Minireview Cellulose Digestion in Insects

I. Cellulose digestion hns been demonstrated in the Thysanura (Lepismatidae), Orthoptera (Cryptocercidae, Blattidae). Isoptera (Mastotermitidae. Kalotermitidae. Hodotermitidae, Rhinotermitdae. Termitidae). Coleoptera (Buprestidae. Anobiidae. Scarabaeidae, Cerambycidae), and Hymenoptera (Siricidne). 2. In all but the scarab beetles. cellulose digestion is brought about by a complex of three types of enzymes (C,-cellulases, C,-cellulases, and cellobiases). as in fungi. 3. Many insects are able to synthesize their own C,-cellulases and cellobiases, but few (if any) can synthesi7e C,-cellulases. 4. Insects compensate for their inability to synthesize C,-cellulases by exploiting the cellulolytlc potcntlal of protozoa. bacteria. or fungi. 5. The maintenance of permanent populations of hindgut protozoa. the maintenance of permanent populations of hindgut bacteria, and the ingestion of fungal cellulases are described as three distinct mechanisms by which insects have been shown to use the cellulolytic potential of microorganisms. 6. A process in which ingested cellulolytic bacteria proliferate in one region of the gut at the expense of ingested cellulose. only to be digested and assimilated in a more posterior section. is a fourth possible mechanism by which insects might accomplish the digestion of cellulose with the help of microorganisms. INSECTS THAT DIGEST CELLULOSE The ability of many insects to thrive on wood, foliage and detritus has naturally stimulated investigations of the extent to which such species are able to digest the structural polysaccharides in their food. Species that have been shown to possess a capacity to digest cellulose are listed in Table I. Based upon the values of the approximate digestibilities or assimilation eficiencies that have been determined for a few species, it appears that termites are more efficient cellulose digesters than the wood-boring beetles and siricid wood wasps. The silverfish, Ctenolepisn~u lineatrr. and the American cockroach, PtvQplmwtu mwricm~~, are omnivorous species that readily include both cellulosic and non-cellulosic items in their diets. The rest of the species listed in Table I are wood-feeders. Foliage and detritus-feeders are conspicuously absent. No evidence for significant digestion of cellulose by foliage-feeding Orthoptera, Coleoptera or Lepidoptera or by detritus-feeding Plecoptera. Coleoptera, Diptera or Trichoptera has been reported. although low levels of hydrolytic activity toward cellulose powder have been detected in the gut fluids of a few species of locusts (Evans & Payne, 1964; Morgan, 1975a, 1976). The grasshopper, Me/unoplus hi~~ittutus. is able to degrade hypocotl cell walls of bean seedlings, but there is no evidence to indicate that cellulose is among the cell wall constituents digested (Talmadge & Albersheim. 1969). The absence of any enzymatic activity toward cellulose powder in the digestive fluids of a number of Lepidopteran larvae has been noted (Shinoda. 1930; Babers & Woke, 1937; Mathur. 1966: Chattoraj & Mall, 1969; Khan & Kasting, 1969; Burton et cd., 1977; Dixit & Mall. 1978; Mall rt N/., 1978). McGinnis & Kasting (1969) conclusively demonstrated the inability of the pale western cutworm, Ayrotis urthqmiu, to digest cellulose by observing that less than 0.5”; of the carbon-14 from ingested labelled cellulose was respired in the form of “%IO,. Although wood-feeders dominate the list of cellulose digesters, there are many species of insects which feed upon bark, phloem tissue. and wood that are unable to digest the cellulose they consume. Beetles from the families Bostrychidae, Curculionidae, Lyctidae and Scolytidae exemplify such species (Parkin. 1940: Chararas, 1979). THE CELLULOLYTIC ENZYMES OF