Eyes on the bramble

The bramble is alive with insects at the moment, its flowers the lure that attracts flies, beetles and a multitude of bees. Although many of the visiting insects are familiar enough it is always worth spending some time at a flowering bramble to see what else might be attracted in. Over the years, for example, bramble has provided me with some of my best views of white admiral and black hairstreak butterflies, not to mention several interesting species of beetle.

Today though, it is the commonplace insects that are my focus, the bramble providing an opportunity to brush up on my bumblebee identification and to try out a new identification ‘app’ developed for the ipad by TouchPress . The most common bumblebee on this particular patch of bramble is the tree bumblebee Bombus hypnorum, a recent colonist whose populations here in the Brecks appear to be going from strength to strength. This attractive bee was first recorded in Britain back in 2001 but was not recorded from Norfolk until 2008, when an individual was found in Earlham Cemetery. Being one of the easier bumblebees to identify, with (in queens, workers and most males) a ginger thorax, black abdomen and white ‘tail’, you stand a good chance of spotting one this summer here in Norfolk. Tree bumblebees, as the name suggest, usually nest above the ground in holes in trees or buildings. They have also taken to using nest boxes of the type erected for hole-nesting birds like blue or great tit.

The arrival of the tree bumblebee is perhaps unsurprising given that its distribution elsewhere in Europe sees it established as far north as the Arctic Circle. The colonies of this species can be sizeable with perhaps as many as 200 workers supporting them, which might explain the numbers that I am seeing here on the bramble where they outnumber the white-tailed and buff-tailed bumblebees flying alongside them. Some of the other bees present will need more careful scrutiny, however, if I am to pick out some of the cuckoo bumblebees that might be present. Cuckoo queens take over the nests of true bumblebees, ousting the resident queens to secure the labour of their workers, which then help to raise cuckoo males and females.

Bumblebees take up residence

While bumblebees have always been regular visitors to my town centre garden, this year is the first that I have actually witnessed a queen bumblebee take up residence. An abandoned small mammal hole in my main flowerbed, most likely that used by a colony of wood mice last summer, has been taken over by a nest of buff-tailed bumblebees. Throughout the day, a growing procession of worker bumblebees enter and leave the tiny hole in the woodchip mulch, near the base of one of my aquilegias. I am amazed by their industry.

The buff-tailed bumblebee is one of our most common and widespread bumblebee species, occurring right across England and Wales, and now expanding its range within Scotland. If you see a large bumblebee in spring, with two yellow bands on a black background and a buff or off-white tail, then the chances are that you have seen this species. The workers that are on the wing now are smaller than their queen, similar in colour and pattern, though with the tail white rather than buff. If you look closely then you may see that the white tail has a thin line of buff between it and the black of the abdomen.

The nest itself will be placed at the bottom of the long tunnel that drops down through the soil, and will consist of an untidy dark-brown comb, more than likely covered by a waxy cap. The nest will be where the first generation of workers would have been reared by the queen before they, in turn, go on to rear subsequent broods. Also present will be the stores of pollen, large quantities of which are stored in tall cylinders of wax. Right now, the colony is probably still rather small; buff-tails are notoriously slow in getting their colonies going. However, numbers will keep growing and, at its peak, the colony may number several hundred workers. The outcome of all this industry is not simply the workers, since these are means to an end. Later in the summer the first males and daughter queens will be produced. It is these that will set forth and continue the bumblebee line. If things go well, the colony may complete two cycles in a single season.

The worker bumblebees can be seen foraging around the garden, visiting many of the flowers that I have planted in the hope of providing suitable nectaring opportunities. Buff-tails are slightly shorter tongued than certain other bumblebee species and so are unable to gain access to every flower. Interestingly, they sometimes overcome this problem by biting through the flower tube (known as the corolla) to steal nectar. It is wonderful to see such resourceful creatures doing so well.

