H. M. Lkfroy 289 



6. Lime, salt and sulphur in vague proportions for apple sucker, etc. 



7. Lime washing for winter use. 



8. Nicotine as a general panacea for all pests, as a stomach poison 

 for caterpillars, as a contact poison for apple sucker, aphis, etc. 



It will be admitted probably that these insecticides have come into 

 use on purely empirical lines; the use of arseniates has probably come 

 from human toxicology, and a very exhaustive inquiry into the action 

 of some mineral compounds in India^ has not produced anything more 

 poisonous; it is probably a question of the form of arsenic to be used. 

 But what has governed the choice of insecticides that do not act on 

 the internal organs, but those acting from without ? I think none but 

 tradition and purely empirical practice, with experience in large scale 

 experiments as a deciding factor. 



Is experience on a large scale experiment of much real use? It is 

 in some cases, such as mussel scale, where the insect is one-brooded; 

 I doubt if it is much use in a many brooded or migrating insect, such as 

 an aphis. The factors that control the increase of aphis are many and 

 it w^ould require very careful examination of the conditions to say that 

 the spraying had done the real work, or that the actual nature of the 

 insecticide had been the deciding factor. Suppose a grower of plums 

 sprays for the Leaf Curling Aphis in June ; a week after he looks for 

 aphis and finds none; he thinks it is the spraying, whereas it is the 

 migration of the aphis from the plums that has produced the 

 absence. 



It is worth while thinking for a moment how these insecticides act; 

 what actually happens when paraffin emulsion is applied; where does 

 the paraffin go to, what tissue does it affect, how does it work ? A 

 Board of Agriculture Leaflet states " The presence of the soft soap causes 

 the wash to touch the aphis, and as it dries the thin layer of soft soap 

 clogs the breathing pores and kills the insect" (Hop aphis). Did the 

 author of this statement really know this, or was it simply the generally 

 accepted statement? If so, why do most growers add quassia, and 

 why does quassia kill aphis? what does quassia actually do? I do 

 not think actually that anyone can tell us what the quassia extract does, 

 and I believe that we shall get much better insecticides when we know 

 how it is that these things kill. What is it that happens ? If we know 

 that, can we not devise insecticides that make this happen, not that 

 happen to kill? 



1 Lefroy and Finlow. Memoirs of the Agricultural Dept., India. Entomology, 

 vol. IV. no. 5. 



