Weekly Witter: Out of the strong came forth sweetness

Spring is a time of joy for all naturalists, leaf burst and blossom delighting all of us, not least the entomologist who has suffered a long and bleak winter with hardly a buzz or a flutter of wings. The search for the rare and spectacular is most definitely on and it’s not in praise of leaf or flower that I devote this blog. I’m celebrating a sweet and intoxicating liquor, a dark brown liquid that oozes, bubbles and even gurgles from trees. Sap-runs or flux as they are sometimes known, prove irresistible to insects and insect hunters alike.

There are many reasons why sap might spring from trees, bacterial disease, physical damage or the attentions of wood boring insects such as the chunky larvae of the goat moth which might spend five years developing on the frugal diet of solid wood. Whatever causes the sap to flow from the tree there are rich pickings for insects. Flies, beetles and wasps are all attracted to the sugary secretions. While some of the species such as red admirals and wasps are fairly ordinary, there’s a chance of finding more rare species per square inch than any other habitat I know.

The dark brown streaks on this oak trunk are a tell-tale sign of a productive sap-run.

The dark brown streaks on this oak trunk are a tell-tale sign of a productive sap-run.

The tree in the photograph was in a field at Maidenhead and Cookham Commons and is clearly in decline although the sap runs are a symptom rather than the cause. One of the first things I check for on such trees are the exit holes of the twin-spot wood-borer, a formerly rare beetle that has become much more common as a result of acute oak decline. These beetles, sometimes implicated in the spread of disease leave holes that are distinctive for being flattened on one side, much like a D. There was no sign at all of where this handsome beetle had been, indeed despite seeing hundreds of holes on scores of trees I’d never seen this species in over ten years of trying. Other species though were there in abundance.

Wasps were frequent, along with their bigger cousins the hornet. These, despite their fearsome reputation are luckily kind-tempered; praiseworthy when poring over the trunk with your nose an inch away from where they feed. Sap-beetles, fungus beetles and hoverflies all flocked to the sweet sap. The rare brown tree-ant, a real southern speciality was busy, scurrying across the trunk, drinking sap and seemingly attending hoverfly larvae that were immersed in the syrupy stream. Several hoverflies are known to breed exclusively in sap runs, some of these are tiny and rather dowdy but the inflated hoverfly is a much more robust beast. This inch long fly is a dapper black and orange affair and always a pleasure to see.

The inflated hoverfly, a distinctive sap-run specialist.

The inflated hoverfly, a distinctive sap-run specialist.

Red admiral butterflies had settled onto the trunk to feed occasionally flashing their wings to startle other insects attempting to muscle in. The smell of the sap was even tempting enough to lure a purple emperor away from its sylvan kingdom. After a good twenty minutes of inspecting the insect life on this once mighty tree I thought that the emperor would be the highlight. Just as I turned to walk away, another insect alighted at the foot of the tree, the wings creating an audible buzz as it did so. Its slim shape and spangled appearance gave it away as one of my most sought after species. Agrilus pannonicus, the oak jewel beetle, the twin-spot wood-borer (so good they named it thrice), a Holy Grail that serendipity (and sap) had seen fit to allow me to meet.

oak j Oak Jewel Beetle

Oak Jewel Beetle

  • Peter Brash is a wildlife and countryside advisor for the National Trust. He specialises in animal ecology; specifically birds, insects and bats. As well as the surveying of habitats, wildlife interpretation and engagement.
  • The Weekly Witter is a new regular Monday mouthpiece for our many specialists to talk about what’s on their minds at the moment.
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