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

Abstract

(1) A model of parasitism and spatial distribution was constructed using field observations of a natural system: The tortricid Epinotia tedella (Cl.) and its two primary parasitoids, Pimplopterus dubius (Hgn.) and Apanteles tedellae Nix. 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. (2) As the distribution of hosts and parasitoid attacks is apparently governed by the preference of the species for certain physical conditions, the distributions were modelled as fixed patterns. Apanteles tedellae showed a stable interaction with the host, due to its aggregation, whereas P. dubius provoked increasing host fluctuations, even when all three species were acting together. (3) Both parasitoids could eliminate each other, depending on their searching constants, and change the system into a stable or unstable two-species system. A stable coexistence of the three species proved not to be attainable. (4) Various distribution patterns of P. dubius were simulated, and it was found that 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. (5) 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. However, 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. (6) As expected, simulated dispersal of P. dubius resulted in coexistence, but the dispersal influenced the apparent log a/log P relation (pseudo-interference) by turning the, otherwise curvilinear, relationship into an almost linear function. Thus, the 'pseudointerference'-model of parasitism may include dispersal as well as spatial and temporal distributions.