Do carnivorous plants warn pollinators of danger?

Sphaerophoria menthastri and Drosera toyoakensis both have problems. S. menthastri is a small hoverfly that likes to visit flowers for nectar and pollen. D. toyoakensis is a plant that wants pollinators to visit its flowers. They should get along. Unfortunately for S. menthastri, Drosera toyoakensis can be clingy. D. toyoakensis is a sundew, a carnivorous plant that grips small insects and digests them. It would be a terrible idea for Sphaerophoria menthastri to land on a sundew leaf, and usually, they don’t. Research by Tagawa, Watanabe and Yahara suggests the hoverflies are careful around the sundew, unusually careful.

Entomologists have seen cautious hoverflies before. Crab spiders like to sit in flowers and ambush hoverflies. This is not good for a hoverfly that would like to visit a second flower. What Yokoi and Fujisaki found was that when they approached flowers Sphaerophoria spp. would hesitate, moving back and forth to get a view of the flower to see if it was safe to land. If they saw a crab spider Thomisus labefactus, even a dead one, they would be more likely to avoid a flower.

What Tagawa and colleagues have done is watch hoverflies approaching sundews, and to see if they do the same thing.

Writing in the Journal of Asia-Pacific Entomology, the authors say:

“When we observed an event that S. menthastri was hovering for a while in a position close to ca. 10 cm or less of an organ before landing, we counted it as one approach. We observed totally 175 approaches made by 57 individuals of S. menthastri. We recorded the target of each approach as (1) trap leaves of D. toyoakensis, (2) flowers of D. toyoakensis, (3) flowers of L. fortunei or (4) leaves of Poaceae and Cyperaceae; Sphaerophoria menthastri did not approach leaves of L. fortunei that were small (ca. 1–2 cm long) and mostly covered by dense leaf layers of Poaceae and Cyperaceae. In each approach, we noted whether it exhibited a hesitation behavior or not and if any we counted the number of hesitation behaviors by defining one hesitation behavior as a sequence of one forward and one backward flight in front of an organ. We then recorded whether or not a hoverfly finally landed on a target organ after a series of hesitation behaviors.”

They found that about half the time they saw S. menthastri hesitate when approaching a flower, for both Drosera and other flowers. For leaves it was different. About a quarter of the time they’d hesitate approaching a leaf unless it was a Drosera leaf. About three-quarters of the time, the fly would hesitate.

What was going on?

Tagawa and colleagues don’t know. The work by Yokoi and Fujisaki shows they have the visual ability to spot danger. Do they recognise and avoid Drosera based on sight? Is the red of Drosera a danger sign to pollinators? Another possibility is chemical signalling. There’s certainly research into whether or not Drosera uses volatile chemicals to attract prey, and there are volatile chemicals released by traps. Could these same chemicals work as an insect repellent as well as an attractor, for the right species?