Guess who's coming to dinner.

Here at Weymouth Wetlands we have probably given the impression once or twice that we hold moths in high esteem for their delicate beauty, benign demeanour, cute little faces and intriguing names. The recent wet weather hasn’t been terribly conducive to moth-trapping and the quite sensibly named November Moths and Winter Moths making infrequent appearances at the window are the only species to set our moth sensibilities trembling of late. However, another species recently made an impression upon our discovering evidence of its summer activities.

Visitors to Radipole may have seen a video clip at the Discovery Centre showing Tree Bees coming and going in the summer at the entrance hole of a bird nest box that was attached to our workshop. The Tree Bee, which also answers to the name Bombus hypnorum, is a bumblebee species originating from continental Europe that was first recorded in the UK in 2001, in Wiltshire. Over the last few years its distribution has quickly spread through much of England and to a lesser extent into Wales.

Bombus hypnorum, Richard Revels (rspb-images.com)

Its growing abundance somewhat bucks the trend experienced by some other bumblebee species which are declining in the face of habitat degradation. Tree Bees have a preference for nesting above ground, in tree cavities or bird boxes, which might have something to do with their success. Being bumblebees, they come in peace, but unlike their close relatives they can quickly gang up and get a bit tetchy if you get too close to their colony. After I unintentionally proved this point, Luke and I had to keep a respectful distance when we pointed the camera at that nest box.

Curious as to what a disused bumblebee nest might look like, a couple of weeks before Christmas I removed the box from the wall to take a look at the contents. Opening the lid wasn’t easy; something was holding it shut and it had to be prised open with a great deal of effort. As these photos show, inside was a dense mass of silk-like threads, rather like cotton wool, a couple of inches deep and firmly stuck to the lid and sides. In the depths of the box was an untidy mess that looked like it might once have been a bird’s nest. The inside surface of the lid seemed to have lots of shallow gouge marks on it as if something had been nibbling at the wood.

Not knowing exactly what to make of this I contacted those nice people at Buglife to see if they could put us on the right track, which they kindly did.  It turns out that the silky mass at the top of the box is the work of a species of moth, or more directly its caterpillars, and the mess at the bottom is the remains of the bees’ nest – they always go to the bottom of a cavity. The moth behind this scenario is a noctural species called the Bee Moth or Aphomia sociella, a member of a small group of moths called Wax Moths. To see what this one looks like, go here: http://ukmoths.org.uk/show.php?id=1331

It has the cunning ability to sneak unnoticed into the bumblebees’ nest hole and lay its eggs somewhere within. When the caterpillars hatch they weave protective silk screens and tunnels which enable them to make forays into the nest where they feed on the old wax cells, pollen, debris, droppings, dead bumblebee remains and sometimes even the bumblebee larvae. When the time comes for them to pupate they retreat from the nest and settle down behind a toughened silk mass, which is what was found inside our nestbox. Whether the gatecrashing caterpillars’ activity brought about a total collapse of this colony or just put a big dent in its productivity is uncertain – we were blissfully unaware of the abuse of hospitality that was going on within the box.

A rummage through the material from the bees nest for anything recognisable revealed some remains of dead bumblebee and other bits and pieces; any wax cells seemed to have been demolished.

Eric the Half-a-Bee perhaps. Are those brown grubs on the right of the picture bee larvae...?

Meanwhile the moth pupae remain blissfully unconcerned by the mess that they’ve left in their wake; they will remain in their cocoons until it’s time to emerge next summer when they’ll take their chances in the bat infested skies over Radipole - they won't have everything their own way.

Many thanks to Steven Falk at Buglife for his kind assistance towards getting to the bottom of this intriguing discovery. www.buglife.org.uk

p.s. I'm still wondering whether the caterpillars have anything to do with those gouge marks in the lid - they closely match the area of the silk…