Dinner of the green bug muncher

After the very rainy April we are having a teasel growing in the garden is looking very luscious. It has grown a large rosette of leaves, and developed two large leaf-cups which are filled with water to the rim. If you want an account on this fascinating carnivorous plant, check here. I had a peek a couple of days ago to check if any drowned invertebrates were visible. And there were indeed! On the bottom cup (above) a 7 spot floated on a very cloudy liquid, while on the top one, a drowned slug was visible at the bottom of the clearer water, and a few small unidentified insects. What a catch!

Wasp and teasel water cup

 Wasps make their nests of paper which requires mixing chewed wood with saliva, so they need to drink a lot. These wasps, however, were drinking from a risky source of water. The water-filled cups of teasels, where many insects and other bugs drown, and whose juices increase the teasel seed set. Although the unfortunate prey of the teasel might have just happen to fall in the cups, the lure of available water on a sunny day could also be a reason insects are lured to the cups, and many bees and wasps must fall prey to this carnivorous plant (check the wonderful blog Cabinet of Curiosities for an example). These wasps were lucky, they held on, quenched their thirst and flew away.

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

A business of hoverflies

A business and a swarm, are the collective terms for flies, in this case, hoverflies. In the last few days, hoverflies, particularly the Marmalade fly, Episyrphus balteatus (above) have descended covering every flower, bush and branch with a stripy fuzziness. The group of Marmalade hoverflies on the flowering teasel above greeted me first thing in the morning. As I have said before, many hoverflies are migrants: numbers are swelled in the summer by migrants from southern Europe. I have taken shots of several species today, including Scaeva pyrastri, which I had only seen once in 2006 before, and now seems to be everywhere.

Syritta pipiens on fennel

Eristalis sp.

Syrphus sp.

Scaeva pyrastri on Knapweed

Myathropa florea on fennel

More information
Stubbs A.E. & Falk, S.J. (2002) British Hoverflies. BENHS.
Hoverfly Recording Scheme.

Bombus hypnorum here to stay

Coming out of work I see the first Bombus hypnorum of the year: a dying, squished female, possibly a queen. B. hypnorum, or the tree bumblebee is a newcomer in the UK. It appears to have arrived on its own steam, possibly an inseminated queen crossed the channel around the turn of the century. The first specimen was collected in the New Forest in July 2001. In mainland Europe it is a species closely associated to human habitats. It is a species quite easy to identify due to its striking coloration: ginger thorax and black abdomen with a white tail. It is a good size bumblebee, similar to B. terrestris.
 In July 2005 I reported the first B. hypnorum in Hull and I have recorded it every year since. In the UK B. hypnorum has expanded steadily through most of England - click here for the 2009 records - and it appears that its status as a British bumblebee is consolidated.
 It nests in trees, often in nest-boxes and also in roofs and while foraging, it has a preference for high flowers. The bumblebee is found from March until September. These are the flowers I have observed it foraging:
March: Mahonia
April: Cherry
May: Rowan, Comfrey, Cotoneaster horizontalis
June: Bramble, Deutzia, Cotoneaster, Comfrey, Angelica, Hebe, Snowberry bush
July: Teasel, Cotoneaster, Lavender, Hebe
August: Snowberry bush, Buddleia
September: Ceanothus

The first recorded B. hypnorum in Hull

The entrance to a B. hypnorum nest in a nest box. Flies appear to be always at the entrance.

BWARS gallery page for B. hypnorum here. You can also send your records to BWARS.

First B. hypnorum record by Dave Goulson and Paul Williams here.