Two summer bees

The Summer season is quickly advancing this year. This week, Stachys sylvatica, the hedge woundwort, came into blossom and it has quickly being followed by the first Anthophora furcata males, which I saw yesterday visiting the sage, the foxgloves and Lamium maculatum. The previous two years I spotted the first males on the 7th and 10th of June, so the 21st of May is very early indeed.

Anthophora furcata male visiting sage

 The second summer bee is the leaf-cutter bee, Megachile. I think I have already seen the two species that visit my garden this year. The first one, a female Megachile centuncularis (tentative ID) cold on a Cat's Ears, Hypochoeris radicata, on the 16th of May.

Megachile centuncularis on Cat's Ears

The second, a just emerged male M. willughbiella, peeking out from its cell in the bee hotel, its white-golden gloves visible (top photo) on the 18th (top photo). A few moments later, a male was sunnying itself, stretching its abdomen, and grooming itself on a branch nearby and flying away into the wide world.

A fresh Megachile willughbiella male stretching its jaws and showing its white gloves

This small bee hotel -placed in a sheltered, sunny fence in the garden - has been very popular with bees this season, and many of last year's cells have produced bees. You can see the muddy remaining of the walls of last years nests around some of the openings in the photo above -, and several others have been completed this year by red mason bees, Osmia rufa, which are still active.

A bug eating plant in the garden

Ever since I've had teasels (Dipsacus fullonum) in the garden, I have wondered why each joined-up pair of leaves catches and holds rainwater like cups. This fact has been remarked by numerous botanists for a long time. As early as 1875, Francis Darwin, son of Charles, observed how these rain-filled cups trapped many invertebrates, including slugs, who drowned and whose bodies decomposed in the water, and described the plants adaptation to catch the insects:

it is quite certain that the plant is well adapted for catching and drowning insects.
The connate leaves form cups holding from 12 to 100 c.c. of fluid ; the leaves are smooth (although those of the seedlings are rough, with large prickly hairs) and are inclined so as to form a large angle with the horizon and a small one with the vertical; they form, therefore, two steep and slippery slides, leading to a pool of water. The stalk of the plant is covered with sharp prickles, but these cease where the stalk dips into the water in the cup. If it were not for the loss of the prickles at this point, a ladder of escape would be provided for the drowning victims. I have seen a beetle struggling to get out, and observed his tarsi slipping over and over again on the smooth stalk. The cups undoubtedly form a most efficient trap. In
some wild teasels the following insects were found:—In one cup six large malacoderm beetles, from half to three quarters of an inch in length, one fair-sized caterpillar, and two flies; in another, seven of the same beetles, one earwig, a bluebottle fly, besides many smaller flies and much debris.

Francis Darwin cited his great-grandfather Erasmus, who, the previous century, hypothesized that the teasel cups actually protected the nectar from insects, being unaware of the relationships between flowers and insects. Frances, who thought the teasel was a carnivorous plant, went on to investigate how teasels could absorb the nutrients afforded by the insects and made numerous, if somewhat puzzling, experiments on some glandular cells and filaments produced by the leaves. He also explained how beetles died faster in teasel water than in pure water, as if the teasel produced some chemical to "narcotize" them. Francis Darwin believed that the teasel benefited from these catched insects, and even designed the experiment to prove it:

I hope to decide by a comparative experiment, in which a number of teasels raised from seed under similar conditions will be divided into two lots, one half being starved and the other fed with insects or pieces of meat.

Unfortunately, apparently, he never carried out or published this experiment and therefore, could not provide the key test to show that the teasel was a carnivorous plant. These key experiments had to wait over a century. A few weeks ago Peter Shaw and Kyle Shackleton published a paper finally showing that the teasel is actually a carnivorous plant. They carried out field experiments treating teasels in one of three ways: (1) removing all insects in the cups one a week; (2) supplementing the cups with a dead maggot larvae a week (fed treatment) or leaving the teasels as they were (control). Their results show that although the treatments had no effect of the size attained by the plant, supplementing the plants did increase the weight of seeds set by 30%.

