ig4 );i:i\i:i'ic tAL ixsix'is. 



hope to creep into the small creases and cracks in the bark of 

 a .scrape vine — the hiding-places of the mealy bug. A very 

 small indigenous species at work in the \'ineyards was found to 

 have a parasite that proved to be a severe handicap. Would it 

 be possible to find a strain of this species that would be dis- 

 tasteful to its parasites? 



The isolation of the first-maturing C. uiontrouzicri impressed 

 me with the practical importance of saving time by breeding from 

 the quick-maturing specimens, and it occurred to me that it 

 would have been a factor of great advantage had it l)een neces- 

 sary to depend on breeding A. fccdata to control /. piiniiasi, and 

 that in time it might have resulted in increasing the number of its 

 annual broods. The practical need for this had disappeared 

 with the advent of Noviiis, and hence remained mere specula- 

 tion, and was all but forgotten till a request was received from 

 Mons. D. d'Emmerez de Charmoy. Government Entomologist 

 for Mauritius; for a colony of Mofiiis cardiiialis, in the hope 

 that it would adapt itself to Iccrya scy.clieUanim and exert a 

 more perfect control than does the indigenous species. J'cdalia 

 chcrmesina, of which, on April 27th, Kji.S, he wrote as fol- 

 lows: " I am not sure whether it is worth while trying to intro- 

 duce this species {Nov'uis cardiiialis) here, as it is not i)r(n-ed 

 that it will live on /. ScychcUaniui. whicli has alreadv a similar 

 enemv in Vcdalia chenncsiiia. Unfortunateh' this ladvbird 

 breeds very slowly, is sedentary, and cannot 1)e relied upon as 

 a serious auxiliary for controlling the Iccrya i)est all the year 

 round, although it is undoubtedly due to it that this ])est is 

 kept within a tolerable limit in its depredations." 



The o])inion of Mons. D. d'Emmerez in regard to I'cdalia 

 chenncsiiia agrees perfectly with my own opinion of the work 

 of Aulis (a-data in South Africa, and my early impression that 

 a desirable strain of it could have been developed if necessary 

 is as vivid as ever, and 1 believe it is perfectly feasible. Vcdalia 

 clicriiicsiiia sui)i:)lies a practical case in point, for want of which 

 i ha\c refrained from discussing the matter seriously, and 1 

 liave therefore decided to record the idea of developing desirable 

 strains of beneficial insects and to make certain suggestions that 

 may ])ossil)ly be of value to others who are in a position to make 

 ])ractical use of them. 



In studying the relationship between host and parasite as 

 illustrated by /. purchasi and Noviits cardiiialis, in the writer's 

 opinion tlie following facts deserve special consideration:— 



1. I''.xperiencc in South Africa has shown that ^V. cardiiialis 

 alone is able to keep its host under satisfactory control, and 

 this is a])parently due to its being a prolific, (|uick-maturing 

 .species that has specialised on one host. In other words, Nature 

 has achieved her most brilliant success on the plan of one host, 

 one parasite. 



2. The host is what 1 shall call a non-fluctuating s])ecies — 

 i.e.. it lias a wide range of host plants, and left to itself, it goes 

 on increasing steadily on any given ])lant rmd breeds throughout 



