xxiv Reinedial Measures and Insecticides. 



introduction of a beneficial insect without its own natural checks 

 will also permit of its rapid increase as long as an ample supply 

 of congenial food is obtainable. When the food supply begins 

 to fail, which means when the pest has been mastered by its 

 imported enemies, then they will both decline together. There 

 need be little fear that, when the food supply has been exhausted, 

 the imported insect will itself become a pest. A predatory insect, 

 by which is understood one that preys upon other insects or 

 animals, will seldom, if ever, alter its diet and become a vegetarian. 



It is noticeable that an insect seldom assumes any importance 

 in its original home, unless through some accidental or artificial 

 interference with the balance of nature in that part. (For instance, 

 it has been asserted that the widespread destruction of moles in 

 England has resulted in a marked increase of damage to pasture 

 land from the grubs of the ' cockchafer ' beetles and ' crane-flies,' 

 upon which the moles fed.) Consequently, if we are to obtain 

 any benefit from the use of natural agents, we must endeavour 

 to reproduce the conditions prevailing in the country where the 

 insect in question is known to occur, though without attracting 

 notice as a pest. Or, if the original home of the injurious insect 

 is unknown, we may reasonably hope for good results from the 

 introduction of an insect that is found to prey upon some allied 

 pest in another country. 



The most important natural enemies of the scale insects, or, at 

 least, those that have attracted most attention, belong to a family 

 of small beetles popularly known as ' lady-birds.' The complete 

 success attending the introduction of an Australian lady bird 

 ( Vedalia cardinalis) into California, where it cleared the orange 

 orchards of the destructive ' Fluted-scale ' {Icerya purchasi) has led 

 to numerous other experiments of a similar kind. These experi- 

 ments have not always been successful. There must, of necessity, 

 be many failures. We are still only in the experimental stage of 

 the work. Even when the beneficial insect has been successfully 

 established in a country, it is by no means certain that it will 

 thrive. There may be climatic or other conditions against it. In 

 that case, all we can do is to try another insect. Occasional, or 

 even repeated, failures should not discourage the repetition of the 

 attempt. The value of a single success will far outweigh the cost 

 of many failures. In the course of such experiments the causes of 

 failure will in time be ascertained, and improved methods be 

 employed. The freezing method recommended by Mr. Koebele 



