The Evolution of Host Suitability in a Polyphagous Parasite with Special Reference to the Role of Parasite Egg Encapsulation1

Abstract

Attempts were made to reconstruct the evolutionary pathways by which suitability to new hosts might be acquired and developed by a polyphagous parasite. Three criteria of host suitability—acceptability for oviposition, incidence of parasite egg encapsulation, and fitness for maturation of parasite larvae—were measured in some new and some regular host scale insects of the encyrtid parasite Metaphycus luteolus (Timberlake). Values of each suitability measure varied within each host species, no single criterion serving as a reliable index of overall suitability. Certain similarities and differences in egg encapsulation capacities of the hosts suggested possible routes for the genesis and progression of this host defense mechanism, and indicated possible routes of parasite accommodation. Parasite egg encapsulative capacity was shown to develop in a host independently of the presence of the parasite involved. Fitness of a host for larval parasite development appeared to be a less flexible attribute than expected. Little tendency could be shown for parasite adaptation to the host suitability factors studied.