I thought I'd post on this topic once I came across some teasels and could get some drowned insect shots. This week I kept an eye on a teasel growing in a neighbours' garden and this is the result. A spider (above) and a dipteran (below) falling prey to this intriguing plant. The shots are from the same set of leaves.

References

Shaw PJ, & Shackleton K (2011). Carnivory in the teasel Dipsacus fullonum--the effect of experimental feeding on growth and seed set. PloS one, 6 (3) PMID: 21445274
Darwin, F. (1877). On the Protrusion of Protoplasmic Filaments from the Glandular Hairs of the Common Teasel (Dipsacus sylvestris). Proceedings of the Royal Society of London, 26 (179-184), 4-8 DOI: 10.1098/rspl.1877.0003

Tiny snails on cockle shells

My daughter found some tiny snails under a cockle shell in a pot yesterday. I took some photos but I could not identify them so today I went to find them again and try and take some shots of the more informative mouth of the shell. There were five or six of them, that were now active after last night's rain.

I looked in the pot and I found dozens of them. I had never seen them before and we had a whole pot population of these 2 mm or so snails happily living in the garden! Fauna, from WAB, identified my photos of the shell mouth today as Lauria cylindracea, a very common and widespread snail in the U.K. A favourite habitat is on stone walls covered on Ivy, but also lives in moss, leaf litter and crevices of old trees, habitats not particularly wet. It feeds on fungus growing on the dead leaves.
 As many snails, individuals of L. cyclindracea are hermaphrodites, which mate with each other to fertilize their eggs. This species, however, is relatively unusual for a snail in that it is ovoviviparous. It retains its eggs - 4 to 6 -inside its body until they hatch, and they are born into miniature snails, even tinier than their parents. Some of these juveniles were also present under the shell near the adults. The snails reach maturity at two years old and can live over 5 years, reaching a maximum size of a mighty 3.5 mm.

 In their study of life history and reproduction of L. cylindracea in Israel, Heller and collaborators discussed what evolutionary forces might have led to the evolution of ovoviviparity in this species. They hypothesize that a combination of small size - which limits the total number and the size of the eggs that can be produced - and high egg mortality, for example due to drought or lack of appropriate egg laying sites or inability to bury the eggs, are the key selective pressures. That way, offspring are born right next to their parents at the conditions most suitable for survival. The snails can also determine up to some point when the birth happen, retaining the juveniles longer when the conditions are very dry, and quickly giving birth when favourable moisture conditions return, that way maximising the chances of their offsprings survival. Heller and collaborators state:

Active hatchlings are advantageous, as compared to eggs, in that they can immediately start to feed, grow, fight off fungi, cope with brief periods of desiccation by moving into deeper layers of litter or temporarily retreating into their shell, and avoid drowning (or being washed away) as a result of occasional flooding of the riparian litter habitat, by moving into higher layers.

They compared this reproductive strategy in minute snails with the situation in larger species:

Large snails can produce eggs in vast numbers. Further, lack of size constraints enables the large adult to lay large eggs, generously coated with thick layers of albumin and mucus to tide the developing embryos over varying periods of drought and hunger. In Helix texfa, a species of 40 mm and 4.3 g (found in Israel very close to our present study site), each single egg averages 5.5 mm in size and 70 mg in wet weight (bigger and heavier than an entire adult Lauria), an adult produces 60 eggs per clutch, and clutch weight represents about 10% of body weight (Heller & Ittiel, 1990). In large snails with large clutches also sib cannibalism may occur, thereby increasing survival chances of the first individual to hatch (Baur, 1994). Further, large oviparous snails frequently deposit their eggs into pockets in moist soil. A deep cavity lined with mucus can protect the eggs against bacteria and fungi (Baur, 1994); it also enables the deposition of eggs in soil layers that remain damp for longer periods of time, and thereby reduces risks of desiccation. The cavity is dug out by the snail’s foot, so its depth is broadly correlated to foot size. Helix can dig 5cm into moist soil and Arclzachatina can dig a 15 cm cavity in one night (Tompa, 1984; Baur, 1994). Minute snails, however, do not have the potential to dig much beyond 0.2cm (except for subterranean genera, see Heller, Pimstein & Vaginsky, 1991). They can either dig minute cavities, or deposit their eggs in damp litter, where they would be exposed to fungi. Ovoviviparity may thus be a mode of parental care for snails that can neither lay many eggs, nor big eggs, nor dig deep egg cavities.

