Phishing Flowers

By Peter Dearden 02/12/2010

SM Morgan

It is surprising how often technology mimics nature, and in the most subtle of ways. Take for example the phenomenon of email phishing — a probe is sent out to entice the recipient with something they want/need; for example, a MillionDollars!, an OnlineEasyDoctorate! or a ready supply of CheapDrugs! Once enticed, the source gets something it needs — most often, your unwitting money, and depending on how savvy you are — you get something back as well. In your garden this same bait-and-reel technique is being employed by your flowers.

Plants achieve sexual reproduction via flowers — and the combination of egg cell and pollen granule. To achieve maximum genetic variation, the flower needs pollen from another plant. This is facilitated by wind, and, to a greater extent, insects transporting the pollen on their legs and bodies. The bloom entices the insects with scent, colour and potential rewards in order to have their pollen dispersed by the insects. Sometimes the insects are rewarded with food; other times not, and they leave as disappointed as the hundreds of people who fall for email scams every year.

The production of nectar separates those plants that reward pollination, and those that trick the insect into visiting the bloom without a reward. Such deceptive flowers use the colour and scents of attraction, without a nectar or prey reward. These flowers can mimic sites that insects like to lay eggs in, offer fake food, or even imitate members of the opposite sex. This sneakiness delights the secret criminal in me — which rejoices in getting something-for-nothing. How smart are these flowers to be achieving pollination through such a deception?

In October a paper was published detailing a new method by which a species of Orchid fools Hoverflies into visiting their blooms. The Orchid gives off a scent normally produced by adult aphids, which the Hoverfly larvae eat, to entice the Hoverfly to visit the flower and pick up some pollen. In this situation the aphids are the reward, food for the next generation, and occasionally a small drop of nectar for the adult as further encouragement.

Imitating a site on which to lay eggs is one of the most-used deceptive tricks by orchids to attract insects. Orchids copy structures where insects lay their eggs, especially using colour or scent to fool the egg-laying insect into thinking that shelter or food exist on their flower.
This research paper examines how a particular species of orchid attracts its pollinator, a hoverfly. Hoverflies are awesome insects; adults feed on pollen and nectar, thus pollinating flowers, while the larval form feeds on aphids. So the flowers the hoverflies visit would ideally have aphids for the larvae to feed on after the eggs have hatched, and nectar to feed the adult while laying. The larvae cannot move far once hatched, thus the site of laying must be chosen wisely by the female hoverfly.

The hoverfly uses scent to locate perfect flowers; semiochemicals, produced by the aphids and plants, used for communication. Female hoverflies can tell the difference between types of plant, and whether they possess a colony of aphids. So a female hoverfly can detect plants that are good hosts for offspring, and contain aphids to feed larvae once hatched. She uses colour to find the flowers, but the colour alone will not induce her to actually lay her eggs.

The researchers in this study noted a type of orchid that was being visited by hoverflies, who were laying eggs, but had no aphids. Knowing that the larvae require aphids to survive they hypothesised that the flower was somehow acting to trick the hoverfly into laying her eggs in a poor location, and thus achieving pollination. This particular orchid has warts on its petals which look a bit like aphids from a distance, but these alone are not enough to induce egg laying. The flowers are producing some form of scent to trick hoverflies into visiting the flower, and laying eggs.

The researchers collected samples of both the plants and the hoverflies, and recorded the volume of nectar produced by each bloom, and most excitingly — the types of scents. They did this experiment by encasing the whole plant in an oven bag with a filtered tap on it. Drawing collected air out of the bag, through the filter, traps any chemicals on it – from where it can be analysed. Scent chemicals from aphids were extracted in a much more ‘barbaric’ manner; whole-body crushing and ‘scent’ extraction. The scents were analysed by a technique called ‘gas chromatography’ which works a bit like how you would separate the different colours out of a sample of black ink — place a drop on some thick paper and put one end in water, the colours are drawn out of the drop sequentially depending on their chemical size.

To test whether the hoverflies were able to detect the scents from the plants, the researchers removed their heads and attached electrodes to the antennae. When the scents are wafted over the head, if the detectors in the antenna recognize that scent, an electrical impulse will be produced and detected by the machines. Chopped off heads and tiny electrodes? Brilliant! It worked; the hoverflies could detect the plant’s false aphid scent.

The final experiment in the paper was to see if the hoverflies would lay eggs in response to the scent produced by the plant. They did this by puffing a synthetic plant scent, made in the lab from the components they had identified from the gas chromatography, on to bean plants. It turned out that the gas does induce egg laying- the hoverflies laid eggs on the bean plants despite their having no flowers, nectar, colour or aphid mimicry.

The researchers managed to show that this orchid (E veratrifolia) was able to mimic the same scents produced by adult aphids (in this case M viciae) to great effect. The flower is sending out chemical signals that pretend aphids are present on the flower. The hoverfly is being fooled into thinking those aphids will make a tasty treat to feed its offspring, when in fact this orchid never has aphids on its blooms at all. The orchid is pollinated, and its job is done — just as email phishing does, occasionally, persuade innocents to part with their money.