[wirasitoid tends to vary proportionally with the dens- 

 ity of the host (Varley 1947). However, it has been 

 shown experimentally that with increase in jwpula- 

 tion density of the parasitoid, there is decreased re- 

 productivity for it, increased difficulty in finding in- 

 dividual hosts not already infected (DeHach and 

 Smith 1941), and increased competition hetw^een 

 duplicate infestations in the same host individual 

 {suf>cr-parasitisiit) so that neither parasitoid sur- 

 vives (Fiske 1910). Of interest in this regard is 

 that in one experiment 50 parasitoids during a lim- 

 ited period of time were able to find and eventually 

 kill 80 ])er cent of the hosts, but that it required 100 

 parasitoids to find 95 per cent of the hosts and 200 

 parasitoids to find them all. This phenomenon is 

 comparable to the law of diminishing returns and is 

 doubtless one reason why a parasitoid rarely ex- 

 terminates a host (DeRach and Smith 1947). In 

 order for a particular parasitoid to regulate the num- 

 bers of a particular host species, it ordinarily needs 

 to have a high intrinsic rate of increase, at least equal 

 to its host (Muir 1914), and to have high searching 

 ability for locating host individuals (Andrewartha 

 and Birch 1954). 



Even though the density of host or prey popula- 

 tion greatly aflfects the success with which a parasitoid 

 or predator finds its victim, searching is not random 

 as far as the individuals are concerned. Predators in 

 general have evolved many adaptations in sense or- 

 gans, methods of attack, and special behavior patterns 

 designed to facilitate the finding of specific prey, 

 avoid unsuitable objects, save time, and increase effi- 

 ciency (Thompson 1939). However, searching is 

 largely at random as far as the area covered by the 

 entire population is concerned since the individuals 

 mostly hunt independently of each other and may 

 cover the same or dififerent areas indiscriminately. 

 Many parasitoids avoid placing their eggs inside the 

 bodies of prey that are already infected, but this be- 

 havior tends to break down when the ratio of number 

 of parasitoids to number of uninfected prey is high. 



The coaction between host and parasitoid may be 

 complicated by differential elTect of environmental 

 conditions, such as temperature, on the two species. 

 Experimental studies have shown that the greenhouse 

 whitefly, a homopteran, at temperatures below 24°C 

 lives longer, lays more eggs, has a higher rate of 

 oviposition, and consequently increases more rapidly 

 in abundance than does it chalcidid parasitoid. At 

 24°C. the rate of population increase is about the 

 same in the two species, but above 24°C, the para- 

 sitoid population increases more rapidly than does the 

 host species. The result is that the percentage of 

 hosts infected increases markedly with rise in tem- 

 perature (Burnett 1949). Similar relations have been 

 demonstrated for other species of hosts and para- 

 sitoids (Payne 1934). 



lUitTer species may be as important with ])ara- 

 sitoids as with true predators. Prior to 1925 in the 

 Fiji Islands, the zygaenid moth Lcvuana iridescens 

 was a serious pest of coconuts, defoliating trees over 

 extensive areas. Outbreaks terminated only when its 

 supply of food became exhausted. In 1925, the tach- 

 inid fiy Ptycliomyia rcuioia was introduced from 

 Malaya and within a year reduced Lcvuana to a rare 

 species, a status which it has had ever since. How- 

 ever, Ptycliomyia requires alternate hosts to main- 

 tain its existence when Lcvuana becomes reduced in 

 numbers. Such alternate hosts, or buffer species, oc- 

 cur on most of the Fiji Islands, but on one island 

 from which they are absent the death rate among the 

 predators became so high that Lciivana has been able 

 again to increase in numbers (Andrewartha and 

 Birch 1954). 



There are several known cases where crop pests 

 in agricultural regions have been controlled or 

 virtually extirpated by introduced parasitoids and 

 predators ( Fleschner 1959, Varley 1959). In the 

 Hawaiian Islands, the sugar cane leafhopper is con- 

 trolled by the capsid bug Cyrtorhinus mundulus, and 

 the sugar cane borer by the tachinid fly Ceromasia 

 sphenophori. The black scale in California is suc- 

 cessfully controlled by the chalcidid Scutillista cyariea 

 imported from South Africa. The silk industry of 

 Italy was apparently saved by importations of Pros- 

 paltclla berlcsi which controlled the scale that w-as 

 destroying mulberry trees, on the leaves of which the 

 silkworm feeds (Thompson 1928). Not all cases of 

 supposed control of pests by parasitoids and preda- 

 tors can be substantiated in critical study. Pests most 

 adequately controlled are usually scale-insects, mealy- 

 bugs, aphids, and leafhoppers of the order Homoptera 

 which are sedentary, gregarious, and limited in the 

 number of host species that they attack. Beneficial 

 parasitoids and predators belong chiefly to the He- 

 miptera, Diptera, Coleoptera, and Hymenoptera 

 (Sweetman 1952). 



The relations between some insect herbivores and 

 plants are somewhat similar as between parasitoids 

 and hosts. Cactoblastis and Dactylopius, introduced 

 from California, are credited with destruction of large 

 areas of tree cactus in the Hawaiian Islands (Huf- 

 faker 1957). 



Attempts at artificial control of pests with in- 

 secticides often bring disorder and unexpected re- 

 sults. Elm trees on the University of Illinois campus 

 were sprayed with DDT to control Scaphoidcus 

 hitcolus. a leafhopper vector of phloem necrosis 

 which was causing considerable destruction of the 

 trees. But the spray also caused a high mortality of 

 Aphytis uiytilaspidis. a hymenopteran parasitoid of 

 the scale Aspidiotus, and allowed the scale to in- 

 crease and do damage in turn to the trees (Tinker 

 1957). 



Regulation of population size 227 



