34 PROTECTION OF PLANTS, 1915-16 



Control Measures. 



Plant lice are capable of doing very serious damage because of their 

 great numbers and their remarkable ability of very rapid reproduction. 

 Although comparatively few stem mothers may hatch in the spring, yet, 

 if conditions are favourable, the increase may be so rapid that severe injury 

 may be done before it is observed. Then the lice may be so protected by 

 the curled leaves that it is difficult to reach them by any spray, even though 

 applied with great care and under very high pressure. From a close examin- 

 ation of the life cycle of these plant lice several vulnerable points of attack 

 may be noted. These are: — (1) the destruction of the over-wintering 

 eggs; (2) the destruction of the stem mothers when they are clustered on 

 the opening buds; (3) the destruction of the lice before the leaves are badly 

 curled, (in late May and early June) ; (4) the killing of the last generations 

 in September to prevent the deposition of eggs. 



Although many attempts have been made to reduce aphis infestations 

 by the destruction of the over-wintering eggs practically all have ended 

 in complete failure. Under normal conditions scarcely more than a small 

 per cent of the eggs ever hatch, yet these, in a few generations, leave such 

 hordes of offspring that in some years very serious damage is done. It 

 seems useless to enter into a critical examination of the experiments so far 

 undertaken, suffice it to say that no satisfactory spray mixture has been de- 

 vised which will destroy the eggs though applied with extreme care. Another 

 factor seriously limiting the usefulness of such a spray, even though capable 

 of destroying the eggs without injuring dormant trees, is the difficulty of 

 completely covering all the twigs and branches bearing the eggs, especially 

 as so many of the eggs lie hidden away in every conceivable crack or crevice 

 in the bark. It would thus seem that so far as our present knowledge 

 goes we may hope to secure better control by attempting to spray when 

 the lice are more vulnerable. 



The next point of attack would be w^hen the young, tender lice are clus- 

 tered on the opening buds. It is possible at that time to destroy all the stem 

 mothers and thus prevent further danger. If we could apply a spray 

 when we know all the eggs have hatched and at the same time none of the 

 leaf or fruit buds are so far opened as to offer protection from the spray, we 

 might hope for complete success. From our life-history studies it is well 

 established that the hatching of the eggs extends over a considerable period 

 of time, at least twelve or more days. As these eggs begin hatching just 

 as the buds are showing green at the tips and continue till the blossom 

 buds show pink it would seem to be impossible to do this by one spray. 