I will keep looking for these snails to see how widespread they are in the garden. I wouldn't be surprised to have overlooked these fascinating creatures due to their minute size. Were not for the white background of the shell where we found them we would have probably missed them altogether.

Reference
Heller, J., Sivan, N. & Hodgson, A. (1997). Reproductive biology and population dynamics of an ovoviviparous land snail (Pupillidae). Journal of Zoology, 243 (2), 263-280 DOI: 10.1111/j.1469-7998.1997.tb02781.x

Tree bees. 1. Holly.

These days I am looking up trying to find bees in trees. Insect pollinated trees can attract large numbers of bees, as they offer a highly concentrated resource: hundreds or thousands of flowers are present in the same spot. The drawback for the bee watcher is that, unless you carry binoculars, identification is not easy. There are two holly trees in my garden and I had noticed patrolling Red mining bees and feeding Bombus hypnorum in them before, but I could not take good shots. Today I found a holly hedge in bloom and I could get close to the bees that pollinate this tree. Holly flowers are small and inconspicuous and I have rarely noticed bees visiting them, but this little bush had many bees in it. Hollies are dioecious, which means that there are male and female trees, self-pollination is not possible and fertilisation requires insects visiting first a male tree and then a female tree. I am not sure if male holly flowers produce nectar as well as pollen. On the photo above a male Red Mining bee sits on the flowers of a male holly, its antennae covered on pollen.
In their Holly monograph, G. F. Peterken and P. S. Lloyd stated:

Entomophilous. Apis mellifera L. is the commonest insect visitor, but the following bees have been observed in southern England (O. W. Richards): Andrena wilkella Kby. (Andrenidae), Osmia rufa L. (Megachilidae) and Bombus lucorum L. (Apidae). B. lucorum and syrphid flies have been seen at the flowers in northern England. Nectar is secreted from tissue at the base of the ovary.

A honeybee visiting the flowers.

An Red Mining bee at the left of the flowers with scopa full of yellow holly pollen

Reference:

Peterken, G., & Lloyd, P. (1967). Ilex Aquifolium L. The Journal of Ecology, 55 (3) DOI: 10.2307/2258429

Vine weevils!

I do not like all bugs. I like my pot plants and that's why one of the villains in the garden is the Black Vine Weevil, Otiorhynchus sulcatus (adult above). The larvae of this weevil feed on the roots of many types of plants. In pots, especially indoors, where there are fewer parasites and predators and root growth is restricted they thrive and cause havoc. They overwinter in the soil as larvae and start feeding underground as temperatures increase. By the time they pupate most plants host to this species will be dead. If they are succulents, their root system is gone and the stems fall out of the pot one by one. I have a pot of strawberries from last year. At the end of the winter they had a few little offshoots which I planned to pot out. But they all died and shrivelled and there was only some ghost strawberry plants clinging to life left. I took the plants out today and came across the culprits: several larvae and dozens of pupae of Vine Weevil.

Three pupae and a fully developed larva of Black Vine Weevil

The adults emerge from the soil around June, they climb clumsily out and wander in search of food. They feed on leaves at night, leaving tell-tale C marks on the edge of leaves. They appear slow and harmless, and are unable to fly, but they can climb very well and feign being dead and drop to the ground if disturbed, so they tend to be left alone. Adults are all females which reproduce by parthenogenesis giving origin to genetically identical daughters. They can start reproducing by themselves as soon as they are ready, no need to waste time looking for a male and mating, and when they are, they go for it in earnest. At 21 oC, their optimal temperature, they can lay over 1000 eggs over their lifetime of four or five months. This mode of reproduction coupled with its preferred niche in the ornamental and agricultural trade has allowed this species to thrive and expand across the world from their native Europe, so, although I do not like it, I do marvel at its successful life history. What next? We should be getting some parasitic nematodes and watering the pots with them in the hope that they will infect the pupae and stop the invasion.