Tuesday, 11 August 2015

Groundsel and Ragworts

The genus Senecio is a huge one with something like 1,300 species. It was once even larger but over the years species have been shunted off into other genera. It is represented in Britain by about fifteen species plus a number of casuals. (Casuals are plants which occur now and again around ports, or as crop impurities or where birdseed has been scattered; they do not persist.)

Some of our most familiar weeds are Senecio members, including groundsel and several ragworts. In my 'Kingsthorpe Meadows' blog of 23 June I briefly mentioned Common Ragwort, Senecio jacobaea, and I was reminded of this recently when examining some specimens in a friend's garden. A considerable number of plants had found a congenial place in her borders and I had a good look at them.

Cinnabar moth in a garden at Doll's Hill, Byfield, Northants.
24 June, 2015
I wasn't the only one interested. A couple of Cinnabar Moths. Tyria jacobaeae, were also checking them out. One of them paused for long enough to be photographed. They lay their eggs on various Senecio species and by looking beneath the leaves I soon found some on a plant of groundsel, Senecio vulgaris. Their striking black and red coloration makes them instantly recognisable. 'Aha, aposematic coloration,' I hear you cry. And of course you would be right.

Cinnabar eggs on groundsel. Doll's Hill, Byfield
Northants.  24 June, 2015

Cinnabar moth caterpillars on waste ground, Daventry.
11 August, 2015

The caterpillars are, of course, a very familiar sight with their bright yellow and black bands. It is not a coincidence that they display the same warning colours as wasps another case of aposematicism.
Packed with noxious chemicals, these caterpillars are certainly not to be eaten 

Once the caterpillars have emerged they grow rapidly. Senecio species contain a poisonous alkaloid, jacobine, and the caterpillars will retain the poison in their tissues where it will remain even when they have metamorphosed into adult moths (imagos). So, larva or imago, the colours give a clear warning.

These brilliant red areas on the wings must surely have inspired its name of cinnabar which is, you may recall from school science lessons, a bright red ore of mercury. Like the moth, cinnabar is highly poisonous, a fact that did not prevent it being used for many centuries as a pigment, when it was known as vermilion. In ancient Rome the soldiers in parades which followed military victories would powder their faces with vermilion even though its toxicity was fully recognised. (For slaves sent to work in Spanish cinnabar mines it was a certain sentence of death.)

However, I digress. 

Common ragwort mined by...what? Doll's Hill, Byfield
Northants. 24 June, 2015

The mine on this Common Ragwort leaf shows that Cinnabar Moths are not alone in exploiting Senecio species as a food plant. I'm not sure which organism has created this mine; my money would be on a tephritid fly, Acidia cognata, but I simply don't know.

Aphids cluster around the stem-base of Common Ragwort.
Doll's Hill, Byfield, Northants.  24 June, 2015
Colonies of aphids were also clustered around the base of the stem. I didn't bring home a specimen but I strongly suspect it is Brachycaudus cardui. The primary host for this aphid is, among other trees, cherry, where their activities cause curling of the leaves. Plants of the Asteraceae Family, which includes Senecio, are the secondary host. There is no doubt that this highly poisonous plant is important to many invertebrates.

Oxford Ragwort growing in Byfield Pocket
Park. 15 July, 2015

The story of Oxford Ragwort, Senecio squalidus, is well-known - of how plants were brought from ash-strewn volcanic slopes of Italy and brought to Oxford, of how seeds were blown from that city's botanic gardens on to nearby railway tracks and of how the plant, finding these conditions amenable, began to spread along railway networks to become a familiar sight across much of Britain. Here it is in the pocket park at Byfield, growing in gravelly material at the side of a path.

Oxford Ragwort, cheerful despite its
 specific epithet of 'squalidus'.

Despite the specific name of 'squalidus' the plant is not unattractive and its bright yellow flowers brighten up many a neglected corner in towns and cities.

Sticky Groundsel in Byfield. 22 July, 2015
Rather like a small-scale version of the Oxford Ragwort is Sticky Groundsel, Senecio viscosus. It is unpleasant to handle, the whole plant being covered in sticky glands. Again like Oxford Ragwort it is an introduction and was first recorded in Northamptonshire in 1822. The latest flora for our county, by Gent & Wilson, describes it as 'occasional' and it seems only to be spreading slowly. Nevertheless there is a flourishing colony in Byfield, just off Edwards Close.

In general Groundsel and Ragworts are opportunists, occupying a patch of waste ground until they are shaded out by more robust and long-lasting plants. This was clearly the case when I visited Daventry earlier today (11 August). A patch of land earmarked for development has been 'invaded' by huge numbers of Common Ragwort to become a magnet for enormous numbers of insects.

I said in the opening paragraph that Senecio is a very large genus and it follows that I could ramble on about its members at length. I must rein myself in and sign off. And in any case I have a mysterious bug sitting under my microscope, demanding attention.

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