Monday, 28 May 2012

Cherry wars

The new shoots of my cherry tree are covered on Black Cherry Aphids, Myzus cerasi. The new leaves are stunted and folded, sheltering the aphid colonies which produce abundant honeydew - loved by bees and bumblebees. Ladybirds have also arrived at the tree in search of the aphids. There are many Harlequin ladybirds, mating, feeding and laying eggs (above).

Today, tens of Harlequin egg clusters were very obvious under the leaves...

...often close to aphid infestations. 
One of the clusters was already hatching: a new generation of Harlequins 
There were also many 2 spot ladybirds on the cherry, I am not sure they are as advanced on their life cycle as the Harlequin, although this pair was mating. Given the strong impact of Harlequins on 2 spot populations, it is obvious there is more than one war going on in the cherry tree. All the photos, except for the top one (26/05/12) taken today.

Thursday, 24 May 2012

A primer on solitary bees

When you read ‘bees’, do you immediately think honeybees? I don’t blame you. Honeybees are fascinating creatures: they have a complex social system which has puzzled biologists for over a century, they are very important pollinators, and they are one of the few domesticated insects, producing honey and wax. We are familiar with them since we are kids. They are also big business, so that means they get a lot of press.

But bees do not mean just honeybees, because them and other social bees (like bumblebees) represent just a tiny fraction of the world bee species. There are 20,000 species of bees in the world, of which about 250 are found in the UK. Most of them are solitary: males and females mate, and females find a nest, lay their own eggs and collect nectar and pollen for their own offspring. Many provision the nest themselves, others, instead ‘cheat’ taking over other ready-made nests laden with pollen and nectar: the cleptoparasitic bees. Solitary bees include very efficient pollinators, like the Red Mason Bee (Osmia rufa or O. bicornis), which are also used commercially to pollinate particular crops (OSR, apples, cherries and strawberries). Other than their economic importance, the diverse morphologies, life histories and behaviour of solitary bees make them fascinating invertebrates to watch. In this post, I will list a few wonderful features of solitary bees using examples of common species often found in gardens. 

A male Hylaeus sp. on Fennel

1. British solitary bees range in size from the tiny Hylaeus (5 mm) to the male Wool Carder Bee Anthidium (14-17 mm), which is larger than many honeybee workers. 

A male Wool Carde Fee, Anthidium manicatum, note the prongs at the end of its abdomen

2. Males often emerge from their nests a few days before females and are smaller (Osmia bicornis male above). There are a few exceptions like the Wool Carder Bee, where males are bigger and armed with a three-pronged abdomen. 
Mating Wool Carde Fee, Anthidium manicatum
3. Male bees use a range of behaviours to secure access to females. Some species mate with females even before they emerge from their nests (Andrena carantonica), others hang around nest entrances waiting for females to emerge (Osmia bicornis), yet others patrol flowery places incessantly to attempt mating with feeding females (Antophora plumipes) and in the most extreme case, males aggressively defend territories containing suitable flowers from other males and even other species of bee, so that they can monopolise mating with females visiting them (Anthidium manicatum). 
Female Osmia bicornis (rufa) carrying a ball of mud to its nest
A leaf-cutter, Megachile centuncularis cutting a piece of leaf
A Tawny Mining bee female Andrena fulva, digging its nest in the soil

4. Bees lay their eggs in cells in the so-called ‘nests’, which they provision with pollen and nectar, and where the larva later develops. Nest building takes a lot of effort and female bees often have specialized structures in the head (the ‘horns’ of Osmia bicornis) or strong or scissor-shaped mandibles (like in the leaf-cutters Megachile), which facilitate nest building. Bees use a bewildering diversity of materials for their nests, and these are often species specific, so that they get from them their common names such as carpenter, mining, mason or leaf-cutter bees. In some species females excavate their nests in wood (Anthophora furcata), soil (Andrena fulva), or soft cliffs or mortar (Anthophora plumipes). Others use existing holes such as old beetle holes or hollow plant stems and line them with diverse materials: leaf fragments (Megachile sp.), mud (Osmia bicornis), downy hairs collected from plants (Anthidium manicatum), or their own cellophane-like secretions (Colletes, Hylaeus). A few British species even use empty snail shells to nest (see the fantastic photo series by Richard Comont here). 

