Hoverflies laying

I posted on hoverfly larvae feeding on aphids a few days ago, but I was able to photograph several species laying eggs and thought that deserved a post on itself. My mock orange (Philadephus) has dense aggregations of black aphids and several hoverflies have been laying eggs on the buds and leaves on Philadelphus and Pyracantha. On the top photo Eupeodes luniger. Her swollen abdomen curling to deposit an egg on a bud. She patiently moved from a clump of buds to another, spreading her eggs in the whole bush. The carnivorous, aphid loving larvae that will hatch from these eggs will contribute to keeping the aphid numbers on check.

Syrphus

Epistrophe eligans

And to end the post, just to say that this is a special post for BugBlog: Number 100. I am quite happy to have taken on blogging, and it has been a lot of fun researching the posts. I hope I can mark many more returns for BugBlog in the future.

The leaf-cutters are back

A male leaf-cutter has been patrolling in the garden the last few days, when the weather has been quite hot. Leaf-cutters are summer bees, emerging from their nests at the end of May-beginning of June and dissapearing for the year at the end of August. The male has been circling around the broom and other flowers, in a regular path, landing to bask for no more than two shots in a sunny leaf or flower. Occasionally, he stopped to feed on some bluebells.

They are Megachile willughbiella (thank you to eucera from WAB for confirming the ID). The males bear white and golden 'boxing gloves' on their forelegs, which are enlarged concave-shaped furry tarsi, contain glands producing odor chemicals and are involved in mating.

Males use the 'gloves' as blind-folds: to cover the female's eyes during mating. They also have various morphological modifications under their heads, mandibles and legs to hold and fold down the female's antenna, head, wings and abdomen in position during mating, so that exposure to the odor glands is maximised. It is not entirely clear why the female should be blinded in such a way, and it has been suggested that the males effectively immobilise the female during mating and that the odors contribute to 'calm' her.

This drawing, taken from a figure in Wittmann and Blochtein's work below) shows the position of the mating pair. The hatched parts are modified to aid during mating.
Females, however, can and do repel advancing males. Sexually dimorphic forelegs of males appear in various bees (some leaf-cutter and carpenter bees) and many sphecid wasps, and are all used during mating to cover female eyes, suggesting a common function.

More information
Wittmann, D. and Blochtein, B. (1995). Why males of leafcutter bees hold the females' antennae with their front legs during mating Apidologie, 26 (3), 181-196 DOI: 10.1051/apido:19950302

Greenflies clouds and networks

There were raining greenflies today, whole clouds of them causing a visible effect on people, talking about them, swatting them and taking them from their clothes, glasses and skin (above). The warm weather has encouraged the winged forms of aphids (greenflies) to take to the air and search for new feeding plants. Aphids are a central part complex ecological networks. For example, a study by Muller and coworkers in an abandoned field in the south of England found:

Twenty-six species of plants were attacked by 25 species of aphid which were parasitized by 18 species of primary parasitoids. The primary parasitoids were attacked by 28 species of secondary parasitoids, of which 18 directly attacked the still living aphid (hyperparasitoids) and 10 attacked the aphid after mummification (mummy parasitoids). The aphids were attended by three species of ants.

Only the bean aphid (Aphis favae) is parasitized (or superparasitized) by no less than 12 species of small wasps which lay single eggs inside individual aphids. Some of the wasps, the secondary parasitoids, lay eggs in parasitized aphids. Then the sweet sap produced by aphids is enjoyed by ants, butterflies and bumblebees. Aphids are eaten by lacewings, ladybirds, hoverflies and small spiders; and swifts and swallows who pick them in the air plankton. You could say that the presence of aphids generates a lot of added biodiversity.

