August, '19] SMITH: BIOLOGICAL CONTROL METHODS 289 



have occurred if left to act under normal conditions. The first 

 embraces only the establishment of a species in the local fauna, while 

 the second involves an attempt to make the entomophagous insects 

 continually dominate their host, a condition which cannot prevail 

 if nature is left to act unaided. 



So far as the first section of biological control is concerned, ento- 

 mologists are, it is believed, pretty well agreed that the introduction 

 of as many new beneficial insects as may be obtained is desirable, 

 provided only that the work is done with an intelligent understanding 

 of the interrelations of entomophagous insects. Dr. L. 0. Howard 

 states that " ... since there exist all over the world beneficial 

 insects, many of which can undoubtedly be acclimatized here, and 

 some of which will undoubtedly prove of value to American agriculture, 

 carefully planned work should be begun looking to the ultimate 

 increase of our insect population by the addition of as manj'^ of the 

 beneficial forms as possible." It is hardly necessary to add that such 

 introductions should be made with a proper appreciation of the pos- 

 sible results to be obtained and, because of its retarding effect on the 

 use of other means of control, over-optimistic and premature advertis- 

 ing should be carefully avoided. 



The other section or type of work with entomophagous insects, 

 that embracing the artificial manipulation of species already estab- 

 lished in the infested area in such a way as to increase greatly their 

 numbers and thus to decrease the numbers of their host, has not been 

 looked upon with equally great favor by entomologists. 



Biological control by this latter method is based on the proposition 

 that fluctuation in abundance of host and parasites may be prevented 

 and the host insect kept permanently in subjugation, by maintaining 

 a super-abundance of natural enemies in the orchard or field at all 

 times. The "balance of nature" is like a pendulum, swinging to and 

 fro, the dominance of any species alternating with that of its natural 

 checks. The method of control here under consideration is based 

 on the assumption that by artificial manipulation the natural checks 

 can be made to permanently dominate the species against which they 

 exert a controlling influence. There may exist a perfect "balance" in 

 the relation between a species and its host and the host still be a pest 

 because the number of individuals occurring at the time of greatest 

 abundance is sufficient to damage cultivated crops. It is a well- 

 known fact that in the relation between host-insect and its natural 

 checks there is a point where the natural enemies, having temporarily 

 dominated the host, themselves become almost extinct, with the 

 result that the remaining individuals of the host are again permitted 

 to breed almost without hindrance. This is the strong vantage-point 



