354 Aiq-NUAL REPORT SMITHSONIAN" IN-STITUTIOIT, 19 3 2 



being merely elements in a stable complex these insects have assumed 

 the status of a veritable plague and have persisted in destructive 

 numbers year after year. This is the condition which we face 

 constantly with practically all of our principal crops in this coun- 

 try. In those parts of the world where agricultural practices have 

 been stabilized for several hundreds or thousands of years the 

 injury is not nearly so great, owing to a partial restoration of the 

 natural equilibrium. 



Among the greatest influences which operate to maintain an equi- 

 librium in the insect world and to restrain the destructive capacities 

 of various species within reasonable bounds are those insects which 

 attack others of their own class. With comparatively few excep- 

 tions, every insect has one or more species which feed upon it, and 

 these in turn may be attacked by others. This adaptation in some 

 cases is so exact that the parasite can live upon its single host species 

 only, and the disappearance of that host consequently results in the 

 extinction of the parasite. For this reason it is undesirable, from 

 the point of view of the parasite itself, that the host should be unduly 

 reduced in numbers. Under the condition of equilibriunii there is a 

 constant numerical rise and fall of the host species, which coincides 

 with, or slightly precedes, the corresponding cycle of its parasites. 

 In times of abundance of the host the conditions for increase of the 

 parasites are at their best and consequently they increase rapidly. 

 This results shortly in a decline of the host species. Eventually a 

 point is reached where no further increase of the parasites is possible, 

 and here the cycle starts once again. From our point of view the 

 question of importance is whether this parasite attack keeps the pest 

 at a sufficiently low level so that crops are not seriously damaged. 

 Wliere the reduction is not sufficient, it is necessary to utilize mechan- 

 ical control measures, such as spraying, dusting, and fumigation. 



Within the United States probably more than half of our most 

 destructive insect pests upon agricultural crops have been brought 

 here from other parts of the world. Among these we might men- 

 tion, as better-known examples, the cotton-boll weevil, the gipsy 

 mioth, the Hessian fly, the European corn borer, the Japanese beetle, 

 the oriental fruit moth, practically all of the scale insects which are 

 so destructive to citrus trees, and many others. In most cases these 

 insects are of minor importance in their native habitats, whereas in 

 this country the injury inflicted to crops is often very great. This 

 greater destructiveness of introduced species may be due to several 

 causes. Among them is the absence of the various natural enemies 

 which assisted in holding the species in check in its native home. 

 Usually when an insect gains entry into a new country it leaves its 

 natural enemies behind, and consequently one of the important re- 

 straining influences upon it is lacking. There then ensues an ex- 



