How to get a snail to go aaaaah!

I have spent quite a lot of time lately tidying up my photo library and tagging photos and I have come across several worth posting about even if they won't be as timely as usual. I wanted to see if I could take some shots of snail's mouths. To do this, I mashed up a few dandelion leaves and painted an area of the outside of one of the conservatory window with the resulting concoction. Then I found a few active snails and this one delivered. It started climbing up the window, its foot showing the muscular ripples that power its slow advance, mouth shut tight.

As soon as the snail felt the dandelion mixture, it opened its mouth and started licking it, showing its tongue (the radula) and the chininous, dark hardened ridge in front of it. The radula is a muscular organ covered in rows of hard little teeth that can scrape surfaces and it was very evident how the snail used it to eat the bits of the dandelion leaves once it got to them.

Dandelion leaves, yum!

These are the marks left by snails or slugs while grazing algae growing on a pot

The snail also showed nicely the opening of its lung, on the right side of its body.

Winter active bumblebees

We had our first frost yesterday, and it was also a frosty morning today. But coming back home this afternoon, with the light already going weaker, I came across a Tree Bumblebee, Bombus hypnorum feeding on a large Mahonia bush. I have posted before on this bumblebee, a recent natural colonist in the U.K. In the last two decades, reports of winter active bumblebees - mostly Bombus terrestris - have steadily been accumulating, especially in the south of Britain. Queen bumblebees are occasionally active in warm winter days, but the reports referred to queens collecting pollen - a sign they are actually nesting, not hibernating - or workers - indicative of active nests. These observations depart of the usual bumblebee life cycle in northern Europe, where winter hibernation of queens is the usual case. Whereas there are many native flowers in bloom during the winter to support a second generation in the Mediterranean, this is not the case in northern Europe. What resources are active winter bumblebees using? Stelzer and coleagues tested the hypothesis that winter bumblebees are making use of non-native flowers in parks and gardens. To do this, they set up B. terrestris colonies and followed them during two winters. In the second winter they introduced a new technique - micro-tagging - which allows automatic recording each time the tag passes nearby the tag reader set at the nest entrance. They attached the small RFID tags to the thorax of 64 individuals, and positioned a micro-balance at the entrance of the nest so that bumblebees also weighed themselves as they went in and out the nest, so that they can estimate the increase of weight of each bumblebee at the return of each foraging trip. Their results showed that bumblebees in their experimental nests were able to forage for nectar successfully during the winter, with nectar returns per hour foraging comparable or higher to yields obtained during spring and summer.

The researchers also carried out transects in the Royal Botanical Gardens in Kew to ascertain which plants were being visited by Bumblebees during the winter:
Only cultivated plants were in flower and the main plants visited by B. terrestris were Arbutus unedo and Salvia uliginosa in October (38.2% and 42.3% of the total recordings during that month, respectively), Arbutus spp. (A. unedo and A. x andrachnoides) (31.2%) and Mahonia spp. (Mahonia x media ‘Winter Sun’, Mahonia x media ‘Charity’ and Mahonia lomariifolia) (43.7%) in November, Mahonia spp. (69.0%) in December, Salix aegyptiaca in January (48.2%) and Lonicera fragrantissima (24.1%) in February.

The high yields obtained by the experimental bumblebees might be explained by the nectar sources being large bushes with large volumes of nectar per flower and the flowers being in large inflorescences, such as the ones in Mahonia (see photo above) and willows. In addition, there is no competition for nectar and pollen in the winter months by other foraging bees. Only a few honeybees were around, and these are not active below 10oC, while the bumblebees can forage at temperatures close to 0oC. Further work is necessary to assess if this generation is actually successful, that is, if males and queens are produced, but these results show how human impacts - gardening possibly coupled with climate change- have unexpected effects on the life cycles of native organisms.

Stelzer RJ, Chittka L, Carlton M, & Ings TC (2010). Winter active bumblebees (Bombus terrestris) achieve high foraging rates in urban Britain. PloS one, 5 (3) PMID: 20221445

Silver Ys feeding

I've got a lavender hedge growing next to my conservatory, which comes very handy to do a daily bumblebee count. Tonight, while inside, I noticed a few silver Y moths feeding on the lavender and everlasting pea flowers which grow amongst the lavender. I went outside and more moths were feeding on the buddleias. As migratory moths, taking advantage of warm winters in high latitudes, silver Y numbers fluctuate a lot from year to year. This has been a warm summer and there are many of them around. When not resting, these moths are a flurry of activity, almost never stopping on the flower, and constantly vibrating their wings. Without using a flash, they are often a blurr, with it, they look like they are frozen in space. I had no choice but to use a flash, as it was too cloudy and dark, and the photos were taken trough glass. So here are my first night photos of the silver Y.