This figure taken from the study mentioned earlier summarised the parasitoid network over two years of monthly study. The middle bar represents the different aphid species (each is a number and the length of each section represents the abundance of each species. The bottom bar are the primary parasitoids and the top bar the secondary parasitoids. The conecting lines represent the strength of the interaction between a particular pair of species.
Back at the garden, on the roses newly growing shoot, covered on rose aphids (Macrosiphum rosae) this was beautifully illustrated.

A 7 spot happily munching a rose aphid 

A hoverfly larvae crawling around also eating them. This one seems to be showing off!

A weeks old garden spider has caught a winged one.

A parasitic wasp lurking about in the rose bush. I cannot say it is an aphid parasitoid for sure, but keep surveying the rose leaves, leaving and coming back to another leaf.

There are lots of fascinating things about aphids and they surely will come back to these pages in the future.

More information

Muller, C., Adriaanse, I., Belshaw, R., & Godfray, H. (1999). The structure of an aphid-parasitoid community. Journal of Animal Ecology, 68 (2), 346-370 DOI: 10.1046/j.1365-2656.1999.00288.x

Ladybirds like nectar too!

At the beginning of spring aphid populations haven't build up yet. When ladybirds emerge from hibernation it is not uncommon to see them enjoying some plant nectar. Here a Harlequin ladybird enjoys licking the nectar gland under the male flowers of a spurge.

This 7 spot is enjoying the same sweet secretions on the buds of peony that ants like.

And on a verge, tens of 7 spots congregate on the flowering heads Garlic Mustard (Alliaria petiolata). I couldn't see any aphids or other pests and the female being mated has her head right into the buds of the plant.
Greenfingers also touched the topic recently on his wonderful blog, full of great shots and information.

Ants helping plants: extrafloral nectaries

In the last couple of weeks I have posted about ants affecting plants negatively one way or another: taking nectar from flowers but failing to fertilise the plant; farming scale insects and defending them from their predators. Ants, however, also have mutualistic relationship with plants. Take cherry trees. As their green cherries are growing, their young, tender leaves are at their juiciest. Many caterpillars and other herbivorous insects would love to have a go at them. But cherries have a sweet way to call for bodyguards: they secrete nectar in red floral glands at the base of each leaf - also called extrafloral nectaries. These are irresistible to black garden ants (Lasius niger) (above) and they secrete nectar in small quantities so they encourage ants to scout around the tree to find the next leaf. Should they encounter a caterpillar trying to get to the leave, or an insect land on it, the ants act aggressively like they would do to defend aphids or scale insects, and keep the insects at bay. Nectaries are produced by a large diversity of flowering plants and experiments have demonstrated that if ants are excluded from a nectary bearing plant, the plant is rapidly damaged by herbivorous insects. In the American black cherry, nectar production in nectaries is seasonal and it is precisely timed to the period when tent caterpillars are small enough to be able to be predated by an ant. When ants are disturbed by an insect, they immediately attacked the insect and if small, they carried it back to their nest. This seasonal match between nectar production and caterpillar attack suggests that the main function of nectaries is ant attraction, as a deterrent from herbivorous caterpillars.

An ant inspecting the nectary, a glistening drop of nectar in the middle of it. Smaller nectaries are also present at the end of each serration in young leaves.

Two ant on nectaries.

More information
Tilman, D. (1978). Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation in Relation to Susceptibility of Catepillars to Ant Predation Ecology, 59 (4) DOI: 10.2307/1938771

Queen Common Wasp

Queen common wasps (Vespula vulgaris) have been around for a few weeks now. I never see them feeding, just sunbathing, flying about the apple or cherry, but not appearing interested in the flowers, or searching for a suitable nest site. Today this one found our hedgehog house (hedgehogs, still snubbing this lovely house, better hurry up before wasps finish it off). The wasp started chewing at the old wood. Wasps make their nests out of saliva mixed with chewed pulp, which becomes pretty much paper mache and this queen is now building up hers.

