about the only data there are to go by. Tothill (9, p. 39) found Compsilura 3 miles 

 away from the colony site two weeks after liberation. This definite record, together with 

 Culver's deductions, would indicate that the tachinid was a strong flier and that under 

 certain conditions a yearly spread of 25 miles might be expected. 



