Interactions Within a One-Host-Two-Parasitoids System, Studied by Simulations of Spatial Patterning

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Abstract
A model of parasitism and partial distributions was constructed using field observations of a natural system: the tortricid Epinotia tedella (Cl.) and its 2 primary parasitoids, Pimplopterus dubius (Hgn.) and Apanteles tedellae Nix [in spruce stands in New Zealand]. The latter species is the inevitable winner in cases of multiparasitism, and neither of the parasitoids shows any discrimination ability. Local search is random, but at a larger scale the attacks of A. tedellae are aggregated. As the distribution of hosts and parasitoid attacks is apparently governed by the preference of the species for certain physical conditions, the distributions were modeled as fixed patterns. A. tedellae showed a stable interaction with the host, due to its aggregation, whereas P. dubius provoked increasing host fluctuations, even when all 3 spp. were acting together. Both parasitoids could eliminate each other, depending on their searching constants, and change the system into a stable or unstable 2-species system. A stable coexistence of the 3 spp. proved unattainable. Various distribution patterns of P. dubius were simulated. Distinct aggregation of this species led to its elimination by A. tedellae, except when P. dubius was much more strongly aggregated and provided with a much higher searching constant than its competitor. Aggregation pattern, in combination with the searching ability, strongly affected competition between the parasitoids. The multiparasitic loser was eliminated from aggregations at high host densities because of multiparasitic overlap, but aggregation in patches of low host density, with low host exploitation, also resulted in competitive exclusion. Thus, the lack of distinct aggregation proved to be beneficial for the multiparasitic loser, P. dubius. As expected, simulated dispersal of P. dubius resulted in coexistence, but the dispersal influenced the apparent log a [area of discovery]/log P [parasitoid density] relation (pseudo-interference) by turning the otherwise curvilinear relationship into an almost linear function. Thus, the pseudo-interference-model of parasitism may include dispersal as well as spatial and temporal distributions.