Rare orchid lures in beetle pollinators using deceitful sexual bait

Longhorn beetles deposit sperm in orchids they pollinate, but why?

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A close up of a longhorn beetle's face

Rooted down at one spot, unable to attract their "mate" or move towards them, plants are at the mercy of animals and the environment to aid pollination and subsequent reproduction. Insects and other animals are responsible for the pollination of 60 to 90 percent of plant species. This is made possible largely due to the food rewards that pollinators get from plants in the form of nectar, fruits and vegetables. But some species exhibit uniquely different methods to attract their pollen shippers, often luring them with a deceitful sexual bait.

An exceedingly rare species of orchid, Disa forficaria, found in southern Africa, does not produce nectar. Yet, it is reliably visited by males of the longhorn beetle, Chorothyse hessei, which carry away its pollen packets attached to their underside. Before they depart, though, the beetles exhibit vigorous copulatory behavior perched on top of the orchid flower. They bite the furry antennae-like petals and extend their aedeagus — an arthropod equivalent of a penis — to fit into a floral notch. These visits often end in ejaculation, with sperm being deposited at the floral tip. The beetles are under the false impression that the orchid flowers are females of their species. But why?

Callan Cohen, from the University of Cape Town, who first observed this behavior in the longhorn beetles, investigated the reason behind this unusual sexual encounter. He separated the chemicals released by the orchid flower and tested the response of the beetle antennae, an organ which they use to detect odors, to each of them. One of these compounds, named "disalactone", produced electrical responses in the antennae of all tested individuals. The odor molecule was exceedingly attractive to male beetles, so much so that disalactone beads treated with it not only drew in the beetles, but also enticed them to try and mate with them. While it is presumed that the scent must be the same as that of female longhorns, until scientists catch and examine one, we won’t know for sure.

D. forficaria plants are so scarce that only 11 sightings have been recorded in over 200 years. Last observed in 2019 in the study region, it was thought to be extinct. But this new research shows it is still out there, budding and thriving.