Abstract:
The omnivorous ground beetle, Pterostichus melanarius Illager (Coleoptera:
Carabidae), is a ubiquitous inhabitant of western Oregon crop land. Because it is often the
most abundant ground beetle, and attains densities that could lead to serious impacts on its
prey sources, I sought to understand the overall diet composition, and specifically the
importance of weed seeds in its diet at three sites in western Oregon. I compared the use of
stable isotope analysis and manual gut dissection as tools for determining diet composition,
and examined the influence of site characteristics.
Visual gut inspection identified broad groups of food items that included arthropods,
worms and plant material, while the analysis of δ¹³C and δ¹⁵N signatures revealed more
defined prey groups including small ground beetles, spiders, soil larvae and C3 and C4 seeds.
Visual examination of gut contents, and comparing the δ¹³C and δ¹⁵N signatures of potential
prey sources with gut content each lead to diet compositions that varied between sites and
seasons. However, both techniques indicated that the primary contributors to the diet of P.
melanarius are ground dwelling arthropods and earthworms. Specifically, the δ¹³C and δ¹⁵N
signatures of small ground beetles, earthworms, spiders, centipedes and soil dwelling larvae
made the largest contributions to the δ¹³C and δ¹⁵N signature of the diet at each of the sites.
Weed seeds did not contribute significantly to the beetle's diet, however there is preliminary
evidence that suggests further examination of the larval diet may reveal that seeds are
important at that stage of life. I did find overall differences in δ¹³C and δ¹⁵N diet signatures
between sites—the isotopic signatures at the two most geographically similar sites were
different than those at the third site. Because general categories of food that were included in
the diet did not differ between sites, we attribute isotopic differences to as yet undetermined
site characteristics.
