290 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



struggle against injurious insects, and its use is particularly general in 

 America, where it is the best-known insecticide. It is used some- 

 times in the pure state, but generally in a state of emulsion with 

 different substances capable of mechanically dividing it. 



Use of Petroleum in the Pure State. — The action of petroleum 

 on insects is radical and infallible ; efforts have therefore been made 

 to use it preferably in this form, so as to free it definitely from 

 dangerous parasites, when there is no risk of destroying the plants 

 treated at the same time. If it did not possess such a poisonous action 

 on the plant it would be perfect. Unfortunately, the plant does not 

 without special precaution stand contact with pure petroleum, and in 

 spring or summer this method should only be used in exceptional 

 circumstances. On the other hand, it is not dangerous to plants 

 during the winter sleep, or as an injection in the soil some time before 

 planting. 



Marlatt recommends its use as a spray or sponge wash in winter 

 on the aged part of trees for the total destruction of Kermes and 

 Scolytes. But it was especially recommended by "Webster for the 

 destruction of Aspidiotus and Lecanium of fruit trees. Webster sub- 

 mitted his observations and his processes to the Congress of American 

 Phytopathologists in 1896. Smith and Alvood write thus on the use 

 of petroleum : — 



" Petroleum sprays may be used with impunity on all fruit trees, 

 even in full vegetation, by taking the following precautions. (If these 

 precautions be neglected there will be a loss of 90 per cent of the 

 trees treated) : — 



" 1. Spraying should be done with an apparatus capable of reducing 

 this liquid into a fine fog, capable of touching the whole surface of the 

 plant without really moistening it. The formation of drops, which 

 might flow along the trunk to the roots, must be avoided. Poisoning 

 always occurs when the roots are drenched with petroleum. 



" 2. The operation must be conducted in warm, dry weather, for 

 the petroleum should evaporate forthwith so as not to kill the plant. 



" 3. It is necessary to operate from March to September. In single 

 treatments September is the month chosen. 



"4. Burning oil (lamp oil) must be used and not crude petroleum, 

 which contains insufficiently volatile ingredients. 



"5. It is well to cover the fi-uit buds before spraying. 



"In these conditions peach-trees and plum-trees are more sensitive 

 than apple-trees and pear-trees. The trees treated have finer shoots, 

 but the treatment appreciably diminishes the crop." 



The report of the 10th Congress of the American Entomologists 

 in 1898 is less enthusiastic on the use of pure petroleum. It con- 

 cluded, after discussion, that pure petroleum is a highly dangerous 

 insecticide for trees, but which may be unhesitatingly used when it is 

 ■a case of the destruction of insects as formidable as the San Jose louse 

 {Aspidiotus perniciosus) and certain kermes. In this case it must 

 only be used on the old wood, and in dry, warm weather, in which it 

 evaporates forthwith. 



The Congress of 1900 was altogether unfavourable to the use of 



