Tuesday, 21 November 2017

Cruel (but fair enough for the) Plant

A little like the Common Milkweed whose propensity to leg-pull I mentioned a few months ago, the Cruel Vine has love-hate relationship with it's pollinating butterflies, moths and bees. To get to the nectar at the base of the flower the pollinator has to extend its proboscis deep into a wedge-shaped groove so narrow that the insect, or part of its proboscis, can remain jammed into the flower.

This is clearly unfortunate for the insect but you'd think equally so for the plant that not only kills its method of pollination but has a decaying animal blocking up its reproductive system. Not so, as local orchid expert Edith Coleman explained to her Victorian Field Naturalist audience back in 1935 ('Pollination in Australia of Araujia sericifera Botero', Vic. Nat. 52: 3-7).

Like the milkweeds, Cruel Vine, or Araujia sericifera, is a member of the Apocynaceae, a family of often milky-sapped, poisonous plants such as Oleander. The Cruel Vine takes this a step further by trapping insects in its flowers then exposing them to a toxic secretion inside the flower.  

We also call Araujia sericifera the White Bladder-flower, on account of the slightly inflated flower shape as you can see in the unopened flower above (it's also a little inflated at the bottom, within those large green sepals).

As for the name False Choko, that arises from the choko-like fruit produced if the flowers don't kill their pollinators before they finish the job. (The true choko, or chayote, is produced by Sechium edule, a member of the melon family, Curcurbitaceae.)

Presumably insecticide is not the primary objective of the Cruel Vine flower. Like milkweeds and other plants closely related to Araujia, the flower structure is quite complicated, with a system of openings and grooves that work like a peg, clipping the pollen mass onto part of the visiting insect.

In Araujia, rigid wings around the anthers - where the pollen is attached - are responsible for trapping proboscises and also whole smaller insects that are incapable of removing the pollen mass anyway. The former may just be collateral to deter the latter (who are of no value to the plant in terms of its future success) or they may be part of a cunning plan....

Edith Coleman spent at least five years watching her Cruel Vine flowers, counting and observing their insect visitors. In her garden, thousands of insects visited the flowers with a total of 12 bees, five moths and two butterflies becoming trapped and dying. Her neighbour's plant (a bigger one, with more flowers) trapped 15 bees in a single year, but no moths or butterflies.

Edith Coleman observed a range of visitations which might be summarised as:
1. The insect proboscis reaches the nectar and is withdrawn without being trapped or getting a clip of pollen. No value to the plant here.
2. The insect pushes its proboscis a little further (to find more nectar), releasing in the process a pair of pollen masses which are clipped to the proboscis on the way out. And there is a way out: the insect isn't trapped when it picks up this load on exit. Half way there for the plant.
3. An insect with pollen attached to its proboscis visits another flower and manages to get in an out without releasing the pollen or becoming jammed. Again no value to the plant.
4. An insect with pollen attached to its proboscis pushes that little bit further (again, to find more nectar), such that the pollen jams the proboscis in the flower slit. The pollen can fall off, allowing the insect to withdraw fully from the plant or, it can be left behind if the proboscis breaks or the insect gets trapped and dies. Bingo, pollination!

Coleman also notes that only once did she see an insect trapped that didn't have pollen attached to its proboscis. So from these observations it seems that the process works fine for the plant, and only sometimes not so well for the insect visitors.

Given the success of Cruel Vine in so much of the world, and the demonstrated rarity of self-pollination, its seems the plant does get pollinated with great regularity. It can clearly 'adopt' various local insects of the right size and shape to do its pollination work. In South Africa, and presumably here in Australia, it's the native honey bee mostly, with only a few moths and butterflies.

Honey bees are not native to where it grows in Brazil, although bumble bees are. However the colour, large size, nocturnal perfume and low nectar offer of the flowers suggest they are more attractive to moths. In a more recent study Gareth Coombs and Craig Peter suggest the Cruel Vine may attract a range of insects in Brazil, including large moths which may be the primary pollinators even though some don't survive the attempt (as observed by Edith Coleman).

The Cruel Vine's ability to accommodate insects other than moths, however, has been a large part of its huge success as a weed around the world. From the plant's point of view, it doesn't seem to matter that a few proboscis-bearing species are killed along the way.

Note: Neville Walsh kindly suggested this charming topic to follow up my  recent story of butterflies leaving a leg or two when they pollinate one of the milkweeds. Because it flowers in summer in the Southern Hemisphere I've had to source my images from the Wikipedia, except for the final image, used with permission from HernĂ¡n Tolosa and from his delightful website Flora Bonaerense.

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