Anthophora furcata on one of its favourite pollen-collecting flowers, Stachys sylvatica

5. Solitary bees need pollen to produce their eggs and feed their larvae. They may either use many species of flowers to obtain pollen (these are called polylectic) like Osmia bicornis, or they might be specialized, using only a few or a single type of flowers (oligolectic) like Anthophora furcata (above). 
Megachile willughbiella female with laden pollen basket under its abdomen

6. Bees use several ways to carry pollen. The specialized brushes to carry pollen are called ‘scopa’. The scopa can be located on their rear legs (Andrena, Anthophora), under their abdomen (Megachile, Anthidium, Osmia), or on both. Some bees (like Hylaeus) carry pollen in their crop, mixed with nectar.
Lasioglossum bee covered on dandelion pollen
7. Bees in general are very hairy. As the bee flies, its branched hair becomes electrically charged, so that it acts as a pollen-collecting device: as the bee contacts the pollen, it sticks to its hairs. To collect pollen, bees can roll in pollen-rich flowers (dandelions), make flowers vibrate using their wings to release the pollen, which falls on their body (poppies, comfrey), or push the flower open with their heads and bodies (irises or vetch flowers). After pollen is collected on the bee’s body hair, the bees groom it using their legs, mix it with some nectar and package it in their scopa to carry to the nest.
Anthophora plumipes collecting Lamium pollen
8. Bees have a specialized tongue to collect nectar. Some bees have a very long tongue - Anthophora plumipes tongue is as long as their body – and use it to reach flowers with deep corollas; others have short tongues (Colletes, Hylaeus, Andrena) and they can only reach shallow, open flowers. The tongue is made of several movable parts, and is tucked under the body when the bee is not using it.
Nomada marshamella, a cuckoo be resembling a wasp, near its host nest entrance

9. About a third of solitary bee species are cleptoparasites to other bees (they are also called ‘cuckoo bees’), laying their eggs on the ready provisioned nests of other species, where their larvae develop. Cuckoo bees are often closely related to their hosts. Cuckoo bees tend to be very species-specific, they parasitize just one or a few species of bee. Because of this, some cleptorarasitic bees are extinct or have become endangered in the UK due to the scarcity of their hosts.

10. Cleptoparasite bees, tend to be less hairy than other bees – as they do not collect pollen they do not need the hairs – and some can be confused with wasps (Nomada bees. They have also hardened cuticles to resist the stings of their hosts defending their nests. 

And there is much, much more: after all, there are just over two dozen species I am familiar with.

More information on bees
Bees in Britain. A superb, freely accessible introduction to British bees by BWARS members.

Tuesday, 22 May 2012

The very pregnant spider

 Most of the very abundant female wolf spiders in the garden (Pardosa sp.) are looking like the one above: their distended abdomen indicating they are about to lay eggs. I only saw one was already carrying an egg sac, the first of the year, sunbathing on the fence.

Monday, 21 May 2012

Communal mining bees

ResearchBlogging.orgA few years ago, a crack in the concrete path at the front of our house became busy with bees coming in and out. We assumed they were honeybees that had settled their hive there, but on closer inspection they turned out to be mining bees instead. Their cleptoparasite bees, Nomada, hung around the nest entrance awaiting an opportunity to get in and lay their eggs. There are many similar species of Andrena mining bees in the UK and several of them looked very similar to mine so I finally gave up with obtaining an ID and I didn't find much more about my front path bee. The bees are back again in the same spot, and a couple of days ago, thanks to tweep @DayMoonRoseDawn I finally knew the identity of these curious bees: they are Andrena carantonica, a large, furry mining bee. Although mining bees are solitary, a handful of species in the UK often nest communally. Communal bees have a simple form of social structure: they share a nest, but individual females will build their own cells, lay eggs and provision their own offspring: they do not have the complex social organisation with workers and division of labour of honeybees. Nesting aggregations of A. carantonica consist of hundreds of females (up to 600) females using the same nest entrance. In my street, cracks on concrete in front garden paths are favoured. Adults emerge in April, with the smaller males doing so just before females and awaiting them inside the nest. In a series of studies on this species, Robert Paxton and collaborators revealed fascinating details of the social structure of this species. In one of these studies they installed nets on nest entrances to be able to capture recently emerged bees: 75% of females were already inseminated when they left the nest for the first time, indicating that copulation happens most of the time inside the communal nests. Although the females in the nest are not highly related, there is a large amount of inbreeding in this species: genetic analysis suggests that 44% of females mate with their full brothers. Emerging females are much more numerous in a given nest (about 1 male to 3 females), although, as Rose Dawn pointed out, it is likely that many males never leave the nest and die inside after having mated, so emergence nets do not capture them. Females must by necessity swap nests after emergence so that the relatedness levels is still low within a given nest, and the bees show a high degree of tolerance to conspecifics, regardless of their nest of origin. If a high level of relatedness is not responsible for the evolution of communality, why do these bees nest communally? A potential benefit is improved nest defence. There are always bees going in or out, and in my own observations, a few bees often sit by the nest entrance (like as in the photo at the top), therefore opportunities for parasitism by cleptoparasites might be reduced. There are still many questions to answer, which probably will have to await to observations being carried out inside nests, but the study of these and other communal species might offer clues as to how and why complex social systems like those of honeybees evolved.
A male Andrena carantonica near the nest entrance
Female Andrena carantonica
A cleptoparasite bee Nomada flava by mining bee nest entrance.

