NOTES ON THE NATURAL CONTROL OF OYSTER-SHELL SCALE. 195 



Conclusion. 



By way of conclusion it may be said that overcrowding, parasitism by Aphelinus 

 inytilaspidis, and destruction by Hemisarcoptes malus, are in Canada the most 

 important factors in the natural control of the mussel scale. Of these the last is by 

 far the most important. As hundreds of the mites can be sent through the mail 

 on an apple twig it should be possible to colonize it in scale-infested places and 

 countries where it may prove to be absent from the local fauna. 



Appendix. 



As in the past several species of mites associated with oyster-shell scales have 

 been confused, it seems advisable to give a brief comparative account of those most 

 commonly found in Canada. These mites have been determined through the 

 kindness of Mr. Nathan Banks. 



The two mites most abundant are whitish or colourless and consequently somewhat 

 resemble the pearly white eggs of the scale. Of these one is Hemisarcoptes malus, 

 Shimer (fig. 2), which feeds on healthy eggs ; the other Monieziella angusta, Banks 

 (fig. 3), a scavenger on the carcase of the females and on the eggs killed and partly 

 eaten by Hemisarcoptes. At first glance these look somewhat alike, but closer 

 observation shows that Hemisarcoptes is turtle-shaped, fairly active, and supplied 

 with conspicuous tarsal suckers ; while Monieziella is an elongate, sluggish creature 

 supplied with weak tarsal claws and no suckers. Hemisarcoptes is invariably found 

 under scales containing at least some healthy eggs, while the scavenger is usually 

 iound under scales with no healthy eggs. The scavenging habit of Monieziella was 

 first established by Lignieres in France. In Canada, of several thousand scales 

 infested with this mite only two were noted in which the Acarid was feeding on living 

 eggs, in all other instances it was feeding on dead eggs or dead material. This 

 •scavenger was often more abundant than the predator, Hemisarcoptes. As many 

 -as eighteen and twenty have been found beneath a single scale, and sometimes 

 they are present under all the scales on a twig. 



The other mites found are coloured, ranging through shades of brown and red. 

 'Of these I have found four in some numbers. The smallest most active, and most 

 abundant, Tydeus gloveri, Ashm., is easily recognised by the presence of a rather 

 striking median vitta on the caudal part of the abdomen. It is more or less gregarious 

 in habits, fifteen or twenty sometimes collecting under a single scale. Though 

 widely distributed over the continent — I have specimens from Massachusetts, New 

 Brunswick, Quebec, Ontario, and British Columbia — it plays little or no part in the 

 ■control of this scale, as I have found it only under old scales where it had gone to 

 Tiibernate. This is probably the species identified as T. coccophagus by Ewing and 

 "Webster. Another of these coloured mites, Gamasus sp. (fig. 6), also found hibernating 

 under old scales and playing no part in control, is much larger than the above species. 

 Of these only three or four can huddle into one scale. It can be easily distinguished 

 by the presence of a pair of conspicuous whitish markings extending fully three- 

 fourths of the length of the body. It has appeared in New Brunswick, Quebec, 

 Ontario, and British Columbia. A third coloured mite, found hibernating in 

 considerable numbers under old scales at Covey Hill, P. Q., is one of the snout 

 (C507) b2 



