Monday, August 9, 2010

Decline of the Cinnabar Moth Suggests Larger Threat

Larva of the Cinnabar moth, Tyria jacobaeae feeding on ragwort

August, and the larvae of the Cinnabar moth (Tyria jacobaeae) are to be seen feeding manfully on ragwort plants (Senecio jacobaeae) throughout the country. The adult moths, with their wonderful charcoal and red wings, prefer the duck and night time where males are often attracted to outdoor lights. The black and yellow stripes of the larvae make a surprisingly good camouflage against the yellow of the ragwort flowers, however where once plants seemed to be infested with them, numbers of T. jacobaeae have decreased steadily in the past 30 years to a point where now only one or two individuals may be spotted on a plant, if at all.

While rarely killing the plant, T. jacobaeae is an important grazer of ragwort, often entirely defoliating individual plants and reducing seed production by 65 to 98% (Cameron, 1935 Journal of Ecology 23 pp. 266-322; Bornemissza, 1966 Australian Journal of Ecology 14, pp. 201-243). this is important as ragwort is classed as a noxious weed in both Ireland and the UK. This is due to the production of toxic alkaloids by the plant which cause cirrhosis in livestock if consumed (Dempster, 1982 Advances in Ecological Research 12 pp. 1-36). The alkaloids do make ragwort unpalatable if consumed fresh, so is avoided by grazers but toxicity is not lost upon drying so plant material ensiled may cause death in livestock.

Larva of the Cinnabar moth, Tyria jacobaeae feeding on ragwort

This toxicity does not effect T. jacobaeae: indeed it uses it to its advantage, as it stores alkaloids from the plants making it distasteful to most predators (Aplin et al., 1968 Nature 219 pp. 747-748). It announces this toxicity in its bold black and yellow stripes, which also warn of high levels of histamines which the moth produces itself (Bisset et al., 1962 Proceedings of the Royal Society 152 pp. 255-262). These are far quicker acting than the alkaloids and would deter a predator before it has killed the moth.

The moth is now under threat. While not in the same category as the endangered Marsh Fritillary Butterfly (Euphydryas aurinia), T. jacobaeae has been placed on the UK Biodiversity Action Plan which recognises a sharp decline in numbers (Whitehouse, 2007 Managing Coastal Soft Cliffs for Invertebrates pp. 12, 17). This reflects a general decline in moth numbers as a whole in Ireland and the UK. Conrad et al. (2006, Biological Conservation 132 pp. 279-291) showed in a 35 year study of 337 most common moth species that two thirds of them had declined, with 21% declining by 30% or more. This is a worry as the decline will surely have a knock on effect on the populations of insectivorous birds and bats. Moreover it suggests that as-yet undetected declines may be widespread among temperate-zone insects.

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