Thanks to Rose Dawn for providing an ID and sharing lots of information on this bee, including results from her own research
More information
Paxton, R., Thoren, P., Tengo, J., Estoup, A., & Pamilo, P. (1996). Mating structure and nestmate relatedness in a communal bee, Andrena jacobi (Hymenoptera, Andrenidae), using microsatellites Molecular Ecology, 5 (4), 511-519 DOI: 10.1046/j.1365-294X.1996.00117.x

PAXTON, R., THOREN, P.A., & GYLLENSTRAND, N. (2000). Microsatellite DNA analysis reveals low diploid male production in a communal bee with inbreeding Biological Journal of the Linnean Society, 69 (4), 483-502 DOI: 10.1006/bijl.1999.0371

Sunday, 13 May 2012

Crane Fly halteres

This Crane Fly or Daddy Long Legs (possibly Tipula scripta) got into the house and I gave a try to getting its portrait on a white background. It is pleasantly surprising how many insects that I expect to fly away immediately do actually stay still through my white bowl session. The little knobbed structures sticking out at the sides of the body are the halteres, typical of all flies but more prominent in these large flies. They evolved from the hindwings and when in flight move in opposite directions as the wings and are involved in balancing and steering and contribute to the exquisite flying abilities in the group, although crane flies themselves are weak, wobbly fliers

Wednesday, 9 May 2012

The Footballer hoverfly

From mid April up to October, you might come across this handsome hoverfly resting on leaves or feeding on flowers either close or away from water. This species is Helophilus pendulus, the commonest of its genus in the UK, distinguished by other species by the parallel yellow stripes in the thorax, black stripe on its face and the mainly yellow hind tibia. The genus Helophilus, which is a wasp mimic, is closely related to Eristalis, the droneflies, which are bee mimics. Both genera share a rat-tailed aquatic or semiaquatic larvae. I found this female sitting on a sage leaf after a light shower. She was cold and reluctant to fly, and with a bit of coaching, she climbed on the white bowl and let me take her portrait.

Tuesday, 8 May 2012

Shiny shieldbug

The little Woundwort Shieldbug Eysarcoris venustissimus might easily pass unnoticed in your garden. It frequents native plants such as Enchanters nightshade, Herb Robert and, mainly Hedge Woundwort, where it lays its eggs and the nymphs develop. It can also complete its life cycle in the widely planted Lamb's Ears. On close up, it is a beautiful insect, iridescent bronze/green and white, heavily speckled with black dots. Today, I found the first mating pair of the year, 10 days after the first ones emerged from overwintering.

Monday, 7 May 2012

Losing old legs and growing new ones

I see lots of Philodromus spiders with missing legs, some of them with up four legs missing. They are - incredibly - able to walk, and presumably find enough food to stay alive. Many spider species have weak points in their legs, when in danger of being predated or otherwise attacked, they shed them, a phenomenom called autotomy. Spider autotomy has actually been shown to be a voluntary act, as anesthetised spiders would not shed their legs. For a spider, it is costly to lose a leg, as it might impair reproductive or fighting ability, or success at hunting, but it is a small price to pay when the alternative is to be eaten. And the best thing is that sometimes legs are not lost forever, as immature spiders maintain the ability to regenerate lost limbs in the next moult  (although long lived spiders that moult when adult, like tarantulas, do keep that ability for life). The spider on the top shot, enjoying a juicy aphid a few days ago, has regenerated the middle pair of legs on the foreground: notice that they are thinner and shorter than the rest, and lack the colour pattern seen on the equivalent legs on the other side. Even in the Philodromus below, which has lost four legs and a palp, the apparent stumps of the missing legs are actually the miniature folded regenerated legs.