Insect herbivory is common and has been shown to drastically limit the reproductive success of many plant species. Most research examining the affects of herbivory has focused on common plants, while very little attention has been given to understanding how herbivory impacts the reproductive success of rare plant species.
Whited’s milk-vetch (Astragalus sinuatus) is a Washington endangered plant species. Its entire distribution is restricted to an 8km square area in the shrub-steppe habitat of Eastern Washington. Ms. Julie Combs’s Master’s Degree research determined that in areas of high Bromus tectorum cover, A. sinuatus is most likely limited by B. tectorum, an invasive annual grass. However, in areas of low B. tectorum cover, A. sinuatus is more likely limited by insect seed predators.
To examine whether competition with B. tectorum inhibits seedling germination and survivorship of A. sinuatus, seeds were added to plots with and without B. tectorum. Overall seedling germination did not differ between control and removal treatments. But overall seedling and juvenile establishment after the two-year monitoring period was significantly higher in plots where B. tectorum was removed. These results show that insect herbivores may strongly affect demographic processes of A. sinuatus.
Primary seed predators were tortricid moth larvae and two species of specialist beetles: the seed weevil, Tychius semisquamosus LeConte, and the seed beetle, Acanthoscelides fraterculus Horn. An insect reduction experiment using insecticide and manual removal techniques suppressed insect herbivores, resulting in 164-345% greater viable seed production. Insect herbivory indirectly led to a seven-to eleven-fold greater incident of fungal attack on plants exposed to herbivores. These results show that insect herbivores directly and indirectly decrease viable seed output of A. sinuatus.
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