Fungus gnats see red when pollinating

Fungus gnats (Mycetophilidae and Sciaridae) are probably never going to win prizes for glamour. Two scientists studying them, Ko Mochizuki and Atsushi Kawakita, have described them as “small, weak-flying insects common in moist forest and riparian [riverside] habitats.” The larvae tend to find their food among the rotting wood, eating the fungi they find growing there. They’re strongly at the mercy of their environment, so they might not seem a very good choice for a pollinator.

Nevertheless, Mochizuki and Kawakita note that eight flowering plant families use these gnats in a variety of ways. Some use sexual deception to lure the gnats to flowers. Others mimic fungi as suitable places to lay eggs and transfer pollen along the way. The gnats aren’t always victims of the plants. Some offer nectar or even seeds for the larvae of pollinators.

What Mochizuki and Kawakita noticed is that while the flowers might exploit the gnats in various ways, they tend to follow a similar pattern for display. Dark red petals. Dark red is a colour often used by sexually-deceptive orchids, and by sapromyiophilous plants, which are plants pollinated by insects attracted by smells of dung or carrion. Mochizuki and Kawakita were familiar with quite a few other plants with dark red flowers and set out to see if fungus gnats are pollinating a lot more flowers than people realise.

They decided to study some Japanese plants, Aucuba japonica (Garryaceae), three Euonymus species (Celastraceae), Disanthus cercidifolius (Hamamelidaceae) and Micranthes fusca (Saxifragaceae). They also looked at Streptopus streptopoides (Liliaceae), because it has dark red at the base of its tepals. They then staked out some sites between 2015 and 2017 and examined them for pollinators.

In all the plants they studied, the insects coming to the dark red flowers were mostly dipteran insects – flies. Of these, fungus gnats were the most frequent visitors. Not only were they visitors, but they were also the most useful pollinators to the plants, taking away pollen from one flower and then delivering it to the right species when they landed elsewhere. Curiously when they looked closer at the gnats, they found that they were around four-fifths female. That would make sense if the plants were trying to deceive the egg-laying insects, but this doesn’t seem to be what was happening.

It would at least seem that dark red works to attract fungus gnats, but even that is an odd result. Mochizuki and Kawakita refer to other research that shows that if you want to trap a fungus gnat, then yellow is a far more attractive colour. They suggest that instead of attracting gnats, the flowers might be repelling bees, who don’t see red so well. That way they avoid getting their pollen taken by a more hungry insect.

Whatever the mechanism, the finding that all the plants they studied used fungus gnats for pollination has implications for the importance of fungus gnats as pollinators. Mochizuki and Kawakita write: “Prior to this study, pollination by fungus gnats was known to occur in 20 genera of eight plant families over a wide geographical range, including North and Central America, the Himalayas, East Asia, New Zealand, Australia and South Africa. This study adds five genera of five families in the flora of Japan alone. The role of fungus gnats as pollinators of angiosperms has clearly been underestimated, and examples are likely to be found in regions other than Japan.”

It would appear that where fungi provide the nursery for insects, plants are happy to take advantage of the adults. It means that conserving fungus gnat pollinated plants will mean conserving the right fungi too.