ipi6 



BETTER FRUIT 



Page 29 



adding to this iinlil they finally are 

 able to take more than six grains at a 

 time, or several times the amount that 

 would normally produce death. If a 

 tree is poorly sprayed, a leaf-eating in- 

 sect might get here and there a particle 

 of arsenic to act as a tonic, and a 

 spraying might therefore conceivably 

 do more harm than good. To test this 

 hypothesis Professor C. T. Brucs of 

 Harvard University and I last year se- 

 cured eggs of the Gypsy Moth from 

 sprayed and fi'om unsprayed wood- 

 lands. The caterpillars developing 

 from these eggs were fed varying but 

 weak dosages of arsenic and showed a 

 measurable resistance to the poison in 

 favor of the caterpillars reared from 

 the sprayed trees. While this experi- 

 ment shows that there is a hereditary 

 ditference in insects in their suscepti- 

 bility to poison, its explanation is per- 

 haps merely this: Where spraying is 

 not thorough, so that not all insects 

 are killed, the ones that escape are 

 most apt to be the hardier, more re- 

 sistant individuals, and such vigor is 

 apt to be transmitted to coming gen- 

 erations. 



Contact insecticides are, generally 

 speaking, of two kinds — those that 

 sutTocate the insect by clogging or poi- 

 soning the brcalhing system, such as 

 soaps, oils, tobacco or fumigants; and 

 those like sulphur-lime or sulphur- 

 soda, that produce a chemical reduction 

 of the insect's body. Insects breathe 

 not through a mouth or nose, but 

 through a series of pores arranged 

 along the sides of the body. These 

 pores open into a system of tubes of 

 microscopic fineness, which permeate 

 every part of the body. Plugging of the 

 pores by oils or soaps results in death. 

 Here the spray must actually wet and 

 adhere to the greasy body of the insect, 

 which explains why nicotine in water 

 solution is much less effective than 

 when combined with soap. 



Sulphur-lime is a very unstable solu- 

 tion. Principally it consists of a chem- 

 ical called calcium polysulphide (CaS4). 

 As soon as exposed to the air this sub- 

 stance absorbs oxygen and is converted 

 into a series of compounds known, in 

 the order of their formation, as calcium 

 thiosulphate (CaSjO:.), calcium sulphite 

 (CaSd.;) and calcium sulphate (CaSO^). 

 This reaction calls for a progressive ab- 

 sorption of oxygen and a simultaneous 

 deijosition of the finest of sulphur. The 

 combined action of the withdrawal of 

 oxygen from the insect sprayed and the 

 effect of the deposited sulphur are sup- 

 posed to account for the insecticidal 

 action of sulphur-lime. 



Sulphur-soda has i)roportionally much 

 more of the thiosulphate compound, 

 which in this case, as sodium thiosul- 

 phate, is the familiar "hypo" used by 

 photographers, and has no insecticidal 

 value, as it is not oxidized further. 

 Theoretically, therefore, a sulphur- 

 soda spray should be less cflicient than 

 sulphur-lime, instead of being much 

 better as claimed by the manufacturers. 

 The decomposition reaction of sprayed 

 sulphur-lime is rapid at first, but con- 



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