168 DESTRUCTIVE INSECTS OF VICTORIA : 



iusiifficieut in a very badly-infested case by an occasional scale escaping- 

 through the protection afforded by a covering of other scales. A 

 diligent search shortly after the treatment has often failed to reveal the 

 presence of a .single snrvivor among thousands of dead scales, but that 

 a few had escaped the general destrnction woxild become apparent after 

 the laps of three or four months. The decreased potency of the gas as 

 it penetrates mnsses was prettily illustrated by its action on the eggs of 

 the Australian bug {Icerya purchasi). This scale insect, be it remem- 

 bered, deposits a very large number of eggs iu a sac-like outgrowth 

 from the posterior of its body, and the eggs within this ovisac are 

 embedded in a mass of protective cotton-like filaments. Comparatively 

 weak gas destroyed the eggs nearest the end of the sac, but had no 

 effect on those further within. As the doses were increased in succes- 

 sive tests, larger and larger proportions of the total number succumbed, 

 but those in the layer immediately against the body survived even one 

 ounce of cyanide to 200 cubic feet, and a very few in the largest sacs 

 withstood even a half hour's exposure of one ounce to 150 cubic feet. 

 A number of young apple trees badly infested with woolly aphis were 

 exposed to the gas after sousing the roots in water to remove the soil. 

 One ounce to 450 cubic feet for one hour appeared at the time to be 

 thoroughly efficacious, but after the lapse of three months the roots of 

 even those exposed to one to 300 for an hour and of those to one to 200 

 for a half hour were again much infested. The film of water about the 

 roots or the traces of soil still clinging to them had evidently served to 

 protect a number of tlie insects. Altogether, the series of tests, though 

 incomplete, indicates that in the disinfection of nursery stock care should 

 be taken to give the gas free play among the trees, and that it is advis- 

 able to employ much stronger doses than Avhat are known to be fatal 

 under ordinary circumstances. Dormant deciduous stock is not injured 

 by exposure to four times the normal orchard proportion for citrus trees 

 — one ounce to 300 cubic feet. The cost of charging a small chamber 

 is a mere trifle, and, this being the case, my present recommendation 

 to nurserymen is to use not less than one ounce to 125 cubic feet of 

 space when treating deciduous trees. The strength which citrus 

 trees will stand without being injured has not been determined, but 

 fully dormant stocks are not injured in the least by one ounce to 

 250, nor more than the tips of growing trees by one ounce to 300 ; 

 under favorable conditions at least, the latter portion is fatal to all 

 stages of the scale insects known to affect citrus stock in the nurseries 

 of this colony, with the exception of the Australian bug and mealy 

 bug. 



Hydrocyanic Acid Gas for Glasshouse Fumigation . — An officer of the 

 United States Department of Agriculti^re has been experimenting off 

 and on for the last four years with hydrocyanic acid gas as an insec- 

 ticide for the destruction of various kinds of insect pests which affect 

 plants in glass houses, and following his lead a number of gardeners 

 have adopted tiiis fumigant. It is said that it has been found possible 

 to destroy practically every insect without injuring so much as a leaf or 



