Abstract:Adaptations of parasitoids to fluctuating host resources are essential for their survival and reproduction. Among these adaptations, host selection behaviors have important consequences for adapting to an ever changing environment. Generalists parasitoids are perhaps less efficient at discovering and using one particular host resource compared to specialists, but they have the advantage of exploiting a wider range of host resources. When one host population is at low density, generalist parasitoids have ability to maintain a relative high population density on other hosts. The effectiveness of biological control statistically tends to be higher when the agents used are generalists than when they are specialists. Sometimes the key to successful biological control may not lie in discovering another species, but instead may rely on the use of different biotypes or geographic races of biocontrol agent. The present paper summarizes the host adaptations of generalist parasitoids and some factors influencing the choice of hosts. Parasitoids will differentiate among populations of themselves not only due to geographic isolates, but also duo to different hosts they parasitized for better adaptations to some particular attributes of hosts′ life and the microhabitats therein. The selection by parasitoids of which species to use as hosts is influenced by multiple factors such as host species, host developmental stages, host sizes, host nutrition, host immunoreactions, host escape responses, competition from other natural enemies, presences of host associated symbionts or mutualists, host plants, learning behaviors of parasitoids, and symbiotic microorganisms of parasitoids, etc. In addition, the potential utilization of novel host resources, the mechanisms of exploitation, and host adaptations of generalist parasitoids are discussed. This discussion may provide new ideas and theoretical support for improving biological control efficiency by enhancing our understanding on host\|parasitoid interactions and artificially selecting for novel parasitoid/host relationships for augmenting the management of target insect pests.