Host Discrimination in Three Ectoparasites (Aphytis Coheni, a. Lingnanensis and a. Melinus) of the Oleander Scale (Aspidiotus Nerii)

1. The host-searching and oviposition behaviour of three Aphytis species (A. coheni, A. lingnanensis and A. melinus; Hymenoptera, Chalcidoidea, Aphelinidae), ectoparasites of armoured scale insects (Homoptera, Coccoidea, Diaspididae) was studied to obtain information about the presence of the ability to distinguish parasitized hosts from unparasitized ones in gregarious parasites. 2. Analysis of frequency distributions of parasite eggs over the hosts showed that extreme care must be exercised when dealing with such indirect results, because differences in host distribution patterns (i.e. hosts occurring in clusters versus isolated hosts) resulted in large differences in the frequency distribution of parasite eggs per host. These differences in egg distributions may prevent detection of the parasite's discrimination ability. 3. Careful analysis of these data suggested, however, that the three Aphytis species are able to distinguish unparasitized from parasitized hosts, and direct observation of the oviposition behaviour confirmed this suggestion. 4. Furthermore, the data indicate that Aphytis adults are able to 'count' the number of parasite eggs present on a previously parasitized host and that they prefer to lay on hosts containing the lowest number of parasite eggs when meeting parasitized hosts. 5. Females of the three Aphytis species apply an external mark on the scale cover of their hosts after parasitization, and they are also able to detect internal changes within a previously parasitized host. Rejection of parasitized hosts usually occurs when the parasite perceives the external mark while she is drumming with her antennae on the scale cover; however, if she provisionally accepts a parasitized host, rejection can still occur when she detects the internal change with the ovipositor inserted into the host's body. 6. When third stage oleander scales are used as hosts, the number of eggs laid per host visit is usually 1 for A. coheni and A. lingnanensis, and about 2 for A. melinus. The first two species are more or less solitary parasites, A. melinus is a gregarious species, usually laying two or more eggs per host visit. More than one parasite may develop per host in A. melinus without a reduction in size of the resulting adult. If more than one parasite develops per host in A. coheni and A. lingnanensis, a reduction in size of the parasite adults occurs. 7. The ability to discriminate is important for both the solitary and gregarious Aphytis species, because it prevents the wasting of parasite eggs, it saves time (rejecting a host takes only a fraction of the time necessary for an entire oviposition sequence) and it may enable the parasite to determine the condition of a patch with hosts (i.e. to leave a site where only parasitized hosts are present).

Affiliations: 1: Division of Biological Control, Department of Entomology, University of California, Riverside, Department of Ecology, Zoological Laboratory, University of Leiden, Leiden, The Netherlands; 2: Division of Biological Control, Department of Entomology, University of California, Riverside