The sticky world of sap-runs

It's well known that dead and decaying wood forms essential habitat for a wide range of species invlved in decay processes, plus many more that use such sites to burrow and nest. However, there are some situations which can be looked at individually and seen as providing even more specialised conditions, sometimes relating to living tree tissue, but siimilar in concept to dead-wood habitats. These include coppice stools, old parkland pollards and deadwood (saproxylic) or epiphytic fungi themselves, plus the one I want to look at here - sap-runs.

Bracket fungi on standing deadwood

Sap runs are living areas on the trunk or branches where sap oozes out for all or part of the growing season (Fry & Lonsdale 1991). These occur on (probably) all tree species - some such as elms and horse-chestnuts more than others - and the one I saw recently was on a branch of an oak. At the time I was looking at, and taking photos of, the rarer Wild Service tree in front of it. However, I noticed a flurry of invertebrate activity on a partly dead branch and initially thought it was a hornet (Vespa carbro) nest as I could see these large, and increasingly rare, wasps darting in and out of crevices and crawling on the bark.

Wild Service tree with the oak behind. The red arrow indicates the branch with a sap-run.

There may indeed have been a hornet nest, but what held my attention further was the behaviour of not only the hornets but also a host of other insects including various flies and Red Admiral (Vanessa atalanta) butterflies. The photos below illustrate some of this (apologies for the grainy images - it was the best my zoom could manage!)

A hornet on the underside of the branch

The most striking behaviour was the amount of aggression between the different species; initially hornets chasing butterflies and flies away, but also butterflies tussling with each other. In itself, this isn't unusual - male Red Admirals are highly territorial, but this happens in spring - the behaviour seen here occurred in late summer so is clearly something else.

Three Red Admirals fighting above while one feeds on the underside. Also note the large flies on the upper surface which may be feeding or basking.

This photo shows the aggression fairly clearly, but more importantly the position of the butterfly below. This is the exact spot occupied by the hornet above and was where any insect that avoided the fighting landed. Clearly there was a valuable resource here - and the only one that seemed plausible was a small sap-run. With sap-runs an important source of nutrient-rich fluid, it is unsurprising that individual insects were competing for it, especially if the run itself was tiny as seems to be the case here. The flies were unidentifiable at a distance and smaller species may well have been present - in fact this is highly likely as sap-runs are known to support specialists such as the gnat-like flies Mycetobia spp. and Sylvicola cinctus, midges Forcipomyia spp., hoverflies Brachyopa spp. (whose males hover near sap-runs and attempt to mate with any females that land on them) and Ferdinandea cuprea, and the beetles Glischrochilus hortensis, Cryptarcha strigata and Epuraea aestiva.

These are only a few examples and there are many others, especially among the hoverflies as the numerous mentions of sap-runs by Rotheray & Gilbert (2011) can testify, including the danger of becoming trapped in sticky fluid that eventually becomes amber. As noted by Kirby (2001), this highlights the importance of sap-runs, along with many other features of trees (rot-holes, dead branches, ivy) that are sometimes removed as signs of 'ill-health', such as by being selectively removed during woodland thinning rather than being selectively retained. In fact, it seems clear that larger and longer-lasting sap-runs support more diverse species assemblages and so trees with large, deep injuries forming sap-runs should be retained just like those with large dead-wood features. It means overturning some of the received (but erroneous) wisdom ingrained in aspects of woodland management, but structural diversity is of key importance and the countryside isn't meant to be neat!

References

Fry, R. & Lonsdale, D. (eds) (1991). Habitat Conservation for Insects - A Neglected Green Issue. AES, Middlesex.
Kirby, P. (2001). Habitat Management for Invertebrates: A Practical Handbook. RSPB, Sandy.
Rotheray, G.E. & Gilbert, F. (2011). The Natural History of Hoverflies. Forrest Text, Tresaith.