NOTES ON THE NATURAL CONTROL OF OYSTER-SHELL SCALE. 193 



©vei-crowding on the upper side, there was scarcely a scale on the lower side ; it seems 

 that the inviting warmth of the sun may have been responsible for the scramble to' 

 the upper side of the twig at the time of migration. With the overcrowding carried 

 to an extreme the twig dies and with it the scales. When the crowding is less, a result 

 is that fewer eggs are laid per scale. Of a hundred female scales taken at random 

 from such, a twig the average number of eggs was five, whereas the normal would 

 have been nearer fifty. Female scales dwarfed in this way by overcrowding are shown 

 in fig. 5. 



Weakened by overcrowding the scales seem more susceptible to a fungus disease 

 that at times helps considerably in the control. L. Caesar (Ont. Agric. Coll. Bull. 

 no. 219) speaks of " a reddish fungus disease " that occasionally does some good. 

 On a small roadside apple at Fredericton the scales were so numerous that over- 

 crowding had occurred and a reddish fungus disease had killed about a third of the 

 females. A few other cases of a similar kind have come to my notice — all from New 

 Brunswick. In one instance the overcrowded scales on an apple near Millville had 

 nearly all been attacked. Twigs showing a similar condition have been given me by 

 R. P. Gorham. 



Parasites. 



Fitch was apparently the first to observe true parasitism of the scale. In 1856 

 he writes " Under these scales I have also repeatedly met with a small maggot . . . 

 of a honey-yellow color, and divided into segments by faintly impressed transverse 

 lines. This is probably the larva of some minute Hymenopterous insect, especially 

 designed by Providence for destroying the eggs of the bark-louse." In 1870 LeBaron 

 described this insect as Chalcis (Aphelinus) mytilaspidis. Since that time its habits 

 have become generally known and have recently formed the subject of an excellent 

 account by A. D. Imms (Quart. Journ. Micr. Sci., vi, pt. 3, March 1916). This species 

 and five less important parasites have been carefully studied on this continent by 

 L. 0. Howard. The five other parasites of the bark-louse are Aspidiotiphagus citrinus, 

 Craw, Aphelinus fuscipennis, A. abnormis, Anaphes gracilis, and Chiloneurus 

 diaspidinarum, all described by Howard. 



In Canada Aphelinus mytilaspidis was reared from the scale by Alfred Eastham 

 at Guelph, Ontario (Forty-first Ann. Rept. Ent. Soc. Ont., p. 74). It has also 

 appeared, often in some numbers, from material gathered for the present studies 

 from New Brunswick, Nova Scotia, Quebec, Ontario, and British Columbia. Here 

 it is the only parasite that seems to be at all efficient in control. As is pointed out 

 by both Dr. Howard and Dr. Imms, its effectiveness is unfortunately somewhat 

 undermined for the reason that the parasitised scales sometimes lay from two to 

 twenty eggs before being killed by the parasites. In England Imms found a parasitism 

 by this species of about 7 per cent. In the Canadian material examined the 

 percentage was usually less. In a few cases however it has been quite high. In 

 collections from Agassiz, B.C. , the percentage ranged from 10 to 40 per cent. Material 

 from the Experimental Farm at Ottawa showed one case of 40 per cent, and one of 

 49 per cent, parasitism by this insect. The highest parasitism that has come to 

 my notice was a case of 75 per cent, destruction of scales in Lake County, Illinois, 

 in September 1914. 



(C507) b 



