FREQUENCY-DEPENDENT HOST SELECTION BY PARASITIC MITES - A MODEL AND ACASE-STUDY

Previous studies on frequency-dependent food selection (changing food preferences in response to changes in relative food abundance) have fo cused on predators and parasitoids. These organisms utilize several vi ctims during their lifetime. We introduce the case of parasites which, having accepted a host, do not change it. We propose two alternative models to explain the biased occurrence of parasites on different host types: (1) through the option of rejecting less-preferred hosts prior to accepting one of them; (2) by differential parasite survival on di fferent host types. These models predict that host rejection, but not differential survival, can create frequency-dependent parasitism (FDP) . Unlike previously described factors responsible for frequency depend ence of food selection, which act through changing the foraging behavi our of individual predators or parasitoids, FDP involves no adjustment of parasite foraging strategy according to previous feeding experienc e. The mite Hemisarcoptes coccophagus is an obligate parasite of armou red scale insects (Homptera: Diaspididae). Our held data show that H. coccophagus is found more frequently on ovipositing than on young host females. Our model, combining the effects of host rejection and diffe rential survival, is used to estimate the relative contribution of the se factors to parasite biased occurrence on different hosts. The contr ibution of differential survival was dominant in H. coccophagus, and o verrode any effect of host rejection. Nevertheless, our prediction tha t FDP may be found in parasites is supported by literature data about a parasitic water mite.