304 INSECTS 



plow and pick up every specimen brought to view. 

 Once a small number of fowls has been trained to this 

 work, the flock will continue the training; the new 

 members following the older without additional trouble 

 to the farmer. 



In the selection of fertilizers considerable benefit 

 is sometimes derived in the use of minerals rather 

 than barn-yard manure. Many insects require the 

 shelter or presence of decaying vegetable material, 

 and do not thrive in soils impregnated with mineral 

 fertilizers. This is a point, however, where the question 

 of farm practice is eminently one for the cultivator, and 

 no general recommendations can be given. 



There are still among our battery of insecticides 

 the gases and vapors, and these are of great impor- 

 tance. Sulphur fumes have been used for many years 

 against household insects but these are being superseded 

 by the hydrocyanic acid already described. 



Bisulphide of carbon is a clean, water-white liquid, 

 very foul in smell, volatilizing rather slowly at ordinary 

 temperatures, the vapor heavier than air and very 

 inflammable. This vapor is fatal to most insects ex- 

 posed to it in a confined space for an hour or more, 

 and it destroys the vitality of seed germs exposed to 

 it much over twenty-four hours. It is rarely used in 

 the field, but for insects infesting stored products is 

 extremely useful. Where entire plants infested by plant 

 lice can be covered by a tight cone, jar or box, one 

 drachm or, roughly speaking, a tablespoonful to every 

 cubic foot of space will kill the insects in one hour. 

 In melon or cucumber fields in which plant lice have 

 just made a start, it is sometimes possible to check their 

 spread by treating the infested hills under hay-caps 

 or similar covers, or even under tubs or large pails. 

 Large clam-shells make good receptacles for the liquid, 



