How long is a bee's tongue, and why do ants protect plants?

ANOTHER LIFETHE TALLEST PLANTS in the tunnel just now are the thicket of broad beans (var Aquadulce) sown last autumn. Fired up by pelleted chicken manure and an early blast of sun, their whorls of bold black, white and pink flowers are swaying shoulder high, while the first pods are swelling from their shrivelled predecessors lower down the stems. I've never had such a promising crop – nor learned so much about one plant and its wildlife.

The beauty of a polytunnel is getting to sit down in the warm and watch things. There’s the robin, when it trusts me, and soon the odd butterfly, but when the beans began to flower I was looking out for bumblebees.

Broad beans will fertilise themselves: the night before the flowers open, the anthers inside are already shedding pollen that can reach the stigma. But a proper, cross-pollinating job between plants needs the forcible entrance of a hairy, nectar-thirsty bumblebee. There are tests to show that plants caged with bees produce longer pods with more and heavier beans than plants caged without bees.

The queen bumblebees of early spring have considerable aerial presence. I welcomed the first through the open tunnel door in early March, making – what else? – a beeline for the first clusters of bean flowers. But hey! This was no good. Instead of entering the open blossoms it perched at the back of their bells and bit through the petals to suck up the nectar within.

To regular bean-watchers, this is old news. There are bumblebees with short tongues and others with long ones. The white-tailed Bombus lucorum and the even bigger Bombus terrestris are both short-tongued. Unable to reach the nectar from the front, they are notorious nectar-robbers around the back, not only with beans but also with honeysuckle, comfrey and other flowers.

This can leave the common garden bumblebee, Bombus hortorum, unfurling its long tongue into more and more flowers, which could actually be good for bean- making. As bee visitors built up among my beans, I was cheering on the ones that chose the flowers’ front door.

Next came the great ant mystery. Veterans of this column may remember that the ants in my tunnel are not welcome. They are testy wood ants, quick to spray acid on my skin and raising big, allergic bumps that itch for days. I am trying to control them with various sinister means, but there they were, mooching around the higher reaches of my beans.

A particular insect pest of broad beans, familiar to many gardeners, is infestation by black aphids, or “blackfly”, multiplying explosively and sucking the sap of the growing shoots. In a phenomenon found worldwide, ants feed on the sugary liquid – “honeydew” – that exudes from bums of aphids, green or black. Finding this an excellent food, the ants look after the aphids, farming them like cattle, moving them around on plants and protecting them from predators. EO Wilson, world expert on ant society, sees this as an evolutionary arrangement, beneficial to both insects.

My beans, however, have no black aphids (and never had, come to that). So were the ants just hanging around, hoping for some to arrive? I fetched a magnifying glass to watch them more closely. I found them, singly and in pairs, sucking away at small black spots, the size of a match head, beneath leafy axils on the stems. Were these an advance guard of black aphids? However hard I looked, the spots didn’t move, or have legs.

An hour later I was still online in my study, tuned to Google Scholar and fascinated by something quite new to me, even though its discovery dates to 1762. Many plants, all over the world, tropical orchids and hibiscus among them, are not content to draw insects by offering nectar in their flowers. They also have features called “extrafloral nectaries”, such as the little black spots on my beans.

They are there to bring hordes of ants (“pugnacious bodyguards” in the title of one paper) to protect the plants against insect predators or parasites. It is, supposedly, another evolutionary symbiosis – plants and insects have evolved together in many mutually beneficial ways. But if black aphids arrive on the beans and the ants protect them, where is the payoff for the plants? Evolutionary biologists are still picking at a global puzzle that extends to pugnacious ants and tropical leafhoppers, caterpillars and bud-destroying wasps. Perhaps the extrafloral nectaries on broad beans (originally from north Africa) divert the ants from the aphids’ honeydew, leaving them to be reduced by predation by ladybird larvae. So much in nature is a delicate balance of odds.

There is another story about broad beans (or fava beans as much of the world calls them), involving a genetic disorder that makes them a dangerous food for some Mediterranean communities. This explains why, asking Google Scholar for “ants fava beans”, it also offered work containing “peasants”, “variants” and “descendants.”

Eye on nature

In my daughter’s garden a sparrowhawk landed very close to me with a blackbird in its talons. It moved over to the hedgerow with its prey, and I heard squealing. Such a beautiful bird, and although I got just a brief glimpse of it, I found myself thinking about it for the rest of the day.

- Finola Murphy, Bray, Co Wicklow

My neighbour’s elderly mare was standing in the field when a sudden downpour of rain and sleet struck. Two very young lambs were standing sheltering under her. When the shower ended the horse began to lick the lambs, and we were convinced that she was drying them.

- Louis and Bonnie Mullen, Riverstown, Co Louth

A robin kept coming to a holly tree outside our living-room window, hovering like a hummingbird and looking into it. Occasionally it stopped and perched on the side of the tree, moving to different levels and positions.

- Martin Crotty, Blackrock, Co Louth

It was checking it out as a nest site.

In Achill at Easter we spotted the two distinctive fins of a basking shark at the edge of the water in Keem Bay. Close by was a large seal.

- Susan Geran, Clontarf, Dublin

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo, or email viney@anu.ie. Please include a postal address