# Sex ratio

> The share of offspring that are female; because only females lay eggs, a female-biased ratio speeds population growth.

- Canonical: https://genetixbay.com/knowledge/glossary/sex-ratio.html
- Also written: female ratio, proportion of females
- 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"

Sex ratio is the proportion of females among offspring, written as the female fraction or as female:male. [Spider mite](https://genetixbay.com/knowledge/glossary/two-spotted-spider-mite.md) and [thrips](https://genetixbay.com/knowledge/glossary/thrips.md) populations are usually female-biased in lab [life tables](https://genetixbay.com/knowledge/glossary/life-table.md), and parthenogenetic [aphids](https://genetixbay.com/knowledge/glossary/aphid.md) indoors are all female.

## In practice

Only females lay eggs, so life tables include sex ratio when computing r_m and [R0](https://genetixbay.com/knowledge/glossary/net-reproductive-rate.md). A population that shifts toward males grows more slowly. Predators that eat mainly eggs and young nymphs cut future females directly.

## Don't confuse with

- **Fecundity**: [eggs per female](https://genetixbay.com/knowledge/glossary/fecundity.md), before applying the sex ratio.

## Related terms

[Arrhenotoky](https://genetixbay.com/knowledge/glossary/arrhenotoky.md), [Haplodiploidy](https://genetixbay.com/knowledge/glossary/haplodiploidy.md), [Life table](https://genetixbay.com/knowledge/glossary/life-table.md)

## Read more

- [Temperature, humidity and pest population growth in cannabis: every major pest compared](https://genetixbay.com/knowledge/pests/temperature-humidity-and-pest-population-growth.md)

## Sources

1. Birch, L.C., 1948. The intrinsic rate of natural increase of an insect population. Journal of Animal Ecology 17:15-26. https://doi.org/10.2307/1605