Variation in herbivore resistance traits among populations of Asclepias speciosa (Asclepiadaceae)

Background/Question/Methods

We investigated variation in levels of herbivory and herbivore resistance traits in populations of Asclepias speciosa from the western slope of Colorado both in the field and in a common garden study. Field surveys in the summer of 2010 cataloged herbivore community composition and levels of herbivore damage. Herbivores were identified and counted on a per stem basis and herbivore damage per stem was recorded. Data on herbivore resistance traits were collected from 20-30 individual plants in each of six populations. We collected leaf discs (28mm² ) from the distal region of the youngest fully expanded leaf and used these discs to quantify specific leaf area (SLA). We quantified latex exudation on all plants by cutting the tip off the youngest, fully expanded, undamaged leaf and collecting the latex on a pre-weighed filter paper disc. In the fall of 2010 we collected mature follicles from 20 plants from four populations, and after germinating the seeds, planted 10 seedlings from each individual and grew them in growth chambers for six weeks. After six weeks we collected leaf discs to determine SLA. We measured latex exudation on all plants as described above.

Results/Conclusions

Field surveys of populations of A. speciosa revealed evidence for variation in herbivore abundance and damage. Significant differences among the six populations surveyed were found in field collections of latex and specific leaf area (SLA). In the common garden study differences in resistance traits among populations were no longer significant, however, family (nested within population) had a significant effect on latex production and SLA. We conclude that, similar to what has been found in other species of Asclepias, constitutive latex production has a strong heritable component and may respond to selection by herbivores. However, we found no evidence for local adaptation in herbivore resistance traits at the geographic scale investigated.