The chestnut has always been considered a wind-pollinated species, and the pollination of chestnuts seemed to be clear without further discussion. The presence of various insects on a flowering chestnut tree was not considered a sign that insects could play any significant role in the process of pollination of female flowers.
The surprising role of insects in chestnut pollination
But is it really the case that chestnuts are pollinated exclusively by the wind, and insects do not help in establishing the crop? So how are chestnuts actually pollinated? By wind, by insects, or by both? Many questions and few convincing answers. The method of pollination of chestnuts, an important genus of forest trees in the beech family that produce edible fruits, has long remained unclear. The mechanisms involved in chestnut pollination have been little studied. In fact, the solution to this issue was the disgruntled chestnut growers who blamed the low harvest on the lack of suitable cultivars that ensured pollination. Therefore, in 2018, a doctoral thesis was launched in France between INRAE and Invenio (experimental station for the fruit and vegetable industry in Nouvelle-Aquitaine) to investigate this issue.
Groundbreaking research reveals the main method of pollen transmission Clément Larue's doctoral thesis, carried out between 2018 and 2021 in the Invenio orchards in Douville, was aimed at identifying the main pollen transmitter of chestnuts.
The experiments clearly demonstrated the dominant role of insects.
It is common knowledge that chestnut orchards serve as bee pastures. Anyone who has stood under a chestnut tree during flowering must have noticed the multitude of bees collecting pollen and nectar from the male flowers. However, in addition to bees, other insects can also be observed on flowering chestnut trees, especially two species of beetles. These are the red soldier beetle - Rhagonycha fulva and the green rose chafer - Cetonia aurata. The bee always unmistakably deals only with male flowers, which provide it with pollen and nectar. It rarely flies to a female flower. But beetles behave differently. They constantly move from male flowers to female flowers and vice versa. The female flower has similar morphological features to the male flower, but does not produce either pollen or nectar. This is probably an adaptation of the chestnut tree with the intention of confusing insects and attracting them to the stigma for the purpose of transferring pollen.
Disbelieving chestnut growers
The fact that the chestnut tree is an entomophilous (insect-pollinated) species was initially not accepted by chestnut growers after the research results were published. Biogeco researchers therefore repeated the same experiments at another site, in Villenave d'Ornon, for a further two years to verify these results. These new experiments confirmed that insects play a major role in chestnut pollination.
All this work now allows us to reconsider the establishment of chestnut orchards and to implement measures to increase biodiversity in order to increase yields. This requires ensuring an abundance of wild pollinating insects by planting different cultivars of chestnut that produce pollen.
The green rose chafer - Cetonia aurata is an important pollinator of the sweet chestnut. Any site near a chestnut orchard where green rose chafer larvae are present in the soil should be protected. Green rose chafer larvae, also known as grubs, feed mainly on dead organic matter in the soil and should therefore not be considered a pest.
From now on, the chestnut is considered the leading entomophilous tree species in France, which poses a major challenge for the protection of wild pollinators.
