404 BIOLOGICAL CONTROL 



in certain instances. For example, Howard (1924) mentions the 

 case of the Hessian fly parasite, Pleurotropis epigonus, which was 

 introduced into the United States from England in 1894. After 

 liberation in the field this species was not recovered until twenty- 

 two years later, and since then it has become one of the dominant 

 parasites of its host over a large area in the eastern States. 

 Thompson (1927) has discussed the rate of progress of introduced 

 parasites from the mathematical standpoint and, although his 

 conclusions are necessarily theoretical, they serve to show the 

 bearing of the relationship between the ratios of the rates of 

 reproduction of parasite and host, together with the initial 

 numbers of parasite and host, and the time involved before a 

 given parasite could exercise a dominant influence. Thus, when 

 the rates of reproduction of parasite and host are equal and the 

 host population is very large in relation to that of the parasite, the 

 advance of the latter will be so slow as to be barely recognisable. 

 Periodical field investigations may reveal the presence of the 

 parasite, but for many years there will be no evidence of increase 

 in its numbers. In cases where a parasite reproduces less rapidly 

 than its host it will never subjugate the latter unless its initial 

 population is much larger in relation to that of the host than is 

 ever the case in any practical undertaking. In certain cases where 

 a multivoltine parasite with a high rate of fecundity is able to 

 attack a univoltine host, or one with a much lower reproductive 

 rate, control has been achieved under favourable conditions in a 

 remarkably short interval of time. This is exemplified in the case 

 of the control of the cottony-cushion scale in various parts of the 

 world, of the mulberry scale in Italy, and of the woolly apple 

 aphis in New Zealand. 



4. Finally, the capacity of a parasite to discover its host is 

 a factor often overlooked and one difficult to evaluate except in 

 a very general way. This capacity appears to be most often 

 inversely proportional to its fecundity, and it affords examples 

 of Nature's prodigality or economy as the case may be. When 

 the chances of a parasite discovering its host are small, the 

 progeny are numerous {e.g. Stylops, leaf-ovipositing Tachinidae, 

 etc.), and where the chances are great {e.g. Scoliid wasps) the 



