Monday, 9 September 2013

Ragwort and its visitors

I have had few opportunities to examine any plants of Common Ragwort (Senecio jacobaea) recently. They seem to have been a little less frequent this year and, if so, farmers will be pleased. As I may have written elsewhere, the plant contains powerful alkaloids, notably jacobine, and is poisonous to stock, although an animal would have to consume a huge quantity for a fatality to occur.

Ragwort, Senecio jacobaea, Solden Hill, Byfield
It is quite handsome in a rather untidy way and its nectar and pollen attract large numbers of insects. You could say that the plant is a typical Senecio but in truth it is a huge genus whose members take on many forms from cactus-like succulents to large shrubs. In reality there is no typical Senecio.

One insect with an unbreakable association with these plants is the Cinnabar Moth (Tyria jacobaeae) This is a familiar day-flying moth, unmistakable with its bold black and red wings. These wings are a warning to would-be predators. and any creature which attacks the butterfly will not quickly do so again. The same is true of the caterpillar. These feed on the Ragwort and store the poisonous alkaloids in its body. These remain stored throughout the metamorphosis and are therefore still present in the imago. Again the colours - this time yellow and black stripes - serve as a warning.

The larva of the Cinnabar Moth. Solden Hill.
When I was examining plants on Solden Hill I was struck by the number of insects being attracted to the flowers. Notable among these was the tachinid fly, Eriothrix rufomaculata. To the non-specialist the tachinids can seen rather nondescript flies but some are very distinctive and this Eriothrix species is one of the first that the newcomer learns to recognise. The specific name 'rufomaculata' means 'red-spotted' and the fly has clear reddish patches on the sides of the abdomen.

Eriothris rufomaculata on Ragwort. Solden Hill.

Tachinid flies are all parasitoids. These use various techniques by which their larvae gain access to a host - generally another insect - and then begin to feed on the unfortunate victim, usually resulting in its death. 

In the case of Eriothrix rufomaculata the host is not known with certainty, a remarkable situation for such a common insect.

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