Observations in Aichi by the University of Tokyo reveal that one of the world's smallest orchids is pollinated by gall midges

Oberonia japonica, an orchid species native to Japan with the world's smallest flowers (about 2 mm), is pollinated by a dipterous insect, the gall midge, as revealed through direct observation by Graduate Student Yuta Sunakawa of the Graduate School of Science at the University of Tokyo, and Assistant Professor Ko Mochizuki and Professor Atsushi Kawakita of the University's Botanical Garden. This is the first demonstrated case of an orchid species pollinated by gall midges. The results were published in Ecology.

Orchidaceae is one of the largest families of angiosperms, including about 2,600 species, and is known for having a wide range of flower morphology. Although the most commonly known species are those with large flowers, such as Phalaenopsis aphrodite and Cattleya spp., the family Orchidaceae has many species of which flowers are as small as about 1 centimeter. Little is known about how these species with small flowers have evolved.

O. japonica grows naturally in Honshu below Miyagi Prefecture to the Ryukyu Islands, Korea, and Taiwan. In 2022, the group conducted a 26.5-hour direct observation in a natural habitat in Aichi Prefecture, Japan, and found that a large number of small dipterous insects visited the flowers during the night. Of a total of 135 visitors collected, 128 were gall midges about 2 mm in length, and all of them were confirmed to be female individuals. Many of the gall midges had multiple lumps of O. japonica pollen on their heads, which are thought to become attached during feeding-like behavior, i.e., the action of gall midges inserting their mouthparts into the concave structures found at the base of the lip of the flower of O. japonica. They observed that gall midges carried lumps of pollen from flower to flower and lumps of pollen were left on stigmas. These observations suggest that gall midges are effective pollinators of O. japonica.

Specialized modes of pollination, in which plants rely on specific animal species for pollination, are known to work excellently for plants in the family Orchidaceae; however, this is the first demonstrated case of specific pollination by gall midges. The family Cecidomyiidae is one of the most species-rich families in the animal kingdom and is well known for their interactions with plants through the formation of insect galls. Meanwhile, only 10 families of plants are currently known to be pollinated by gall midges, making this study valuable in understanding the ecology of gall midges as pollinators.

Journal Information
Publication: Ecology
Title: Pollination of Oberonia japonica (Orchidaceae) by gall midges (Cecidomyiidae)
DOI: 10.1002/ecy.4293