Bumblebees herald spring

The sight and sound of queen bumblebees is a sure sign of spring. Newly emerged from winter hibernation, these large and fascinating insects will soon be searching for a suitable site in which to establish a nest. With their internal fat stores almost completely exhausted, the first priority is to seek out sources of early season nectar and pollen. Although the buff-tailed bumblebee may be seen visiting the catkins of sallow or garden flowers as early as February, the other common species may not be on the wing quite so early. It is a precarious time; early emergence may use up valuable resources if there is insufficient nectar and pollen to be found. If the weather turns cold, as it did towards the end of March, then the bees will seek shelter and become torpid, re-emerging once the weather improves. By the beginning of April all the common species will be on the wing, engaged in finding food. Interspersed with these bouts of foraging are periods spent sunbathing on exposed leaves, walls or stones, a behaviour that is thought to help with the development of the ovaries in readiness for egg laying.

There is then a change in bumblebee behaviour, with the introduction of low, contour-hugging flights, as individuals prospect for a suitable nest site. Preferred nest sites vary with species; while some bumblebees nest underground others may use old bird nests, nest boxes or tree cavities. The underground nests tend to be in the abandoned burrows made by small mammals, like mice and voles. These can be a scarce resource and competition for nests may be fierce. It is sometimes possible to find several dead queen bumblebees around the entrance of a small mammal nest being used by a seemingly victorious queen. One species, the red-tailed bumblebee, uses holes in bare ground, or adjacent to stones or concrete, which act as heat stores and aid the regulation of temperature within the nest.

The whole process of nest site location and selection may take several weeks but, once established, the queen soon gets on with the business of producing her first batch of eggs. Typically, the queen will line the nest with fine material, before constructing a brood chamber. This chamber is made of wax, extruded from her abdomen. It is into this that she will lay a batch of eggs, usually between eight and twelve in number; the queen having mated back in the autumn before going into hibernation. Alongside the brood chamber the queen will also construct a nectar store. This initial batch of eggs will become the first infertile workers that will tend subsequent broods but the queen will need to tend this first brood herself.

Springtime buzz

A visit to Thetford Warren Lodge the other lunchtime not only produced my second Brimstone butterfly of the year but also my first bumblebee. Although queen bumblebees that have overwintered successfully can be seen on warm days earlier in the year (as early as February in species like the familiar buff-tailed bumblebee), the cold weather of recent weeks has held things back somewhat. Queens of the six most common and widespread species are likely to have emerged from hibernation by now and will be searching out early flowers in order to replenish energy supplies. Over the coming weeks, the flowers of both red and white deadnettle will prove attractive to many species. However, if the weather deteriorates (and at night) the queens will seek shelter, often in deep leaf litter.

One pattern that you may begin to notice over coming weeks is for queens to fly very low, just above the contours of the ground as they search out a suitable place for their nest. Many species will seek out disused small mammal burrows in which they will lay their first batch of eggs. These eggs, usually 8-14 in number, produce the first generation of offspring, smaller females that are usually infertile and who serve as a worker caste, helping to raise later broods as the colony begins to grow in size. One quite common species, known as the common carder bee, is more adaptable when it comes to nest site selection and may be found using compost heaps. Not all bumblebees make their own nest. There is one group of bumblebees, known as cuckoo bumblebees, which will invade an already established nest, often killing the incumbent queen in the process, prior to laying their eggs.

Some of our bumblebee species emerge later in the year, including the rather localised red-shanked carder bee. There are records of this species from the north Norfolk coast and Breckland but it appears to be in decline. This species, like many other carder bumblebees, emerges in late May or early June. This means that there is quite an extended period of emergence across our 25 or so species, something that provides plenty of scope for the interested naturalist. Those individuals on the wing at the moment face many challenges if they are to successfully found a colony from which fertile males and females will be produced later in the year. Mortality rates of queens are very high (perhaps 80%) because of predation, inclement weather and a nematode worm. The queens that you are seeing on the wing now will be busy getting into condition before producing the larvae which will form the basis of their colony over the coming months.