U. S. EXPERIMENT STATION BULLETINS. 411 



later. The first brood of the codlin moth lays its eggs in the flower end 

 of the young apple, and the worms upon hatching gnaw their way into the 

 interior of the apple, and on sprayed trees are poisoned in so doing, an 

 infinitesimal amount being sufficient to destroy so minute a worm. The 

 second spraying is for the purpose of destroying larva? hatching from eggs 

 which may be laid after the first spraying, as the arsenic is gradually 

 washed off by rains. 



For the plum curculio on the plum, cherry, peach, etc., two or three 

 applications should be made during the latter part of May and the first 

 half of June. The poison in this case is applied for the purpose of destroy- 

 ing the adult curcuiios which hibernate and gnaw into the young growth 

 of the trees and even into the hard young fruit before laying their eggs. 

 The eggs are pushed under the skin so that the larvae are not ordinarily 

 affected by the poisoning. 



In the case of most leaf-feeding insects one should spray on the first 

 indication of their presence. 



Caution necessary in the use of these insecticides. — The relative suscept- 

 ibility of apple, plum, and peach has just been indicated under the head of 

 arsenical poisons, and these remarks apply equally well to the use of kero- 

 sene emulsions. In the case of other plants, thorough experiments are still 

 necessary, and all insecticides should be used in comparatively high dilu- 

 tion. Tender-leaved plants, such as melons and cucumbers, are more 

 readily injured; while plants with firmer and smooth leaves, like the 

 orange, are least affected. Annual plants, such as cabbages and other 

 garden vegetables, are more susceptible than perennials; but in the case of 

 root crops, such as beets, turnips, radishes, and potatoes, there is not the 

 same need of caution as to damage to foliage. Damage to foliage is not 

 shown at once, and in case of rain following an application another appli- 

 cation should not be made for several davs. Fruit trees should not be 

 sprayed with arsenical poisons while in blossom, as there is no advantage 

 in doing so, and honey bees are reported to be at times killed by working 

 in the sprayed blossoms. 



SPRAYING FROM THE HYGIENIC STANDPOINT. 



The only insecticide sprays which are at all dangerous to use are the 

 arsenic compounds, and even here the danger is greatly exaggerated by 

 those not conversant with the facts. Paris green and L@ndon purple 

 have for many years been extensively used in this country as insecticides 

 and a case of fatal poisoning from their use as such has never been sub- 

 stantiated. The only danger lies in having the poison about a farm or 

 plantation in bulk. In the early days of the use of Paris green against the 

 Colorado potato beetle a great deal of opposition was developed on account 

 of the supposed danger, and only recently the sale of American apples in 

 England has received a set-back owing to the supposed danger of arsenic 

 poisoning from their consumption. The question as to whether arsenic 

 may be absorbed by the growing plant in any degree was long ago settled 

 in the negative by the best chemists in the country. Dr. William Mc- 

 Murtrie, formerly chemist of this Department, in 1878, showed that even 

 where Paris green was applied to the soil in such quantities as to cause 

 the wilting or death of the plants, the most rigorous chemical analysis 

 could detect no arsenic in the composition of the plants themselves. Other 

 experiments in a similar direction, by Prof. R. C. Kedzie of the Michigan 



