Some jack-in-the-desk plants can use sex to attract pollinators when they die

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False – and fatal – invitations to romance could be the latest trickery discovered among some jack-in-the-desk wildflowers.

The fatal part is not the surprise. Jack-in-the-desks (Arisaema) are the only plants known to routinely kill their own pollinating insects, says evolutionary ecologist Kenji Suetsugu of Kobe University in Japan. The new twist, if confirmed, would be use sexual deception to lure pollinators into death traps.

So far, biologists have only found three plant families with species that claim to offer sex to insects, Suetsugu said online March 28 in Plants, people, planet. But unlike the deception in the jack-in-the-desks, these other attractions aren’t fatal, just fake.

The orchid family has proven deceptive, some so alluring that a male insect leaves wasted sperm as well as pollen on a flower. Yet he doesn’t even get a sip of nectar (SN: 3/5/08; SN: 03/27/08). Similar scams have happened among daisies: A few dark bumps that a poorly lit human might mistake for an insect can drive male flies crazy over yellow, orange, or red. Gorteria petals. Enthusiasm wanes with repeated disappointments however (SN: 01/29/14). And among the irises, one species dangles velvety purple petals where deluded insects wallow.

So far, luring pollinators with fake insect sex offers has only appeared in three plant families. Hundreds of cheat orchids (including speculum of Ophrys, left). The same goes for a daisy with alluring insect-like bumps of petals (Diffuse Gorteria, middle) and an iris (Iris paradox, right) with a few drooping dark petals.Steven Johnson (daisy) and Jorun Tharaldsen (orchid, iris) by DCJ Wong, J. Perkins and R. Peakall/Frontiers in plant science 2022

Two species of jack-in-the-desk in Japan have now raised suspicion that their family, the calla lilies, should be added to the list of sex cheaters. For visually oriented humans, the approximately 180 Arisaema the species look like a cheerful reminder of the endless weirdness of evolution. A sort of floating canopy, sometimes striped, bends over a small cup-shaped “pulpit” with a little finger tip or mushroom bulge of vegetable flesh peeping over the edge. Below the rim, bands of flowers open in succession – male flowers overtaken by flowers with female parts – as the plant grows from a young, slender jack to a large mummy.

These weird blooms mostly rely on pollinators that deserve a much larger fanbase: fungus gnats. These gnats, small as punctuation marks and difficult to identify, are real flies. But don’t hold it against them. They don’t stalk picnic spreads or buzz against windows. Pollinating midges “are very fragile,” says Suetsugu, and their wings don’t make any noise a human can hear.

A human also cannot always sense what attracts fungus gnats. It’s clear, however, that variegated canopied pulpits can have a strong appeal to cruising pollinators looking to encounter the right gnat. It will go horribly wrong.

A tiny escape hatch in the bottom of the trap stays open during the male phase of flowering, but that two-millimeter hole disappears during the big mum phase. A gnat cannot overcome the slippery, peeling wax of the inner wall of the plant to get out. Thus, any gnat deceived twice is doomed.

Biologists had assumed that flesh bugs looking for fungus gnats perfumed the air with the smells of mushrooms and pleasant places for children. Many guys seem to do this, but the intimate smells don’t explain a strange sighting by Suetsugu and his colleagues. Among the important pollinator species for two Japanese jack-in-the-desks (A. angustatum and A. peninsula), almost all of the spots found in the traps were males.

A scent lure targeting males could mimic the scent of female gnats, the researchers propose. It’s pure fraud. Even if the hopeful males found a mate in the waxy green dungeon, they and their offspring would starve. They are stuck in a plant with no fungus to eat. Whatever that ruinous scent is, a human nose can barely detect it, reports Suetsugu.

The idea that biologists have so far overlooked a scent important to other animals seems “more than possible” to Kelsey JRP Byers of the John Innes Center in Norwich, England. Byers’ work overturned a common assumption that monkey flowers (mimics) had no scent even though hawkmoths, flying at night and known to follow scents, visit the flowers.

“We’re such visual creatures,” says Byers, who studies floral scents. We can laugh at how insects mistake a blob of discolored plant tissue for a fabulous female, but we miss the smells. Fungus gnats, however, even resemble the citizens of a smellier world, with giant guy-like antennae “like an ostrich feather on a hat.”

At least now, modern analytical laboratory techniques and equipment are opening up the vast sensory world of communication floating around us. To see if even familiar plants like jack-in-the-desks are up to something strange, scientists need to identify the lure itself. Then maybe we’ll understand the irresistible Valentine’s Day scent of a female gnat.

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