Herbivore Defenses of Senecio viscosus L.

Senecio viscosus Sticky groundsel, Senecio viscosus, is rare in the Ottawa area. According to Gray's Manual of Botany, a major environmental niche for it on this continent is railway ballast, and that's where I almost always find it here. Its fine roots wind over stone surfaces, apparently to scavenge moisture that condenses between the stones most nights. Its leaves are succulent to store water. A specimen that I collected had no visible soil at all on its roots. Yet, after planting in my garden, it continued to develop buds, bloom, and set seed as if nothing had ever disturbed it.

The compound that makes its leaves sticky (and scented; people who don't like plants call it stinking groundsel!) is a pesticide - it helps to defend the plant against herbivores. Grazing animals are pests to plants, after all, and the leaves of most Senecio species contain chemicals which can kill most insects and grazing animals in a single meal. However, caterpillars of the genus Tyria actually seek out Senecio. They store the chemicals in their bodies, presumably to make them in turn unpalatable to their own predators. The hairs of sticky groundsel, however, exude an additional compound, which specifically deters Tyria caterpillars. The strategy seems to be successful here, for there was no visible insect damage to any of the plants I found.

Sticky groundsel seems to require something very specific (and obscure) to produce its defensive chemicals. My specimen produced 43 blooms, with an average of 52 seeds per seed head. Each seed has a gossamer-fine cone of 6mm hairs that catches the slightest breeze. There is nothing I can discern about the roadbed where it grew that differs from other parts of the adjacent 10km of groundsel-free roadbed that I have walked. The same puzzlement was expressed by Cole, who reported the first known occurrence of this plant in the Ottawa District (Trail & Landscape 4:16-17;1970).

Senecio viscosus is a fascinating little plant, that raises all sorts of natural questions. Are pesticides such as it produces really as much less common in land plants than in coral reefs as literature indicates? Or, perhaps, have people had blinkers on with regard to such questions? Nature may be green in 'tooth and claw' too, not just red!

The stages of bloom: bud, fertile flower daytime, fertile flower nightime, fertilized flower, mature flower, seed dispersal day 2, seed dispersal day 4, airborne seed, remaining bracts morning, remaining bracts evening

Reference notes: (BA references are Biological Abstracts)


John Sankey
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