Cuckoo bumblebee chemical weapons

This time of the year there is an abundance of cuckoo bumblebees. These bumblebees form a group of closely related species, the subgenus Psithyrus, each specialising on one or a few nesting bumblebee species. For example, Bombus vestalis, above, specialises on the common Bombus terrestis, the buff-tailed bumblebee. Cuckoo bumblebees are social parasites. They lack a worker caste and instead rely of workers of their host species for the rearing the next generation of males and female cuckoos. Fertilised female cuckoo bumblebees emerge from hibernation a little later than the host, usually when their host queens have already stated a nest and already have a few workers. Some species 'infiltrate' the hosts nest passively, with little aggression involved. They simply remain peripheral to the combs, initially thought to get the workers used to their smell, and little by little the workers and queen accept them. Then they start participating on the colony workings by grooming larvae or incubating cocoons. At the same time, however, they lay their own eggs, and eat eggs and larvae of the host species, until only their own males and females are reared instead of host worker. The cuckoo female often engages in 'head-rubbing' with the workers in the nest, a behaviour thought to involve some transfer of chemicals between cuckoo and host. Often host queens and males are produced together with the cuckoo's offspring. In other cases there is quite a lot of aggression from the workers to the cuckoo and the other way round. More peaceful nests produce a higher number of cuckoo bumblebees.

Cuckoo bumblebee Bombus rupestris

Queen Bombus lapidarius, its host.

Cuckoo bumblebees have often similar colour patterns to their hosts, and this type of mimicry could help them enter a nest. As an example, see above the cuckoo Bombus rupestris and its host Bombus lapidarius. But, given the high levels of aggression involved when conspecific worker intruders try and enter a nest, how do cuckoo females manage to be accepted into the hosts nest? They have another, more recently discovered form of mimicry: chemical weapons. Research by Stephen Martin and collaborators investigated the  surface chemicals of 14 bumblebee species, including 5 cuckoos, from the U.K. and Sweden.

They found out a striking pattern: several of the cuckoo bumblebees match strikingly their hosts chemical profile, in particular the alkene isoforms, which are species specific. A few species differ from this pattern. These species are probably being involved in a host switch, and instead of mimicking their hosts chemical bouquet, they produce a worker repellent chemical. Workers then will avoid, and therefore won't attack the cuckoo female when she enters the nest. This is the case of B. sylvestris, whose normal hosts, B. jonellus and B. monticola, are quite rare in the U.K. and more commonly attacks the common B. pratorum.
All bumblebee photos taken today.
More information
Küpper, G. & Schwammberger, K. (1995). Social parasitism in bumble bees (Hymenoptera, Apidae): observations of Psithyrus sylvestris in Bombus pratorum nests Apidologie, 26 (3), 245-254 DOI: 10.1051/apido:19950306
Martin, S., Carruthers, J., Williams, P. & Drijfhout, F. (2010). Host Specific Social Parasites (Psithyrus) Indicate Chemical Recognition System in Bumblebees Journal of Chemical Ecology, 36 (8), 855-863 DOI: 10.1007/s10886-010-9805-3

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.

More on maternal spiders

I posted recently on maternal behaviour in spiders, but I when I came across this I knew another post was in order. The photo above shows what is is most likely a Gnaphosidae spider with her eggs in her nest on the 19th of July. The spider has wrapped herself inside a silken nest she has made for her eggs, and she will remain there until they hatch and the spiderlings disperse. This behaviour, called 'egg guarding' is present in many spiders. Why would a spider do this? Do the eggs benefit in a any way from their mothers behaviour? There is anecdotal evidence in related species that unattended eggs are predated, but little information was available on who the predators are until Simon Pollard carried out some experiments on Clubiona cambridgei, an endemic species from New Zealand displaying this egg guarding behaviour, and the results were surprising.
 Simon had noticed that sometimes Clubiona spiders were found inside or just outside nests with eaten eggs and suspected that individuals from the same species were actually the egg predators. Egg predation seems an unusual behaviour for spiders, who are considered hunters of active organisms. He wanted to test the hypotheses that (1) spiders eat eggs from their own species - a form of cannibalism - and (2) egg guarding represents a defensive mechanism that decreases the chances of conspecific predation. He carried out simple experiments with egg nests - easily collected in this species as it is abundant and nests in folded out flax leaves. His results were conclusive. Nests where mothers had been carefully removed (therefore, unattended) and then were exposed to non-breeding females were all predated, with all eggs eaten within 16 h. Males or breeding females did not eat unatended eggs. When nests were attended, males and non-breeding females quickly retreated when noticing the activity of the guarding mother and no egg predation took place.
 Therefore egg guarding in Clubiona is indeed a strategy to protect the eggs from other spiders predation. More recently, cannibalistic egg predation has been documented in 12 spider families - including Gnaphosidae -, and therefore, egg guarding behaviour is likely to have the same defensive function in these spiders against cannibalistic predation.
My spider successfully defended her eggs, and yesterday, I could see the pale, tiny spiderlings through the nest walls.

