So far I have been unable to find any suitable references. I want to compare the nutritional value of two host plants based on their mineral content (e.g. iron, copper, manganese, calcium, etc.).
There's not a vast amount on this topic as far as I know, but these may help as starting points:
Chararas, C., & Courtois, J.E. 1976. Nutrition and digestive enzymes activity of Dendroctonus-micans xylophagous Coleoptera Scolytidae. C. R. Seances. Soc. Biol. Fil. 170 (6). 1155-1158.
Chipoulet J M., 1979: Nutrition and glycosidase activity of 2 scolytidae coleopteran parasites of the elm ulmus campestris. Bulletin Mensuel De La Societe Linneenne De Lyon: 144, 177-192.
Mattanovich, J. , Ehrenhöfer, M. , Schafellner, C. , Tausz, M. and Führer, E. (2001), The role of sulphur compounds for breeding success of Ips typographus L. (Col., Scolytidae) on Norway Spruce (Picea abies [L.] Karst.). Journal of Applied Entomology, 125: 425–431.
Tittiger, C., Keeling, C.I, Blomquist, G.J. (2005). Chapter Three: Some insights into the remarkable metabolism of the bark beetle midgut. Recent Advances in Phytochemistry, 39: 57–78.
I was able to get the last two references, but not the first two. I also managed to get a hand on Slansky (1987) Nutritional Ecology or Insect, mites, spiders and related invertebrates, which has a chapter on mineral requirements of foliage feeding insects. Not quite bark beetles, but at least the possible functions of certain minerals in insects are mentioned. There is also a chapter on phloem and wood feeder, but mineral requirements aren't mentioned in those two chapters.