76 DISEASES OF CROP-PLANTS 



discussed the question as to how far their efficiency can be in- 

 creased by artificial distribution. As a consequence of the 

 success, since shown to be due to very special circumstances, of 

 the introduction of Australian lady-bird beetles into California 

 to prey on the cottony cushion scale, a very large amount of 

 attention has been given to the question of natural enemies of 

 injurious insects. The results have been usually disappointing, 

 but the rules of the game are now pretty clear. The rapid 

 increase of a pest which may take place when it is newly intro- 

 duced into a country where natural enemies adapted to destroy 

 it are few or absent may be checked in some cases by bringing 

 in such enemies from countries where they exist. To use a 

 relevant illustration : should some kind of introduced white 

 fly become troublesome in the West Indies, it would be worth 

 while to introduce the fungi which are parasitic on certain 

 species in Florida, in the hope of their exercising a check on the 

 insect in these islands. Were the introduction successful, 

 the control might be more efficient or less so than it is in Florida, 

 according to the extent to which the conditions favour the fungi. 

 It would not be expected to be complete. Once given a start, 

 however, the fungus parasites might establish themselves and 

 constitute permanent factors modifying the activity of the 

 insect. 



When such a factor is already well-established in the country, 

 and it is desired to increase its effect, the element of artificiality 

 becomes much greater. The fungus (or other parasite) already 

 has its place, its settled lines of action, so to speak. By cultivat- 

 ing it, by distributing it, it may be increased for the time, but 

 when such means are no longer used it will settle back to much 

 its old position, which is determined by natural conditions, and 

 changes only in response to their fluctuations. This has been 

 and is the situation with regard to most work on the fungus 

 control of insects. 



The method has an insidious appeal, in that it seems to 

 promise to the planter a self-acting remedy, but experience shows 

 that, as might have been expected, results are not to be obtained 

 without an adequate amount of trouble. 



The use of fungi does afford an alternative to the employment 

 of poisonous sprays, but save that in the one case a living organism 

 is used, one method is as artificial as the other, and it depends 

 entirely on the particular circumstances of the case which is 

 more efficient or less troublesome. That is a question for the 

 entomologist and for the planter. In Florida, in connection with 

 white fly control, each method has its advocates. In Trinidad, 

 periodic dusting with fungus spores has been tried against 

 froghopper on sugar-cane, spraying with insecticides in this case 

 being out of the question. 



In one respect the use of fungi has a disadvantage as compared 

 with the use of poisons. The latter may be applied with advan- 



