198 SCIENCE AND FRUIT GROWING 



an abundance of their food supply does not appear to ensure their 

 spread and multiplication. In 1904-5, when the mussel scale 

 was first investigated at Woburn, the chalcids were not very 

 prevalent, but in 1906-7 they were so numerous as to interfere 

 materially with the experiments : on the average 50 per cent, 

 of the scale eggs were destroyed by them, often every egg under 

 a scale having been depleted of its contents, though many of the 

 scales had escaped altogether. 1 Generally a more important 

 enemy of the scale is the tit, aided, perhaps, by some other 

 birds, and branches will often be found to have been quite cleared 

 of scale by them. Unfortunately the birds can reach only cer- 

 tain parts of the trees, so that their good offices can never be 

 effectual in ridding these of scale. 



From this brief account of the mussel scale it will be seen 

 that any spray-fluid used between September and the end of 

 May can do no good, unless it destroys the eggs, the insect itself 

 being already dead ; moreover the fluid, in order to reach the 

 eggs, must soak in under the scale. Perhaps the chalcididez just 

 mentioned are of more use indirectly than they are directly, 

 for they bore a hole in the scale, and this facilitates the entry 

 of any spray-fluid. A greater destruction of eggs by insecticides 

 was found in the case of scales which had been thus bored, than 

 of those which were intact. Under normal conditions, and in 

 the absence of spraying, less than 20 per cent, of the eggs hatch 

 out. 



It is evident that the time when the scale-insect is most vulner- 

 able is in June, shortly after it has hatched, and, if it can be 

 reached then, a mild insecticide, such as a weak paraffin emul- 

 sion, or even quassia and soap, might be sufficient; but it is 

 hardly possible to effectually spray the branches and spurs when 

 the trees are in leaf, and, moreover, the period in the summer 

 when the scale is vulnerable is a limited and variable one, and 

 the proper time for doing it might easily be missed by the grower. 

 The inefficiency of summer spraying in the corresponding case 

 of the San Jose scale has been recognised in America. 2 



Although it must be admitted that an insect which lives on 

 the sap of the tree cannot be otherwise than injurious, it is ques- 

 tionable whether the damage generally done by scale may not 

 have been exaggerated. In a plantation where scale exists, it 



1 See also Canada Expt. Farms' Report, 1903, 188. 



2 The case of this very serious pest is not quite similar to that of the 

 mussel scale, as the insect is generally viviparous. Much work has been 

 done on it in Canada and the United States, a summary of which will be 

 found in the Woburn Report, VI. p. 150. 



