Experimental Studies in Insect Parasitism. VI.—Host Suitability

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Abstract
1. For the purposes of this paper a suitable host is defined as one on which the parasitoid can generally reproduce fertile offspring.2. To be suitable forTrichogramma, a host must satisfy conditions required by two generations of the parasite: it must be such that it can be actually parasitized by the adult parasitoid; and it must provide an environment in which the parasite offspring can develop.3. Some hosts prevent the attack ofTrichogrammaleading to parasitization by the physical resistance of their chorion; others by inhibiting the impulse to oviposition.4. Five physical characteristics of the hosts ofTrichogrammaare investigated: the permeability, rigidity, and hardness of the egg-shell; and the fluidity and quantity of the egg-contents (pp. 226 to 231).5. Eggs ofSialis lutaria,Tenebrio molitor, andBruchus obtectusare chemically unsuitable forTrichogramma evanescensas food.6. Two biological characteristics of the host, its viability and its age, do not directly affect its suitability forTrichogramma; a third, the ability of the embryo to move, is of some importance.7. The various known factors of host unsuitability are given in tabular form (p. 239).8. In spite of numerous attempts during several years it has not been possible to develop a strain ofTrichogramma evanescenswhich could useBruchus obtectusas a host.9. The host list of most groups of parasites is greatly limited by the inaccessibility of many species; it is further restricted in some groups (e.g., the parasitoid Hymenoptera) by the selection exercised by the parasite; it is finally limited in all groups (and especially the parasitoid Diptera) by the unsuitability of many infected animals.10. The division of the problem of host specificity into the three parts, host finding, host selection, and host suitability, is fundamental.

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