INSECTS 225 



may be quoted, the numbers entered giving the percentage 

 mortality consequent on the spraying 



Nicotine '075 '038 '019 'oio '005 per cent. 



Mortality caused. 



July 2 . . 100 100 TOO 106 

 23 . .30 5 90 10 



August 6 . . 100 70 5 i o 



September I . 90 63 53 30 35 



The general results confirmed the previous conclusion, that the 

 strength of solution most suitable for use was 0*075 per cent. ; 

 in some instances a weaker solution might be used, and in others 

 a stronger solution would give better results ; but the variations 

 in the effect due to extraneous or unknown conditions were too 

 great to render any deviation from 0^075 per cent, advisable. 



(4) CATERPILLARS (Report X, 35) 



Series of laboratory experiments were conducted in 1906 to 

 ascertain the relative effect of various insecticides on caterpillars. 

 Leaves were wetted with the insecticides in question, and enclosed 

 in jars with a certain number of caterpillars, the mortality 

 amongst these being observed after various intervals. The 

 interval allowed had to be varied with the various kinds of cater- 

 pillars, owing to their different susceptibility to the insecticides, 

 and to the different rapidity with which they were passing into 

 the chrysalis form, after which passage, of course, observations 

 were no longer possible. In most cases 30 to 40 insects were 

 taken for each trial, being representatives of the winter moth, 

 lackey moth, tent moth, common vapourer -and Vanessa utica. 

 These, naturally, varied much in their susceptibility to poisons, 

 the winter moth caterpillar being the most susceptible, and 

 the tent moth caterpillar the least so. An actual measure of 

 susceptibility, however, could not be obtained, for the results 

 depended, not only on the susceptibility of the insect, but also 

 on the readiness with which it fed on the poisoned leaves. 



Amongst the substances examined was methylated spirits, 

 though this could hardly be used in practical spraying, even if it 

 had less effect than it has on the leaves. When strong, or diluted 

 with an equal volume of water, it was completely fatal to the 

 caterpillars in nearly every case. All the other substances were 

 examined at various strengths, and it was noticeable that an 

 increase in strength had very little effect on the results. 

 Details need not be given here, and the following values for 

 Q 



