Feeding habits of generalist herbivores in soil are largely unknown due to a paucity of sufficient methods. Here we present a fatty acids (FAs) analyses to examine diets of the generalist, a larva of Anomala cuprea (Coleoptera: Scarabaeidae).

A larva was grown with single diets (i.e., humus, wood, and roots of Lolium perenne or Plantago lanceolata grown in sand) or mixture of the roots and humus or wood for a month.

Among the single diets, humus had the highest total nitrogen content while wood had the lowest. Increment of larva fresh biomass was the largest in humus while the smallest in wood. The mixture of roots and humus enhanced the larva growth, but those of roots and wood did not. FA profiles of single diets significantly differed, and that profiles of a larva significantly differed from its diets. The typical marker FAs known as bacteria specific FAs, were found in humus, and some of them were also found in a larva. This indicates the larva fed on humus. The FA 20:0 in plant roots also acted as a marker FA. Content of the plant marker in the larva grown with wood mixed diets was higher than that grown with humus mixed diets.

FAs analysis should be an effective approach to reveal diets of a generalist if diets contain typical marker FAs. The generalist will selectively feed on the diets in a given environment to enhance its growth.