An interaction between a specialized seed predator moth and its dioecious host plant shifting from parasitism to mutualism

Abstract

Westerbergh, A. 2004. An interaction between a specialized seed predator moth and its dioecious host plant shifting from parasitism to mutualism. Á/ Oikos 105: 564 Á/574. Seed predator/pollinator and host plant interactions, which may be considered as antagonistic, have the potential to provide good model systems for the study of the early stages of evolution towards mutualism. I describe a relationship between a seed predator, the geometrid moth Perizoma affinitatum , and the dioecious plant Silene dioica . The moth is an obligate seed predator on its host plant. The searching and ovipositing behaviour of the female moths, number of eggs deposited per flower, the pollinating ability of the moths and the seed consumption by the larvae are described as different parameters and studied in two Finnish coastal populations. A high pollinating ability and limited seed consumption by the predator was found and discussed in relation to fitness models of P. affinitatum and S. dioica . In a mutualistic relationship there must be a balance between the costs and benefits so that the seed production by the moths is larger than the seed consumption by the larvae, given a net seed output larger than zero. The data of the parameters included in a seed production/ consumption model give a positive seed output when the proportion of S. dioica flowers pollinated by other non-predating insects is less than 60%. Accordingly, even if P. affinitatum would become the exclusive pollinator it would not endanger the survival of the host plant and both partners would benefit from this interaction. Limited seed consumption, high pollinating ability and host specificity as seen in the P. affinitatum and S. dioica interaction are considered to have been important pre-existing qualities in the evolution of the obligate mutualisms between yucca and yucca moths and fig and fig wasps. In isolated serpentine populations where the gene flow is restricted and copollinators are rare the interaction between P. affinitatum and S. dioica has the potential to shift from parasitism to mutualism.

Repeated independent evolution of obligate pollination mutualism in the Phyllantheae– Epicephala association

“…In this sense, the active pollination behaviour in Epicephala has been of critical importance for the establishment and maintenance of the Phyllantheae-Epicephala mutualism and thus represents a key innovation in this association. Second, the outcome of a species interaction can vary greatly depending on the community context in which it occurs ( Thompson & Pellmyr 1992;Thompson & Cunningham 2002;Westerbergh 2004); thus, transitions between mutualism and antagonism can occur repeatedly within a single phylogenetic lineage. This parallels recent findings in other mutualisms where derived parasitic taxa are nested within ancestrally mutualistic clades (Pellmyr et al 1996b;Machado et al 2001;Als et al 2004).…”

Benefits and costs to pollinating, seed-eating insects: the effect of flower size and fruit abortion on larval performance

“…In nursery pollination, the pollinator acts as a seed predator during its larval stage (Dufaÿ and Anstett 2003;Kephart et al 2006;Westerbergh 2004). This should lead to plant defenses to reduce the costs imposed by seed predation.…”

Density‐dependence at multiple scales in experimental and natural plant populations

“…One explanation is that they are related to the positive effects of density on seed herbivory at similar scales. Hadena moths are a significant pollen vector as well as seed herbivore for S. latifolia flowers (Westerbergh 2004), so if moths tended to seek out denser patches of female plants in the experiment, they may also have increased their pollination. Alternatively, the positive fine‐scale effect of female flowers may be an artefact of the positive correlation between a focal plant's seed production and the number of flowers that it bore itself, because flowers on focal plants could not be reliably distinguished from other flowers in the surveys of the experiment.…”