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Frass refers loosely to the more or less solid excreta of insects, and to certain other related matter.
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Definition and etymology
Frass is an informal term and accordingly it is variously used and variously defined. It is derived from the German Fraß, a past participle verbal noun of fressen, which means to gobble or to feed as an animal might. The English usage derives the idea of excrement from what larvae had eaten, and similarly also, the refuse left behind by insects. Such usage dates back to the mid nineteenth century. In modern technical English sources differ on the precise definition, though there is little actual direct contradiction. One glossary from the early 20th century speaks of "...excrement; usually the excreted pellets of caterpillars." In some contexts frass refers primarily to fine, masticated material, often powdery, that phytophagous insects pass as indigestible waste after they have processed plant tissues as well as their physiology would permit. Other common examples include the fecal material that insects such as the larvae of codling moths leave as they feed inside fruit, or that the likes of Terastia meticulosalis leave as they bore in the pith of Erythrina twigs, or the larvae of Cerambycidae or powder post beetles leave inside or below their tunnels when boring in solid or rotting wood.
In a significantly different sense the term also may refer to excavated wood shavings that carpenter ants, carpenter bees and other insects with similar wood-boring habits kick out of their galleries during the tunneling process. Such material differs from the frass residues of foods, because insects that tunnel to construct such nests do not eat the wood, so the material that they discard as they tunnel has not passed through their gut.
Various forms of frass often may be seen in the tunnels of leaf miners, in rotting wood, under dead bark, or under plants infested with caterpillars.
Ecological considerations
Contact with frass causes plants to secrete chitinase in response to its high chitin levels. Frass is a natural bloom stimulant, and has high nutrient levels. Frass contains abundant amoebae, beneficial bacteria, and fungi. Accordingly, it is a microbial inoculant, in particular a soil inoculant, a source of desirable microbes, that promotes the formation of compost. It is an important recycler of nutrients in rainforests, and favours plant health.