Ever wonder how your holiday nuts get pollinated? Chestnuts are pollinated by FLIES and BEETLES—not bees. Pecans rely entirely on wind. Walnuts use wind with help from cold-hardy native bees. In this special holiday episode, we explore the surprising, unglamorous pollination strategies of nut trees. Spoiler: it's not the story anyone expects, but it's the one that actually works.

www.secretpollinators.com

KEY SOURCES

Chestnut pollination

• Larue, C., Austruy, E., Basset, G., & Petit, R. J. (2021). Revisiting pollination mode in chestnut (Castanea spp.): an integrated approach. Botany Letters, 168(3), 348–372. DOI: 10.1080/23818107.2021.1872041 — cultivated C. sativa and hybrids in southwestern France; large-scale insect isolation experiment plus monitoring of 16 trees, half male-sterile; pollinator exclusion demonstrated a predominant role for insects; female flowers have erect styles resembling stamens, a probable case of intersexual mimicry; the highest pollen/ovule ratio ever reported in plants, consistent with a beetle pollination syndrome.
• Larue, C., & Petit, R. J. (2024). Insect pollination in chestnut: an organized mess? Acta Horticulturae, 1400, 331–340. DOI: 10.17660/ActaHortic.2024.1400.40 — wild bees, honeybees and hoverflies visit male flowers but fail to visit rewardless female flowers; the male parts of bisexual catkins attract walking insects onto erect female flowers; installing beehives will not improve pollination service in chestnut orchards; preserving non-bee pollinators, especially calyptrate flies, is critical.
• International Society for Horticultural Science, Diversity of pollinators and pollenizers is key to successful chestnut pollination (2023) — within insect-proof nets, scarcely any fruit formed; chestnut is strictly entomophilous; only insects regularly visiting both male and female flowers were counted as pollinators.
• Bounous, G., et al. (1991). Investigations on chestnut pollination. Acta Horticulturae, 288 — earlier caging work across European, Japanese and Euro-Japanese cultivars.
• Larue, C., et al. (2021). An intensive study plot to investigate chestnut tree reproduction. Annals of Forest Science, 78, 87. DOI: 10.1007/s13595-021-01104-w

Pecan pollination

• Wood, B. W., Grauke, L. J., & Payne, J. A. (1998). Provenance variation in pecan. Journal of the American Society for Horticultural Science.
• New Mexico State University Extension, Flowering Habits of Pecan Trees, Guide H-622 — roughly 2,000 pollen grains per anther, four anthers per flower, up to 110–115 flowers per stalk, three stalks per catkin, two catkins per primary bud.
• Noble Research Institute, Cross pollination is essential for pecan production — a single catkin can produce as many as 2.64 million pollen grains; only one pollen grain is required to produce one pecan.
• Noble Research Institute, It Takes Two (or More): Understanding Pecan Tree Dichogamy — protandrous (Type I) versus protogynous (Type II); complete versus incomplete dichogamy; high winds and low humidity shorten the effective pollination period.
• Clemson HGIC, Pecan Planting & Fertilization — plant trees within 200 feet; at least three varieties for maximum production.
• Zhu, X., et al. (2023). Comparative transcriptome analyses reveal insights into catkin bloom patterns in pecan protogynous and protandrous cultivars. — pollination in pecan is anemophilous; dichogamy favors outcrossing.

Walnut pollination

• Polito, V. S. (1998). Floral biology: flower structure, development and pollination. In Walnut Production Manual, UC ANR Publication 3373.
• Catlin, P. B., & Olsson, E. A. Walnut pollination and pistillate flower abscission research, UC Davis.

Beetle and fly pollination generally

• Rader, R., Cunningham, S. A., Howlett, B. G., & Inouye, D. W. (2020). Non-Bee Insects as Visitors and Pollinators of Crops. Annual Review of Entomology, 65, 391–407. DOI: 10.1146/annurev-ento-011019-025055

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