228 SCIENCE AND FRUIT GROWING 



In all such work with insecticides, the need for caution in 

 drawing any definite conclusions is continually being impressed 

 on the worker ; the relative superiority of two given insecticides 

 may repeatedly be found to reverse itself in different seasons, 

 according to the special conditions obtaining at the moment. 

 Thus the superiority of arsenate over paraffin emulsion, indicated 

 by the experiments on 300 apple trees quoted on p. 226, is not 

 borne out by the results with the gooseberry sawfly, nor by other 

 results obtained in 1908 with the caterpillar of the common 

 vapourer moth and the small ermine moth (X, 35) ; whilst the 

 effectiveness of nicotine as indicated by the laboratory experi- 

 ments, was not borne out by field trials with it on the winter 

 moth caterpillar, no mortality having been caused by solutions 

 containing up to o-ii per cent, of that substance (X, 39). 

 Similarly, though emulsions made with iron were as effective as 

 those made with copper according to the experiments already 

 quoted, the reverse was the case in some subsequent trials 

 (X, 38), indicating that the copper may have to a certain extent 

 a poisonous action on the insects. 



The superiority of a paraffin emulsion to lead arsenate, in cases 

 where such superiority was noticed, is, however, partially due to 

 the fact that a stronger emulsion was used in the later than in 

 the earlier experiments (1-5 instead of i per cent, of oil), and the 

 superiority was noticeable both as regards the percentage of 

 mortality ultimately caused, and the rapidity of the action (X, 38). 

 The emulsion has proved eminently, satisfactory in dealing with 

 the winter moth, but, unfortunately, it cannot be used when 

 the blossoms are expanded, and, where psylla and winter moth 

 have both to be dealt with, and where separate sprayings for each 

 cannot be afforded, it would be best to defer the spraying till 

 after the blossoms have fallen, and then to use an emulsion with 

 0*075 per cent, nicotine added to it. Nicotine and arsenate 

 may be applied to trees in blossom without injuring them, but 

 such a mixture would injure the bees visiting the flowers. 



In combating caterpillar and other insect pests we can hardly 

 overrate the truth of the saying that " Prevention is better 

 than cure." The insects ought, if possible, not to be allowed to 

 come into existence at all, for to kill them once they are swarming 

 on the trees, and these trees are in leaf, is by no means a simple 

 problem. The efficacy of winter washes in actually destroying 

 the eggs of many caterpillars, etc., is very doubtful, but there is 

 no question that winter washing is the most effective way of 

 dealing with such pests, acting chiefly, in all probability, by 



