Many
Flowering Plants use Insect vectors to carry pollen from one flower
to the next. The majority of these use brightly coloured flowers and
strong scents to communicate directly with the Insects, providing the
vectors with a food source, such as nectar or excess pollen, in
return for their services. However between four and six percent of
Flowering Plants use a different strategy to attract pollinators,
relying on deception to attract Insects that would not otherwise
visit flowers. This has two distinct advantages over the more usual
method, firstly the Flower is able to avoid paying the pollinator
with a reward of food, and secondly the pollinator is unlikely to
carry the pollen to inappropriate flowers, as it has no interest it
flowers as such, but are being lured to the Plant as they try to
follow some other instinct.
In a
paper published in the journal Current Biology on 6 October 2016,
Annemarie Heiduk of the Department of Ecology and Evolution at the
University of Salzburg and the Department of Plant Systematics at the
University of Bayreuth, Irina Brake of the Department of LifeSciences at the Natural History Museum, Michael von Tschirnhaus of
the Department of Biology at the University of Bielefeld, Matthias
Göhl of the Department of Organic Chemistry at the University of Bayreuth, Andreas Jürgens
of the Department of Biology at the Technische Universität Darmstadt and the School of Life Sciences at the University of KwaZulu-Natal, Steven Johnson, also of the School of Life Sciences
at the University of KwaZulu-Natal, Ulrich Meve, also of the
Department of Plant Systematics at the University of Bayreuth, and
Stefan Dötterl, also of
the Department of Ecology and Evolution at the University of
Salzburg, document an instance of deceptive pollinator attraction in the Umbrella Flower,
Ceropegia sandersonii, a
smal evergreen succulantl Apocynaceaen Plant native to South Africa and Mozambique.
Other
members of the genus Ceropegia are
known to use deceit to attract Flies (Diptera) as pollinators, and
the flowers of Ceropegia sandersonii
are known to be visited by small Flies (rather than more typical
pollinating Insects, making it a potential deceptive pollinator.
Heiduk
et al. recorded Flies
visiting these flowers, finding a variety of Dipteran goups present,
but only members of two families, the Grass Flies, Chloropidae and Jackal Flies, Milichiidae,
carried pollinaria. These Flies were also the largest Flies visiting
the flowers, suggesting that these are the only visitors large enough
to dislodge the pollinaria from the flowers. The most abundant Flies
were members of the genus Desmometopa
(Milichiidae), with about 26% of these flies carrying pollinaria.
Flower
of Ceropegia sandersonii. Heiduk
et al. (2016).
Flies of the genus Desmometopa are kleptoparasites, which is
to say they rely on food stolen from another animal for food;
specifically in this instance the females feed on the bodily fluids
of Bees caught by spiders as a source of nutrients prior to laying
their eggs. They do not feed on pollen or nectar, and do not
otherwise visit flowers.
Honeybee being eaten by a Spider, with kleptoparasitic Flies
feeding on fluids leaking from the Bee. Heiduk et al. (2016).
How these Flies find their prey has not previously been established,
though it is generally thought that they are attracted to odours
produced by distressed Bees, possibly combined with odours or other
signals from the predator. Heiduk et al. reasoned that if the
flowers of Ceropegia sandersonii do rely on mimicking the
Flies food source as a way of attracting pollinators, then is should
be possible to identify compounds or combinations of compounds
produced by these flowers but not usually by other Flowering Plants,
and that these compounds should be the same as, or very similar too,
compounds produced by Honeybees when attacked by predators.
Analysis of scent compounds produced by Ceropegia sandersonii
did indeed reveal the presence of a large number of highly unusual
(for a flower) substances, 60% of which, including all of the main
compounds, were found to be produced by both European and African
Honeybees under attack. Notably 2-heptanone, an anaesthetic compound
released from the salivary glands of Bees which is their main defence
against Arthropods to small to sting, several alcohols and acetate
esters released from the glands during defensive bites or when a
Honeybee extrudes its stinger for defense, and geraniol, geranial,
and neral, pheromones released from the Nasonov gland, which is
associated with communication between worker Bees. The use of
compounds released from the Nasonov gland as part of a defensive
strategy in Honeybees has not previously been suggested, however
secretions from this gland are known to be associated with colony
defence in other Bee species, so such a mechanism would not be
surprising.
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