INDIGENOUS PARASITES 409 



localities, where they were liberated. Notwithstanding the 

 considerable mortality which occurred while in artificial storage, 

 upwards of 75,000,000 have been distributed among farmers in 

 a single season. The operation was carried on by the State 

 Horticultural Commission for seven years in succession before the 

 subject was given expert study as to its value as a commercial 

 undertaking. In 1918 and 1919 W. M. Davidson, of the United 

 States Bureau of Entomology, made a series of experiments in 

 order to ascertain the distribution of the Coccinellids after 

 liberation. Large numbers were collected and sprayed with 

 " silver " or " gold " aluminium paint which rendered them 

 recognisable, and caused only slight mortality. It was found that 

 the insects distributed themselves over large areas so rapidly that 

 they were of little value in controlling local aphid outbreaks. 

 Only one out of every 65,000 sprayed individuals was recovered 

 (Davidson, 1924). 



Transference of Parasites to New Areas. The conception of the 

 transfer and colonisation of parasites, from an area of a given 

 country where they are prevalent into a district where they are 

 apparently non-existent, dates from about 1872. Theoretically, 

 the method appears feasible if physiographical barriers have 

 precluded the entry of a parasite into a given area. On the other 

 hand, if its absence is to be ascribed to climatic causes, the chances 

 of its successful colonisation would seem to be remote. 



Three examples of the application of this method may be 

 instanced. The incidence of the Braconid Macrocentrus ancylivorus 

 has already been referred to on p. 383 in connection with the 

 control of the Oriental fruit moth. It appears that the New Jersc}^ 

 strain of this parasite has a higher efficiency in bringing about host 

 mortality than examples derived from other localities. The result 

 has been that this so-called race has been widely disseminated 

 artificially in many states where it has been subsequently bred in 

 large numbers followed by liberation. Its colonisation is now an 

 established fact, and it is regarded as the most effective biological 

 agent in the control of its host so far discovered. 



The colonisation of Aphelinus 7nali, another North American 

 indigenous parasite, in the Pacific North-West, has, it is stated, 



