POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



383 



In a general, and not necessarily a popu- 

 lational, sense the student of insect para- 

 sitology has been concerned with two as- 

 pects of parasitism; the first, the eflFects of 

 the parasite upon the host; the second, the 

 eflFects of the host upon the parasite. The 

 first approach is traditional and has been 

 intensively investigated. The second or con- 

 ('erse approach has only recently been for- 



gramma evanescens. Adult Trichogramma 

 oviposit within the moth eggs in which the 

 parasite larvae hatch. These larvae feed 

 upon the host eggs and later pupate there. 

 In ten days (at 25° C.) the parasite is fully 

 developed, bites its way through the egg 

 membrane, and emerges as an imago 

 chalcid fly. If a moth egg contains two or 

 more parasite eggs, it is said to be "super- 



C/) 



cr 



Ld 

 DO 



TIME 



Fig. 134. Schematic representation of the course of an epidemic wave in terms of numbers of 

 susceptibles and numbers of cases. (After Jordan and Burrows.) 



mally stated (see Salt, 1941). These rela- 

 tions between host and parasite can be 

 morphological, physiological, or behavioral, 

 and they may be studied from either an 

 organismic or populational point of view 

 (see pp. 253-262). 



Out of such studies there is emerging a 

 growing emphasis upon population aspects. 

 The significance of the interactions be- 

 tween hosts and parasites considered as in- 

 teracting, interspecies groups is becoming 

 recognized. This has gained primary im- 

 petus at the hands of economic entomolo- 

 gists interested in the use of parasites as 

 agents in the biological control of insect 

 pests. 



An illustration of an experimental analy- 

 sis of certain phases of host-parasite inter- 

 actions is aflForded by Salt's (1936) study. 

 Salt set up populations consisting of hosts, 

 the moth Sitotroga cerealella, and of para- 

 sites, the chalcid hymenopteran, Tricho- 



parasitized." Under such conditions some 

 or all of the parasite larvae are under- 

 nourished, fail to develop, and die. In an 

 earlier paper Salt (1934) showed that 

 these larvae actually compete with one an- 

 other for food, and typically one wins out 

 at the expense of its neighbors and emerges 

 as an adult. Sometimes, when the parasite 

 larvae are numerous within the egg (as 

 many as nineteen Trichogramma eggs have 

 been noted in one Sitotroga egg), the com- 

 petition becomes so severe that all die. In 

 this instance there is total mortality of both 

 host and parasite. 



In one section of his pa^er Salt experi- 

 mentally analyzes the effect of varying the 

 density of parasites upon the development 

 of Trichogramma when exposed to a fixed 

 number of hosts. In these studies 0, 1, 5, 

 10, 25, and 50 fertilized female Tricho- 

 gramma were exposed under standard con- 

 ditions to 100 Sitotroga eggs. 



