Insects and Thistles

Thistles are common plants found throughout the British Isles, often invading crops and pastures as weeds. Most of the information here concerns the two commonest species, Cirsium arvense creeping thistle and C. vulgare spear thistle. Despite their spines, thistles support a rich and varied fauna of insect herbivores with their associated predators, parasitoids and inquilines. Thistles have spread rapidly in countries to which they have been introduced without their insect fauna, often reaching pest status, suggesting that the fauna plays an important part in limiting their numbers (see chapter 5).Most species dependent on thistles are herbivores. Most are restricted to particular microhabitats on the thistle and there are distinct guilds of insects associated with different parts of the plant. The flower heads, the insides of stems, roots and leaves carry characteristic concealed faunas of gall-inducers, borers and miners, apart from the more conspicuous insects which visit flowers, browse leaves or suck sap. Within each guild there are differences in timing of life cycle, methods of feeding and causes of mortality which may minimise competition. Some species are monophagous and others are highly polyphagous, but most show varying degrees of specialisation between the two extremes, being found on a number of species of Asteraceae (=Compositae) (Zwolfer 1965*).

Predators are less restricted, feeding on a range of insect species and selecting microhabitats which may occur on many plant species. Insect parasites (parasitoids) are more conservative; each develops on a single host insect, and most species are restricted to a few related host species, particularly those with similar habits.

The plants themselves provide a succession of microenvironments. Communities associated with the flower heads develop during summer, detrital feeders colonise the brown, dead heads during late autumn and winter, and more enter when the heads and stems have fallen.Thistles contain a number of independent communities, connected only by their common dependence on the plant and by the relatively few roving predators which wander all over the plant. These miniature communities offer opportunities for studying a variety of ecological and entomological topics and the fauna of parts of thistle plants (or closely related species such as knapweeds) has been used to study food webs and life cycle interactions (Redfem 1968; Michaelis 1984), energetics (Cameron and Redfem 1974; Michaelis 1984, 1985b), population dynamics (Varley 1947; Michaelis 1985a; Redfem and others 1992), competition and parasite-host interactions (Zwolfer 1970, 1979,1987). Zwolfer (1988) reviews current literature on insect-thistle interactions and community ecology. "

Author: Margaret Redfern
Publisher: Richmond Publishing
Extra Details: Pictorial Card Covers 6 x 9 inches tall.70 Pages.Illustrated with Colour plates and B&W line drawings throughout.NEW BOOK.