Utilization of the nutrients in the soluble and insoluble fractions of fungal mycelium by larvae of the stag beetle, Dorcus rectus (Coleoptera: Lucanidae)

Masahiko Tanahashi,Kôhei Kubota

doi:10.14411/eje.2013.083

Abstract

Larvae of the stag beetle, Dorcus rectus, feed on decaying wood, which they digest with the aid of symbiotic yeasts; however, they can be successfully reared on artificial diets containing only fungal tissue. In this study we tested whether D. rectus larvae can utilize fungal cell walls, which are an insoluble component of mycelium. Lyophilized Bjerkandera adusta mycelium cultured in potato-dextrose liquid medium consisted of a 47.6% hot-water insoluble fraction by mass, which contains 53.7% of the total nitrogen in the mycelium. D. rectus larvae that hatched from surface-sterilized eggs were reared for 14 days on agar-based diets containing either the soluble fraction, insoluble fraction or both, extracted from 100 mg of mycelium. The larvae increased in mass most on the mixed diet, and there was no difference in their growth on the mixed and positive control diets. Both the soluble and insoluble fractions improved larval growth compared to the negative control diet; however, the growth rates were much lower than those expected from the nitrogen dose-growth response curve obtained in a previous study. Addition of b-chitin to the soluble fraction did not positively affect larval growth. Therefore, we conclude that (1) D. rectus larvae need both the soluble and insoluble fractions of mycelium and (2) the larvae digest the insoluble fraction using their own enzymes.

Highlights

Solubility is one of the key factors determining food quality
We show that (1) the agar-based artificial diet can be used to determine the nutrient requirements of stag beetle larvae, (2) they require both soluble and insoluble fractions of fungal mycelium and (3) they can digest the insoluble fraction without the aid of gut microbes
Larval growth recorded on the mixed diet (50.2 ± 3.4 mg) was greater than that recorded on the positive control diet (46.7 ± 7.9 mg) but they are not significantly different

Summary

Introduction

Solubility is one of the key factors determining food quality. Since soluble nutrients are more digested and quickly absorbed than insoluble ones, the amount and balance of soluble nutrients dramatically affect the growth, survival and fecundity of animals such as cows (Janicki et al, 1985), rats (Bronner, 1993), birds (Jefferies & Edwards, 2008) and insects (Awmack & Leather, 2002). Phytophagous insects feed mainly on the soluble components of plant tissues and rarely digest lignocelluloses, which are the main constituents of plant cell walls (Risebrow & Dixon, 1987; Haack & Slansky, 1987). Since wood is composed of almost 90% lignocelluloses (Parkin, 1940) and contains only 2–10% water-soluble substances (Mahood & Cable, 1922; Browning, 1963; Kilic & Niemz, 2012), it is far more difficult for insects to utilize wood directly as food. Many wood-inhabiting insects have relationships with microorganisms, which help them digest wood. Most other wood-feeding insects are thought to be saproxylophagous, which means they feed on decaying wood that is infested with wood-rotting fungi. Saproxylophagous insects are likely to utilize both the fungal tissues and the woody substrates that are partially decomposed and solubilized by the wood-rotting fungi

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Results

Conclusion