Andrena clarkella - one of our early Spring bees

06th March 2014
We are fortunate in having over 60 species of Andrena mining bees in the UK. Some species are very common, others scarce and some endangered. Andrena bees nest in the ground. Some nests are isolated but others can occur in large aggregations. Typically, the nest entrance leads to a burrow; with lateral smaller brood "cells" where the eggs are laid.

Andrena bees constitute about 25% of our "solitary" bee species. The term "solitary" relates to the fact that their nesting behaviour is not social (like honeybees and bumblebees). After completing a series of nest burrows, the females die and their offspring do not emerge until the following season. The females never get to see their offspring.

Andrena bees are generally very hairy and can vary in length from around 5mm to 15mm. Males are usually smaller and slimmer than the females. Some species look very similar and it can be challenging to identify them from photographs. Close examination of features and the time of year the bee is active, can help with this though.

One of the earliest mining bees to emerge in spring is Clarke's Mining Bee (Andrena clarkella). Depending on the weather, they can start appearing from their nests in mid-February. With this year's (2014) wet and chilly weather, I wasn't too hopeful when I set out to a local site on 24th February to check on their status. I was pleased though, to find a small number of males.

This is typical male of the larger Andrena. Males are generally smaller and slimmer than females and often have a prominent "moustache" of hairs on the face. Males also have thirteen segments to their antennae and the females have twelve segments. It's not always easy to differentiate and count the segments though! In Andrena clarkella (and many other species), the facial hairs of the male are paler than in the female. The males emerge before the females and then actively patrol the nesting and foraging sites waiting for females.

The females are largish (about 10mm) and have very characteristic colouration. There are dense black hairs across the face and abdomen, reddish/orange hairs on the thorax and yellow/orange hairs on the hind leg - both tibia and basitarsus (the pollen basket and area below). These features are nicely illustrated in the image to the left.

This mating pair shows the difference in size, build and colouration nicely. This difference (sexual dimorphism), can be very marked in some species. All hymenopterans can control whether the eggs they lay will be develop into males or females. Eggs fertilised with the male's sperm will develop into females. Those not fertilised will develop into males. Mated females can control whether eggs are laid fertilised or non-fertilised. The first (and deepest) laid eggs will be fertilised ones and the last (those nearest the nest entrance), non-fertilised ones. This ensures that the males emerge first and are ready to mate with the females once they emerge.

This activity is definite confirmation that Spring is on the way. We'll see what the next bee species to emerge locally is. Watch this blog for news!

[Click on any image for a larger version]

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