VARIOUS FACTORS 403 



in a new area or country. Investigations conducted in the 

 country of origin of a parasite are of prime importance, but actual 

 experiment alone can provide the solution of its behaviour in a 

 fresh environment. 



The introduction of a multivoltine parasite to attack a univoltine 

 host may be selected as an example of a particular type of host- 

 relationship. In an eventuality of this kind success may largely 

 depend upon the availability of other suitable hosts at whose 

 expense the parasite will be able to maintain itself during the 

 period when the requisite stage of the intended host is no longer 

 in evidence. A comparison of possible hosts indigenous to the 

 country of introduction with the known hosts in the country of 

 origin of the parasite will obviously provide tentative information, 

 but only experiment will determine the extent to which any 

 forecast of this kind will be realised. 



2. The effects of artificial control measures upon the progress 

 of introduced parasites can only be studied in a broad general 

 way when reduced to mathematical terms. No data exist which 

 permit of this problem being considered from its experimental 

 aspect. In the theoretical analyses given by Thompson the 

 initial numbers of reproducing parasites and hosts present at the 

 beginning of a given experiment, the effective rates of reproduction, 

 and sex ratios of the two species and the proportions of each 

 eliminated by artificial control before the period of reproduction 

 are taken into consideration. It is shown that if artificial con- 

 trolling measures eliminate at least as many hosts as parasites, 

 precautions for conserving the latter are unnecessary. If, on the 

 other hand, conditions are such that artificial measures of control 

 result in the destruction of appreciably more parasites than hosts, 

 they may exercise a disastrous effect upon the progress of an 

 introduced parasite and delay its gaining effective control for 

 many years. For a full discussion of the theoretical aspects of this 

 subject the reader is referred to Thompson's original paper 

 (1927a), and also to a recent paper by Clausen (1936). 



3. What may be termed the time factor is of particular import- 

 ance, and lack of its due recognition may lead to erroneous 

 conclusions respecting the supposed failure of parasite activities 



