Cellulolytic insects and their potentials for viable biofuels: a new frontier discipline in Entomology and Bioengineering

Key Words： lignocellulose-feeding insects, biomass, biofuels, biocatalysis and biodegradation, wood-feeding termites

Abstract：        Most insects are unable to use plant cell walls as their main food sources, but some insects subsist on lignocellulosic biomass from agricultural crops to forest woody substrates as their only foods, such as in the case of termites (all seven families), wood-feeding roaches (Blattidae, Cryptoceridae), beetles (Anobiidae, Buprestidae, Cerambycidae, Scarabaeidae), wood wasps (Siricidae), leaf-shredding aquatic insects (Pteronarcidae, Limnephilidae, Tipulidae), silver fish (Lepismatidae), leaf-cutting ants (Formicidae), etc. The ability of these insects to feed on wood, foliage and detritus has recently stimulated extensive investigations into the mechanisms of how these insects digest the structural and recalcitrant lignocellulose in their foods. With these studies, scientists would possibly advance biofuel technologies with the discovery of novel lignocellulolytic enzymes and a better understanding of the bioconversion mechanisms that breakdown plant cell walls inside the insect’s gut. Producing monomeric sugars from cellulose or hemicellulose with high yields and low cost is far more difficult than deriving them from sugar- or starch-containing crops. This difficulty is primarily due to a lack of efficient and economic lignocellulolytic enzymes that convert rigid plant cell walls to their monomeric pentose and hexose sugar subunits. However, cellulolytic insects, especially wood-feeding termites, are a unique group of lignocellulose-feeding animals exhibiting incredible wood degradation capabilities, which accomplish lignocellulose digestion using specialized gut physiology, endogenously produced digestive enzymes, and via their specialized association with prokaryotic and eukaryotic gut symbionts. It is believed that the guts of these lignocellulose-feeding insects harbor diverse symbiotic microbes and endogenous enzymes that could be used as a rich source of lignocellulases as well as functional gene resources for improving the conversion of wood or waste plant biomass to valuable biofuels. Thus, as a frontier and emerged discipline crossing entomology and bioengineering science, utilization of lignocellulose-feeding insects would be very valuable for viable biofuels production made from lignocellulosic biomass. Clearly, understanding the mechanisms of the biomass digestion in these insect guts could potentially shed light on efficient, low cost, lignocellulose-based biofuel production systems. This mini-review addresses various lignocellulolytic systems, the potential values, various challenges, and opportunities that exist for investigating lignocellulose-feeding insects in biofuels production, as well as possible future research directions.