Ants score again

Frustrated by not getting the shot of the Bombus terrestris queen chewing the comfrey flowers the other day, I check the comfrey quite often. I am surprised - OK, not much I must say - by an ant (Lasius niger) walking on the flowers and entering them using the convenient hole made by the bumblebee queen. So, ants are also secondary nectar robbers!

Ant peeks into the hole

Getting in

and she is in sucking the lovely sweet nectar...

Flower engineers

Yesterday I was following a queen Bombus terrestris, as usual, with the faint hope one will nest in my garden when she landed on a comfrey patch. This was the first time I have seen this species on comfrey, their deep flowers are mostly visited by Bombus pascuorum, B. hypnorum and B. pratorum*, in addition to Anthophora plumipes. The heavy queen landed on top of the flowers, and I was able to approach without disturbing her. She noisily bit the top of a flower and then inserted her tongue on the resulting hole. She then moved to another flower and repeated this behaviour quite a few times before leaving the patch. I was too fascinated watching and I didn't take any photos, fortunately, forensic evidence was left behind and I could take a shot of the actual holes easily spotted in the flowers (above). This queen was engaging in 'nectar robbing', a not uncommon behaviour amongst Bombus terrestris, by which individuals can feed from flowers where nectar is not accessible via the corolla opening. Flowers that are 'robbed' might have a complex floral morphology adapted to attract specific pollinators - for example, nectar can be located in a deep spur; or flowers might have lips that need force opening for entering, or the corolla could be just too deep to reach the nectar inside. Nectar robbing - described by Darwin - is another of the aspects of the foraging repertoire of B. terrestris, which, despite its short-tongue it is an impressively resourceful species. Research has shown how queens learn by experience where exactly to perforate the flowers to reach the nectar optimally, and how they switch to use previously made holes to feed when the proportion of perforated flowers reaches a threshold (secondary nectar robbing, of which honeybees and other bumblebees also benefit).

 Surprisingly, and despite its negative name, nectar robbing is not necessarily detrimental to the plant. The studies comparing seed set on 'robbed' vs. normally visited flowers in bumblebees show that either there is no effect on seed set or an increased seed set when 'robbed', indicating that nectar robbers are indeed involved in pollination. In addition, nectar robbing can happen at the margins of the plant distribution, where the usual long-tongued pollinators might be absent and the 'robbing' could be the only chance for seed set in self-incompatible species. Although to avoid this 'robber-like pollinators' has been proposed, I myself thin that a more suitable name for hole-making bumblebees could be 'floral engineers'.

A B. pratorum feeding on comfrey. Despite its short tongue it feeds via the corolla opening, I guess it must be able to reach the nectar due to its small size, maybe able to stick its head into the corolla itself, or it might be just collecting pollen. B. pratorum has also been reported as a primary nectar robber.

Bombus pascuorum feeding on comfrey flowers through the corolla opening

A close up of a Bombus terrestris queen showing her powerful set of mandibles

More information
Olesen, J.M. (1996) From Naïveté to Experience: Bumblebee Queens (Bombus terrestris) Foraging on Corydalis cava (Fumariaceae) Journal of the Kansas Entomological Society, 69, No. 4, Supplement: Special Publication Number 2: Proceedings of the Eickwort Memorial Symposium (Oct., 1996), pp. 274 -286. http://www.jstor.org/stable/25085724

Goulson, D., Stout, J., Hawson, S., & Allen, J. (1998). Floral display size in comfrey, Symphytum officinale L. (Boraginaceae): relationships with visitation by three bumblebee species and subsequent seed set Oecologia, 113 (4), 502-508 DOI: 10.1007/s004420050402

Maloof, J., & Inouye, D. (2000). ARE NECTAR ROBBERS CHEATERS OR MUTUALISTS? Ecology, 81 (10), 2651-2661 DOI: 10.2307/177331

Why are common bumblebees common?