 Inserting its large proboscis on a lavender flower.

Caught in flight between flowers. This is a migrating species, found mainly from July to September.

Atop a lavender spike.

Several were feeding on the buddleia high up.

A silver Y resting next to the tomatoes earlier in the day.

Little footprints on petals

I find it difficult not to write about bumblebees these days. We've had a week of showers, and bumblebees do not seem to mind much, going about their business rain or shine. They have the advantage of being warm-blooded insects, they keep their body temperature above the environment, and therefore, they do not need the sun to warm up and forage, like other insects. When the flowering bushes are buzzing, how do bumblebees decide what flowers to visit and when? Do they follow a pattern or do they choose specific flowers? If you watch a single bumblebee visiting a plant, say, a foxglove or a Stachys spike, it is easy to see that they follow a pattern: from the bottom up, and that might suffice to avoid visiting the same flowers twice; but what if there are many bumblebees feeding together? Surely they will end up visit flowers that have just been sucked up dry of nectar? Well, research shows us an unexpected aspect of bumblebee's - and bee's - natural history. Bumblebees actually mark, with scents detectable by other bumblebees, each flower they visit with glands located on their tarsi - at the end of their legs. Some of these marks are repellent, so that bumblebees avoid flowers that themselves - or others - have recently visited and, as the chemicals are volatile, by the time they have faded, the flower is full with nectar again. Interestingly, flower scent marks seem to be recognised not only by bumblebees of the same species, but also by several other bumblebee species, and bees visiting the same flowers, despite being different chemical compositions. Jane Stout and Dave Goulson carried out simple, but labour intensive, experiments by with Bombus lapidarius and Apis mellifera (the honey bee) feeding on a large patch of Melilotus officinalis, showing that both species recognise each other marks on flowers and avoided recently visited flowers. What I found really interesting around this topic is that, in a different study, Gawleta and coworkers showed that bumblebees avoid to a larger extent the flowers previously visited by Anthidium manicatum, the Wool-Carder bee. This could be a response to the aggressive behaviour of males of this territorial bee.

Bombus pascuorum leaving a Stachys flower. Wool-carder bees have been quite late this year and it is the first time I see B. pascuorum feeding on these flowers.

Female Wool-Carder bee feeding on Stachys. Males have not emerged yet and both bumblebees and Wool-Carder bees seem happy to feed together.

Bumblebee home ranges

Have you ever wondered where do the bumblebees visiting your garden come from? How far do they travel from their nests? How can scientists find this out? It would appear that an obvious strategy would be to mark bumblebee workers from single nests, survey the surroundings of the nest and map where they go. In practice, this proves difficult as it is difficult to find enough marked workers in a reasonable amount of observation time. Another option is to mark bumblebees, release them at increasing distances from their nests and see how well they return home. When this last experiment was performed bumblebees -Bombus terrestris in particular - could return home from the astonishing distance of 9.8 km! However, this does not show how far from their nest they normally forage. In this paper, Mairi Knight and co-workers used an elegant, sophisticated, but cost-effective approach which yielded a lot of data. The premise of their work is that the workers from a nest are expected to be full or half sisters - offspring of the same queen. What they did is to follow a linear transect capturing workers of four common bumblebee species at regular intervals.

The sampling design: across agricultural land, with some woods, hedgerows, and a nearby town in Hertfordshire. The sampling points (in red) are approximately 250 m apart.

They screened the workers using 8 to 9 very variable genetic markers (microsatellites, which are also used in human paternity testing) and identified sisters by assessing the number of markers sisters should be expected to share. Their results were then used to calculate the maximum foraging distances of nestmates of the four species. It was 449 m for B. pascuorum, 758 m for B. terrestris, 450 m for B. lapidarius and 674 m for B. pratorum. Quite a long way, I would say. B. terrestris is the species flying longer to forage, and also the species with the larger workers and more workers per nest. A much more striking result of this study, though, is the number of nests from which workers come to feed in a specific sampling point. Knight and coworkers estimated that, on average, B. lapidarius workers came from 75 different nests to a specific sampling point, these figures were 43 nests for B. pascuorum, 52 for B. terrestris and 37 for B. pratorum. Of course this study applies to farmland, but for B. terrestris, nest densities might be even higher in urban areas. It makes me feel good to know that even when, frustratingly, there are no bumblebee nests in my garden, it still supports a very high number of nests from a range of species.