Frass

"[4] 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 completely as their physiology would permit.

Various forms of frass may result from the nature of the food and the digestive systems of the species of insect that excreted the material.

Many other species of wood borers also leave the tunnels behind them tightly packed with dry frass, which may be either finely powdery or coarsely sawdusty.

Possibly this is a defence against other borer larvae, many species of which are cannibalistic, or it might reduce attacks from some kinds of predatory mites or soak up fluids that a live tree might secrete into the tunnel.

Borer tunnels may occur either in dry or rotting wood or under bark, in the comparatively soft, nutritious bast tissue, either dead or living.

Such tunnels obviously cannot be permitted to become clogged, or the insects could not access their own pastures, so they must either eject at least part of their frass, or otherwise leave room for the edible growth.