BY i. HARVEY JOHNSTON. 23 
who has been steadily experimenting with a view to deter- 
mining the most efficient method of destroying the pest 
by chemical means. In estimating the question of efficiency 
it has been necessary to take into consideration various 
factors such as the cost of labour and material ; the method 
of application of the poisons, whether as infectious in a 
liquid or solid form or as sprays; the time of the year ; 
the effect of rainfall just prior to or just after treatment ; 
the advantage (if any) of chopping down the pear in order 
to facilitate the action of the poison, etc. The record 
of the work done has been published in the Annual Reports 
of the Department of Public Lands for 1912, 1913 and 
1914, and one is now in course of preparation for 1915. 
The object aimed at is the complete destruction of 
the plant, not merely the death of the above-ground portion 
of it, as the “bulb” is apt to retain its vitality. The 
chief result of the work has been the discovery that 
solutions of arsenic pentoxide are much more efficient 
as a prickly pear poison, than any other known chemicals, 
and no doubt when a supply of this compound becomes 
available it will supersede the well known ‘ Briinnich 
preparation,” consisting of arsenic trioxide and caustic soda, 
which is at present in common use for this purpose. 
Her work is summarised in the Annual Report of 
the Department of Public Lands 1914 (1915), p. 80, and 
may be briefly summed up as follows :—Arsenic pentoxide 
is the most effective poison yet used, whether employed 
in the form of a spray, solid injection or liquid injection, 
whilst the best result obtained by using gaseous specifics, 
is produced by the vapours of arsenic trichloride. It was 
also found that summer and early autumn are the best 
seasons for applying poisons in any of the forms mentioned, 
and, moreover, the success of the operation is largely 
dependent on the fall of rain just before or just after poison- 
ing, probably more especially the former. 
We congratulate Dr. White-Haney on her success. 
It will be remembered that the Government com- 
missioned Mr. Henry Tryon, Government Entomologist, 
and myself to visit various countries where prickly pear 
occurred either indigenously or as naturalised weeds in 
