Insect pollination in chestnut: an organized mess?

How are chestnuts pollinated? By wind, insects, or both? For almost 150 years, this question has been in the air. The huge production of tiny pollen grains transported by wind over long distances seems to point towards wind pollination. However, the brightly coloured flowers, the strong spermatic odour emitted by flowering trees, and the sticky pollen grains of chestnuts are characteristics of insect pollination. We present here the results of five years of research on chestnut pollination aiming at answering this question. We performed several insect exclusion experiments using insect-proof nets and two large-scale insect monitoring experiments. In netting experiments, in which access of insects to female flowers is limited, fruit set collapses. Direct observations indicate that only beetles and calyptrate flies regularly visit both male and female flowers. Wild bees, honeybees and hoverflies visit male flowers but fail to visit rewardless female flowers. The male parts of bisexual catkins play a key role in pollination: they attract walking insects on erect female flowers, increasing the probability of a contact with the tiny stigmas. Installation of beehives will not improve pollination service in chestnut orchards. Instead, the preservation of non-bee pollinators, especially calyptrate flies, is critical to the sustainable management of chestnut orchards.