# Frass

> Insect excrement: the dark pellets left by caterpillars, the tiny black specks left by thrips, and the fine debris left by other feeders; finding it is a key sign of a hidden pest.

- Canonical: https://genetixbay.com/knowledge/glossary/frass.html
- Also written: insect frass, caterpillar droppings, insect droppings
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Frass is the excrement and feeding debris left by insects. On cannabis the two kinds that matter most are the dark green to black pellets of caterpillars, often found on leaves below a damaged bud, and the tiny black specks of thrips on silvered leaf surfaces (Oregon State University Extension, EM 9626).

## In practice

Frass on a leaf below a [cola](https://genetixbay.com/knowledge/glossary/cola.md) means look inside the bud: [budworm](https://genetixbay.com/knowledge/glossary/corn-earworm.md) caterpillars tunnel in, and their feeding wounds are a common entry point for [bud rot](https://genetixbay.com/knowledge/glossary/botrytis.md). Check daily in flower, gently parting dense colas. Fresh caterpillar frass is moist and green; old frass is dry and dark.

## Don't confuse with

- **Soil or perlite specks**: no pattern, and no feeding damage nearby.
- **Sooty mould**: a black film growing on [honeydew](https://genetixbay.com/knowledge/glossary/honeydew.md), not pellets.
- **Septoria fruiting bodies**: tiny black dots inside [leaf spots](https://genetixbay.com/knowledge/glossary/leaf-spot.md).

## Related terms

[Caterpillar](https://genetixbay.com/knowledge/glossary/caterpillar.md), [Thrips](https://genetixbay.com/knowledge/glossary/thrips.md), [Scouting](https://genetixbay.com/knowledge/glossary/scouting.md), [Bud rot check](https://genetixbay.com/knowledge/glossary/bud-rot-check.md)

## Read more

- [Caterpillars on cannabis: species, degree-days and control](https://genetixbay.com/knowledge/pests/caterpillars.md)
- [Thrips on cannabis: life table, temperature, scouting and biological control](https://genetixbay.com/knowledge/pests/thrips.md)
- [Botrytis (bud rot / grey mould) in cannabis](https://genetixbay.com/knowledge/diseases/botrytis-bud-rot.md)

## Sources

1. Oregon State University Extension. Corn earworm management in hemp (EM 9626). https://extension.oregonstate.edu/catalog/em-9626-corn-earworm-management-hemp