Under the skin of a sycamore

Thursday's Guardian Country Diary is about the remarkable range of small invertebrates that live under the flaking bark of old sycamore trees.

When the bole of an old sycamore tree expands the old rigid bark cracks and begins to curl at the edges, while ....

.... a fresh, new layer of bark forms underneath. It might not look very attractive but these bark flakes, some as large as a slice of toast, take a long time to fall off and while they are still attached harbour a remarkable fauna of small invertebrates underneath. Here's a selection, most of which I've yet to ID.

Millipedes, a hatched moth pupa and some unidentified cocooons 

Lots more millipedes - these were just a few from under two bark scales - if this sample is representative there must have been hundreds sheltering under the bark of this tree

A minute scarlet mite

A rather beautiful little money spider, Gonatium rubens

A huddle of earwigs ....

... that raised their tail forceps in defence when they were suddenly exposed to the light. Male left, female on the right.

A spider that lives in a silken tent under the bark

Lots of slugs in areas where the bark is permanently damp, where rain water trickles down the tree trunk ..

.... together with snails ......

...... and woodlice

Another moth pupa, that looks as though it hatched successfully.


Sycamore sits low in the league table for tree foliage that supports insect biodiversity - a 1961 research paper on the subject found only 15 species, compared with the 284 hosted by oak (click here for details). But a quick look under the flaky bark of old sycamores casts them in a more favourable light, as a sheltered habitat for a host of invertebrates. 

The few examples shown here were just from a height that I could reach - there may well be a different array of species higher up the trunk and the hosted species most probably vary depending on the aspect (sunny & south facing or shaded & north-facing). Moisture must play a role too because there are well defined runnels where rainwater flows down the trunk from the branches and the bark there is always moist and often very wet. All in all, flaking sycamore bark is probably quite a complex habitat, with many interesting interactions between species that inhabit it.

Red Mites

These tiny red acarine mites, just a couple of millimetres in diameter, seem to thrive in this hot weather, racing over sunlit surfaces and only pausing to pick up food morsels. 

The even hunt on the lids of the black compost bins in the garden, which are almost too hot to touch on the sunniest days. 

Bumblebee mites

Today's Guardian Country Diary is about this queen bumblebee Bombus pratorum that's infested with mites.

The mites belong to the genus Parasitellus and this individual is particularly heavily infested, but her plight isn't as dire as it might seem. These mites don't feed on the bee or transmit disease in the way that Varroa mites of honeybees do. They're commensals, living in the bees' nests and eating the sticky coating from pollen grains, as well as consuming debris that accumulates in the nest. Other than using some of the pollen that the bee collects, they don't seem to do any serious harm and may indeed be beneficial.

It's usually newly-emerged queens that are as heavily infested as this and it does sometimes seem that they are struggling under the load of hitch-hikers, but after a bit of a rest this one took to the air again without much difficulty. The mites tend to congregate in parts of the bee where it's difficult for their host to comb them off with its legs.

The mites spread between bumblebee nests via flowers. When as infested bee visits a flower a few hitch-hikers dismount and hide in the blossom until another pollinating bee arrives, then they climb on board and are carried back to its nest.

Chaffinch with a poorly leg ...

This chaffinch with a swollen, deformed leg has been hanging around the bird table all day. For comparison, the bird below has a normal leg. The deformity is probably caused by tiny mites called Knemidocoptes, that burrow into the leg and cause symptoms similar to scabies in mammals, although chaffinch leg deformities can also be cause by a virus and a fungus.

There is an image of the Knemidocoptes mite here.

This weather makes life tough for healthy birds, so it must be especially difficult for an ailing one.

Hitchhikers

Harvestman spiders like this Mitopus morio turn up quite frequently in our garden at this time of year and quite a few of them carry these little scarlet parasitic spider mites - which apparently are mite larvae belong to a genus called Leptus - attached to their legs and body. 

I really need to catch one of these and take a look under the microscope - at this magnification they just look like little blobs of red sealing wax.


You can see another kind of parasitic mite, on bumblebees, here

Tormented

This weary bumblebee landed on our conservatory windowledge this afternoon and began grooming. While I was photographing it a tiny pink acarine mite (which I think is Parasitellus) crawled out of the fur under the bee's wing, climbed down under its abdomen and then climbed up its thorax...

... before crawling down into the fur again between the head and thorax. The bee was clearly irritated by the mite and tried to comb it off, with no success.

Here you can see it burrowing down behind the bee's head. Opinion seems to be divided about how much damage these mites do. It may be that - like the Varroa mite that infests honeybees - some species transmit diseases between bumblebees and some species may weaken their host when they suck its body fluids through weak points in the bee's joints, where they attach themselves. Apparently Parasitellus doesn't feed on bees but merely uses them for transport between bees' nests, where it feeds on the wax of the brood cells. I suspect that pink patch on this bee's knee is another mite, tucked in between the joints in the leg. Some mite species actually enter their hosts body through the spiracles and live attached to the respiratory organs, as internal parasites. Such mites weaken their host, although their effect isn't likely to be as devastating as thick-headed flies, whose larvae live as internal parasites of bumblebees.