Resource Partitioning of the Sex Communication Channel in Clearwing Moths (Lepidoptera: Sesiidae) of Wisconsin

Clearwing moths (Lepidoptera: Sesiidae) are a cosmopolitan family which use female- emitted pheromones as long-range sex communication signals. The pheromones of two sesiids have been identified as ZZ- and EZ-3,13-octadecadien-l-ol-acetate. Through field trapping in Wisconsin during 1975, 1976, and 1977, males of 21 of 29 sesiids known to occur in the state were found to respond to the above compounds or similar ones. The data obtained from this field trapping were used to evaluate the contributions to reproductive isolation in Sesiidae provided by differences in pheromone, seasonal and diel activity, year of emergence, and habitat. On the basis of these data, Wisconsin Sesiidae can be grouped into six sets such that reproductive isolation between species assigned to different sets is maintained by pheromone differences. In three of the six sets more than one species are found. The data show that segregation of species within these sets is accomplished by differences in seasonal or diel activity. An objective analysis of this segregation is accomplished by considering the channel along which sex communication occurs as a resource. The chemical, temporal, and spatial axes along which the communication channel resource is partitioned among sesiid species are viewed as niche dimensions. Differential resource utilization along a niche dimension then constitutes an isolating mechanism. This conceptual framework and the computation of niche breadth and niche overlap along all dimensions allow the quantification of the degree of potential reproductive isolation (=complement of niche overlap) along all niche dimensions. Niche breadth along the various dimensions varies greatly between species, but mean (of all species) niche breadth tends to be approximately equal along each dimension. In 93% of all pairs of common Wisconsin sesiid species, relatively high reproductive isolation (niche overlap < 0.05) is achieved by partitioning along a single (chemical, seasonal activity, or diel activity) niche dimension. The remaining 7% of species pairs achieve reproductive isolation by simultaneous partitioning along more than one of the above niche dimensions. An analysis of the dispersion of niches suggests that this resource partitioning evolved as a means of achieving reproductive isolation. Pairs of Wisconsin sesiid species never depend upon year of emergence or habitat partitioning to achieve reproductive isolation. Many sesiid species pairs maintain low niche overlap along more than one dimension simulta- neously. This redundancy may possibly result from "environmental noise." Such "noise" could arise when different species exhibit differing degrees of plasticity in their response to changing environ- mental factors. Niche overlap between a species and all other species in its guild pooled together as one (diffuse niche overlap) is low for all Wisconsin sesiids when partitioning along four niche dimen- sions (pheromone, seasonal, diel, spatial) is considered overall. Reproductive isolating mechanisms in Sesiidae are "long-range" in that they operate before the male and female enter the same courtship arena. Sesiidae and many other moths generally have short adult lifespans during which they do not feed, as opposed to the nectarivory of adult butterflies. Long- range reproductive isolating mechanisms allow Sesiidae and other moths to conserve time and energy during mate seeking.

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