Effects of Insect Seed Predators on Astragalus cibarius and Astragalus utahensis (Leguminosae)
1975
Green, Thomas W. | Palmbald, Ivan G.
The genus Astragalus (Leguminosae) is noted for a wide diversity of compounds toxic to animals and for varied fruit morphology. Seed predation by insects was measured in mixed populations of Astragalus cibarius (browse milkvetch or locoweed), a toxic species with glabrous fruits, and Astragalus utahensis (Utah milkvetch), a nontoxic species with pubescent fruits. The study was conducted for 3 yr in areas where both species were sympatric, allowing direct comparison of insect faunas. The major seed predator on both species was a seed beetle (Acanthoscelides fraterculus, Coleoptera: Bruchidae) which destroyed 74% of the A. cibarius seeds and 60% of the A. utahensis seeds examined. Additional A. cibarius seed destruction, principally the result of predation by a seed chalcid wasp (Bruchophagus mexicanus, Hymenoptera: Eurytomidae), averaged 25%. Differential predation on the two plant species may be due to differences in phenology, seed and pod energy content, internal pod temperature, chemicals in the pods, and pod morphology. As A. cibarius and A. utahensis individuals apparently compete for a number of resources when they occur sympatrically, the differential predation rate probably influences the population dynamics of both species. Selection pressure by seed—eating insects may account for much of the chemical an morphological diversity exhibited by the genus Astragalus at the present time.
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