August, '19] SMITH: BIOLOGICAL CONTROL METHODS 291 



tant factors, and probably is the one which will, more often than any 

 ■of the others, work against the success of such undertakings. Unless 

 the beneficial insects can be either reared or collected in great numbers 

 success will not be possible. In cases where the entomophagous 

 insect takes readily to rearing in confinement, the possibility of rearing 

 or obtaining a host insect in quantity is what limits the undertaking. 

 This is in turn limited by the availability of a host plant upon which 

 to grow the host-insect. In any event the beneficial insects must be 

 had in sufficiently large quantities that they will be enabled to over- 

 come the pest in the field. 



5. Cost of Producing Natural Enemies in Comparison with 

 Value of Crop and with Artificial Control, if any. The factor 

 of cost of production is, of course, one of the most important, since 

 the main objective sought in biological control is economy. The cost 

 should not even closely approach that of effective artificial control 

 if such exists. In the case of the mealy-bugs, where there is no very 

 satisfactory artificial control known, the cost factor is not of such very 

 great importance, yet it obviously must remain well within the bounds 

 of profitable crop production. 



6. Presence of Secondary Parasites in the Local Fauna. The 

 retarding effect of the existence in the local fauna of secondary para- 

 sites that will strongly attack those with which the work is being 

 carried on, must be taken into consideration. Undoubtedly it will 

 in some cases be sufficient to render the work unsuccessful, because if 

 by artificial manipulation a superabundance of natural enemies of the 

 pest is created, conditions will then be ideal for the secondary parasites. 

 In most cases it will be practically impossible to foresee just what 

 would occur in cases of this kind and a practical attempt would have 

 to be made in order to ascertain just what part the secondary parasites, 

 and the primary parasites of the predaceous insects, would play. 

 Undoubtedly this factor will limit, or entirely prevent, the control of 

 many of our insect pests by this method. 



7. Unfavorable Agricultural Practice. Under conditions 

 where certain agricultural practices are essential, these will in some 

 ■cases perhaps make efforts at biological control inadvisable. For 

 example, if an orchard is infested with two or more insect posts, one 

 of whicli can be controlled by the biological method and tlic other 

 cannot be, the latter requiring artificial treatment such as fumigation 

 or spraying, such practices, affecting adversely the breeding of natural 

 enemies, would make it impractical to attempt to apply the biological 

 method as a control for the other pest. 



These are, it is believed, the most important factors which must 

 be taken into consideration, in contemplation of any project on the 