Six species of bumblebees in the UK buck the trend for population declines in other species. Of the 25 species of bumblebees, 3 are considered extinct, and a few others are restricted to a few sites. The common six (with the Tree Bumblebee soon set to become a 7th), however, are widespread and often very abundant in gardens. Why is that given how similar bumblebees are? All species have similar life cycles, feed on nectar and pollen and most lack specific habitat requirements. Their morphology is also remarkably similar, with the most important differences being tongue length (associated to head shape) and body size.

Species	Tongue length (mm)	Season
B. pratorum	7.3	Early
B. lucorum	7.5	Early
B. terrestris	7.6	Early
B. lapidarius	7.7	Middle
B. pascuorum	8.5	Middle
B. hortorum	12.5	Middle

 Do these differences reflect different foraging preferences? Foraging preferences might be very apparent to gardeners, with some species having a strong preference for a particular plant. For example only Bombus hortorum (above, on Phlomis) and Bombus pascuorum, the two species with the longest tongue, visit foxgloves. Surprisingly, bumblebee foraging preferences were not known until recently, when Goulson and colleagues studied the foraging preferences of most UK bumblebee species on a number of sites on South England, Scotland and New Zealand (where several species were introduced at the end of the 19th century).
The first interesting result they found is that pollen and nectar was collected by bumblebees from a different array of plants. Fabaceae (bean family) was preferred both for pollen and nectar collection whereas asteraceae was very used for nectar collecting but rarely for pollen. This could indicate differences in pollen quality amongst plant families.

When investigating the differences between common and rare species, pollen use was most important. A group of species are Fabaceae pollen collecting specialists, and others are heath specialists. Common species appear in the middle of the plot, indicating less specialisation and broader diets.
Goulson and collaborators suggest that the abundance of B. hortorum, a relatively specialised, long-tongued species, could be due to there being ecological space for only a single species specialised on long-corolla species given the declining numbers of long-corolla species in the countryside.
Emergence time was also correlated with rarity. The common six are all either early species or mid season species. Late emerging species seem to have suffered disproportionately. These late species are medium or long tongued and they appear late as they feed on unimproved grasslands (hay meadows for example), and this is a declining habitat in the last decades. In the paper they discuss that the loss of these habitats increased when vehicles replaced horses as means of transport and the decline of bumblebee species might be an unexpected legacy of the mechanisation of the countryside.

More information

Goulson, D., Hanley, M., Darvill, B., Ellis, J., & Knight, M. (2005). Causes of rarity in bumblebees Biological Conservation, 122 (1), 1-8 DOI: 10.1016/j.biocon.2004.06.017

Holly Blues

The last week of April or the first week of May sees the emergence of the first adult generation of Holly Blues (Celastrina argiolus) from their hibernating chrysalis in our area. Almost invariably, they are fluttering around Holly trees. The Holly is the larval foodplant for the first brood and it is relatively easy to spot females laying their eggs on the tender flower buds of Holly in the spring (above). Males also flutter around holly and sunbathe on it, and adults feed also on the tiny holly flowers. The second brood feeds on Ivy. I have found very little published information on the behaviour of this butterfly despite being a common butterfly in gardens.

This male was feeding on Viburnum tinus this morning

The adults feed on a range of nectar producing plants, I have compiled this list from several sources:
Aubrieta, Choisya, Buddleia, Hebe, Hyssop, Red Valerian, Ragwort, Garlic Mustard, Viburnum tinus, Bramble (Rubus fruticosus agg.), Bugle (Ajuga reptans), Buttercups (Ranunculus spp.), Forget-me-nots (Myosotis spp.), Holly (Ilex spp.), Honeydew / Sap, Ivy (Hedera helix), Privet (Ligustrum vulgare), Thistles (Cirsium spp. and Carduus spp.) and Water Mint (Mentha aquatica), Hemp-agrimony (Eupatorium cannabinum), Rosebay Willowherb (Chamerion angustifolium).

This is the weekly distribution of my records, clearly showing the presence of two flying generations: