358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 2 



The rate of reproduction which has been mentioned in Schizaspidia 

 and in the Trigonalidae is not at all unusual and is equaled in many 

 other groups, particularly among the parasitic flies, and also in some 

 beetles. The greatest potential increase, however, is found in the 

 relatively small groups of parasitic wasps of the families Encyrtidae 

 and Platygasteridae which, among others, have developed the very 

 remarkable habit of polyembryonic reproduction, where a single egg 

 may give rise to a number of individuals. In some species this num- 

 ber is only 2, in others 12 to 16, and it nmj attain a total of several 

 hundred or possibly thousands. From a single caterpillar as many 

 as 3,500 of these minute wasps of the genus Copidosoma have been 

 reared. The number of eggs required to provide for this total is not 

 known, but it is quite certain that this one species, at least, is theo- 

 retically capable of increasing itself thousands of times each genera- 

 tion. As in the preceding cases, the capacity for reproduction is 

 based on the limiting factors, and, as in the case of the Trigonalidae, 

 this group likewise is quite uncommon. These forms are seldom as 

 important in the control of the host as are other parasites which have 

 a very much loAver reproductive capacity but which are free from 

 these handicaps. 



The rates of increase which have been mentioned may appear to 

 be very high, and the}^ are greatly in excess of that which is possible 

 with the great majority of parasite species. It is of very little signif- 

 icance whether or not the parasite has a greater rate of increase than 

 the host. The important point is whether it will continue to in- 

 crease until the host species is overcome. Some parasite species have 

 a considerable advantage in being able to produce several genera- 

 tions to each one of the host. These may all be passed upon the same 

 host species or each one may be on a different host. This latter habit, 

 however, may be a handicap rather than an advantage, as some one 

 of these hosts may occur in only veiy small numbers and conse- 

 quently^ increase upon it, or even the maintenance of its numerical 

 status, is impossible. 



In the case of those insects which have come to us from other 

 parts of the world, it is desirable if possible to establish here the 

 restraining influences which operate in their native homes. In other 

 words, this is an effort to restore the natural equilibrium. We have 

 no control over climatic conditions, and where this is the principal 

 factor involved there is little hope of remedying the situation. If, 

 however, the lack of its natural enemies has been responsible for the 

 increase in numbers of a species, then we can import them and pos- 

 sibly reduce the pest to a position of minor importance. 



The problem is oftentimes quite complex and the outcome of these 

 importations can seldom or never be anticipated with confidence. A 

 parasite which is \erj effective against its host in its native habitat 



