Saturday, 8 April 2006

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.

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