29 



females before these are full size. This probably takes place early in July, the 

 exact date unfortunately was not determined. In August the female has reached 

 its full development, and its body is so distended with eggs that it occupies the 

 whole of the cavity beneath the covering. It then begins to lay its eggs, and as it 

 does so its body gradually gets smaller and smaller, until by the time they are all 

 laid it is so small that it can only with difficulty be seen beneath the small end of 

 the scale close to the eggs. The female dies very soon after laying all her eggs. 

 There is only one brood a year in Ontario, though in counties farther south there 

 fire two broods. 



Fig. 16. 1. Adult parasite {Aphelinus mytilas- 

 pidis) of Oyster-shell Scale. 2. Larva of para- 

 site in scale with scale insect on one side and 

 eggs on the other. 3. Pupa of parasite. All 

 much enlarged, (after Sherman and Metcalfe). 



Rate of Annual Increase. — Jis stated above, the average number of eggs per 

 female is between 40 and 50. If, as in the case of the San Jose Scale, we allow 

 for a mortality of about 50 per cent. — which I think is not too high for the Oyster- 

 shell Scale, though probably much too high for the San Jose — there will not be 

 more than 25 offspring from a single female in a year. The rate of increase there- 

 fore is small. Comparing this increase of 25 from one with 1,000,000 from one 

 on the part of the San Jose Scale, we can easily see why the latter scale is so much 

 more destructive and to be feared. 



Means of Distribution, 



The chief means of distribution for the Oyster-shell Scale, just as for 

 the San Jose, is the shipment of infested nursery stock. It is also spread 

 by the little cream-colored larvae crawling on 'birds, insects of various kinds, or 

 almost any kind of moving thing that comes in contact with them, and being carried 

 by these to other trees. As the time when larvae are active is only a few days 

 around the first of June, compared with a long period of over three months for 

 the San Jose Scale, it is clear that this latter method of distribution would not be 

 so rapid in the case of the Oyster-shell. 



Natural Enemies. — The natural enemies of the scale in Ontario are more 

 important than those of the San Jose Scale. Certain Ladybird beetle adults and 

 larvae feed to a small extent upon them, as do also a few mites; a reddish fungus 

 disease also occasionally does some good, but the only foes of real importance are 