More information
Pollard, S.D. (1984). Egg guarding by Clubiona cambridgei (Araneae, Clubionidae) against conspecific predators Journal of Arachnology, 11, 323-326
Nyffeler, M., Breene, R., Dean, D., & Sterling, W. (1990). Spiders as predators of arthropod eggs Journal of Applied Entomology, 109 (1-5), 490-501 DOI: 10.1111/j.1439-0418.1990.tb00080.x

A hornet-mimic hoverfly

The sun shone briefly just after breakfast and I mechanically opened the back door and stepped outside. I disturbed a few insects sunbathing on the east-facing ivy-covered wall. One called my attention, a hornet? No, a stunning, giant hoverfly, with a loud hum, settling again amongst a few bumblebees. It patiently waited, surrounded by my family, while I fetched the camera, and then posed nicely for shots.
A quick check in the British Hoverflies identification guide pointed to Volucella zonaria, one of the largest and more striking British species. As other brightly coloured hoverflies, the guide drawing does not do it justice, possibly as it was made from faded, pinned specimens. This species was virtually unknown in the UK before the '40s, and then gradually established itself around the south coast, London and Bristol. Recently there are more scattered records to the north, suggesting a northern expansion, possibly linked to climate change, but nothing north of the Humber. Check its distribution map in the NBN Gateway.
 Apparently it is strongly urban, thriving in parks and gardens, where adults find a broad range of flowers to feed - buddleia, hebes, brambles, ivy, hemp agrimony to name a few. As other hoverflies of the same genus, the females somehow get into wasps nests to lay their eggs, unmolested by the wasps. Once hatched, the larvae fall to the  bottom of the nest where they are scavengers of the colony's debris and feed on dead wasps - and possibly live larvae - on the autumn when the nest is abandoned. The species is migrating as well, and males are territorial.
I look forward to having this around more often.

Mating red-tailed bumblebees

Amongst bees, bumblebees have very long copulations when compared to honeybees and solitary bees. Up to three hours have been recorded for Bombus lapidarius, the Red-Tailed bumblebee, closer to half an hour for B. terrestris. I came across these paired Red-Tailed bumblebees (above) yesterday in the wildlife garden. The brightly coloured, smaller male was simply riding the female, and she bumbled along, feeding on knapweed and heavily flying between flower heads. Females would be expected to be very vulnerable to predators during this time, where they are also very obvious. Although they can actually fly, slowly and clumsily, their sting is immobilised by the male's genitalia. Illustrating this increased vulnerability, the couple of the same species below was squished on a busy pavement last summer.

Why do bumblebees mate for such a long time? Could it be that they need a long time to successfully inseminate the queen? Apparently not, as successful sperm transfer happens in a few minutes when it has been measured. Another reason, is that males transfer not only sperm during copulation, but a 'copulatory plug', which physically hampers further copulation by the female and includes chemicals that make her unattractive to other males for several days, although apparently this plug is also transferred in a matter of 10 minutes. A lengthy copulation might also include the time necessary for the sperm to travel to the female's spermatheca, so the male would be increasing his chances of paternity by guarding his partner. Due to the dangerously long copulation, the copulatory plug dissuading further partners, and the still puzzling effects on her hibernation success and colony fitness due to multiple mating, Bumblebee queens only mate once, with one of the few exceptions being the tree bumblebee (Bombus hypnorum).

More information:
Brown, M., & Baer, B. (2005). The evolutionary significance of long copulation duration
in bumble bees. Apidologie, 36 (2), 157-167 DOI: 10.1051/apido:2005008

Big Butterfly Count

In the last few days, I have been carrying out surveys for the Big Butterfly Count. I did a count in my garden (5 sp.), two in a local wildlife garden (5 and 6 sp.) and two in a local cemetery (3 and 6 sp.). Given the weather, mostly cloudy with some sunny spells, I am quite pleased with the results. Their results website is quite good with an interactive map where you can check the results submitted at a local scale or for a particular species (over 10,000 surveys already submitted) here. The following photos illustrate my results.

 A Common Blue male today, feeding on oregano. The male regularly patrolled his territory in the wildlife garden, stopping occasionally to feed.

Gatekeepers were plentiful today. This female rested on a Knapweed plant.

My local cemetery has a thiving population of Speckled Wood. Lots of territorial disputes were going on yesterday.

This Red Admiral took advantage from a brief sunny spell yesterday near a buddleia growing in the local cemetery.

This is the first year I see Small Coppers in the wildlife garden.

A Holly Blue feeding on lavender

A Common Blue female on Oregano.

Two small whites courting, the females abdomen raised meaning 'not interested'.

Two female Small Whites sunbathing.