184 JOHN D. TOTHILL. 



the officers and friends of the Entomological Branch and mailed to the Dominion 

 Entomological Laboratory at Fredericton, N.B., for examination. The names of 

 these gentlemen appear in the analysis table (on page 191) opposite their respective 

 collections and it is a pleasure to acknowledge their kindly assistance. Each 100 

 of these egg-masses was, so far as practicable, an average 100 selected from 

 several twigs of one tree or bush. So far as possible, collections were made from 

 five trees or bushes at each observation point. 



From a study of the material, an analysis of which is given on page 191 , it was found 

 that a mite was by far the most important single factor in control, and most of the 

 paper is given over to a discussion of this organism. It has been found convenient 

 to speak first of the various agents destroying the egg stage and then of those 

 destroying the postembryonic or summer stages. 



Control in the Egg Stage. 



Desiccation. 



It seems to be fairly certain that desiccation of eggs in the fall occasionally plays 

 a small part in control. In scales that are not in uniformly firm contact with the 

 bark — owing to wrinkling or other irregularities of the surface — some or all of the 

 eggs are exposed to an air current, and one is apt to find that they have shrivelled 

 up and turned brown. In one case as many as 25 per cent, of the eggs on a roadside 

 apple bush at Fredericton had shrivelled in this way. As a rule, however, one rarely 

 finds more than 2 per cent, or 3 per cent, of such eggs. They have been found in 

 material from New Brunswick, Quebec, Ontario, and British Columbia. That the 

 cause of death is desiccation is indicated by the fact that healthy eggs have been 

 induced to shrivel and turn brown when exposed on the laboratory table. 



Weather. 



Winter killing of eggs has been recorded in Iowa by B. L. Webster {Jl. Earn. Ent., 

 Dec. 1912 and June 1915). At Ames, on 12th January 1912, the temperature dropped 

 to 35° F. In the spring Webster noticed " that many eggs under the oyster-shell 

 scales were yellowish in color instead of the usual white " and infers that the low 

 temperature in January was the cause ; he also says " it would appear that eggs 

 might safely withstand a temperature of -31° F., and yet succumb at -32° F." 

 A temperature of -32° F. is seldom experienced in the apple-growing sections of 

 Canada and consequently the winter killing of eggs here is probably quite unusuaL 



Parasites. 

 The eggs of this scale appear to be free from parasites. So far as I have been able 

 to discover there are no published accounts of true egg parasites, and in my own 

 examinations no cases have come to hand. This freedom is not remarkable in view 

 of the minute size of the mussel scale egg. Sometimes one finds a few healthy eggs 

 at one end of a scale and the lemon-yellow hibernating larva of Aphelinus 

 mytilaspidis at the other. This condition, as A. D. Imms points out in his interesting 

 paper on this Chalcid (Quart Jl. Micr. Sci. lxvi, pt. 3, March 1916), is probably due 

 to the fact that the female managed to lay a few eggs before being killed by the 

 parasite. It is therefore not a case of egg parasitism or even of egg destruction. 



