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A Study of the Natural Control of Ragwort (Senecio Jacobaea L.)

Ewen Cameron
Journal of Ecology
Vol. 23, No. 2 (Aug., 1935), pp. 265-322
DOI: 10.2307/2256123
Stable URL: http://www.jstor.org/stable/2256123
Page Count: 60
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A Study of the Natural Control of Ragwort (Senecio Jacobaea L.)
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Abstract

1. Ragwort ( Senecio jacobaea L.) was introduced into New Zealand from Europe prior to 1874, since when it has become a very serious pest in the Dominion. Its wide and rapid spread was attributed to the absence of the European insect fauna, which exercised a controlling effect on the weed in its native lands. Accordingly, the present investigation was undertaken in order to discover the factors, insect or otherwise, which kept ragwort under control in Britain, with a view to their ultimate utilisation in suppressing the pest in New Zealand. 2. The paper opens with an account of the initiation of the New Zealand Noxious Weed Control Scheme, and a summary of previous attempts to subjugate weeds by the biological method. 3. The main principles underlying the biological control of weeds are summarised and the procedure to be followed in work of this kind indicated. One of the dangers in weed control by insects is the possibility that the introduced species may migrate from the weed to plants of economic value. With proper safeguards this risk is greatly minimised. Several experiments dealing with this problem are quoted, and the subject discussed. 4. Ragwort is described in its various stages, while details of the life history of the weed, the history of its introduction into New Zealand, its botanical status, relation to plants of economic value, its poisonous properties, together with the diseases which it causes in stock are fully related. 5. A fairly complete ecological study of ragwort, showing the effect of climatic, edaphic, and biotic factors on the establishment and distribution of the weed, reveals the biotic factor to be of the greatest importance in its control. The effect of plant competition on the establishment of the seedling is extremely important. Long grass and short continuous turf prevents the establishment of ragwort altogether, while overgrazed pastures, owing to the partial exposure of the soil surface, carry a high infestation of the weed. Man, Insects, Sheep, and Rabbits are the chief zoological factors influencing ragwort in this country. The three former are controlling agents, while the action of rabbits in breaking the vegetation cover and exposing the soil, is, in general, distinctly favourable to the increase of the plant. Owing to the absence of the European insect enemies of ragwort in New Zealand, they are of particular interest from an economic standpoint. 6. Over sixty insects, from five different orders, are recorded from ragwort, and a list of these, with notes on their life histories and alternative hosts, is made. 7. Two insects, the Arctiid moth, Tyria jacobaeae L. (cinnabar), and the Anthomyiid seed-fly, Pegohylemyia seneciella Meade, because of their extensive damage to the plant and their specific habits, are selected for special study. A number of other insects which cause a small amount of damage, also receive some attention. 8. Details of the life history of Tyria are worked out. 9. An account of the parasites reared from the larvae and pupae of Tyria, and their percentage infestation is given. This is the first rearing record of Ichneumon perscrutator Wsm., a parasite of the pupa. 10. Several fungi, which cause a mortality of 16-20 per cent. in Tyria pupae, were cultured and identified. 11. The mortality in the pupae of the cinnabar, due to the action of predators is found to be very high--about 60 per cent. Evidence points to the mole as the chief culprit. 12. An account is given of the life history, synonymy, and parasites of Pegohylemyia seneciella Meade. The mouth-parts and spiracles of the larva are figured. 13. The following species of insects cause a small amount of damage to ragwort: Agromyza aeneiventris Fln. (dipterous stem-borer), Homeosoma nimbella Dup. (lepidopterous stem-borer), Phytomyza atricornis Mg., and Spilographa zoe Loew. (leaf-miners), Sphenella marginata Fln. (flower-head fly), and Aphis jacobaeae Schr. A short account of the damage caused by these and the parasites reared from some of them is given. 14. A census of the ragwort and Tyria populations is made in several areas, and the methods employed described. Notes are made on the very extensive damage caused to the weed by the larvae of Tyria. 15. It is found that ragwort reacts to insect attack by producing a secondary crop of flowers and seeds. This reaction reduces the effectiveness of controlling insects. The whole of this aspect of the problem, showing how weed control by insects is more difficult of attainment than insect control by means of their entomophagous enemies is discussed. 16. Extensive experiments on the effect of cutting the whole plant or different parts of the plant, to simulate insect attack, were carried out. The following conclusions confirmed by observations on actual insect attack were reached: (a) A badly attacked plant, if originally vigorous and if the growing season is good, has enough reserve energy to produce a second crop of seeds equal to 34.7 per cent. of the original potential yield producible in the absence of attack, thus reducing a total loss of 100 per cent. to one of 65.3 per cent. (b) Plants with little or no reserve energy, growing on very poor soil, when badly attacked, do not produce seeds secondarily. (c) Many plants which are unable to produce a second crop of flowers and seeds in the year of attack, may send out small shoots from the base of the stem. These shoots grow vigorously in the following year and are often capable of producing more seed than the original plant. Field observations show that this method of secondary growth has the effect of prolonging the life of the plant beyond the ordinary biennial period. (d) Damage to any part of the plant (leaves, stem and flowers) will lower the seed yield for that season. 17. It is shown that Tyria, under certain conditions and in certain areas, has a controlling effect on ragwort in Britain, but in a wide, sandy area, like the Breckland of Norfolk, on account of its distribution not being uniformly general, it is less effective. 18. Pegohylemyia seneciella, in the south of England, infests 8-9 per cent. of the capitula of ragwort, and each larva destroys 75 per cent. of the seeds in the capitulum which it occupies. In the North of Scotland it is much more effective, 33-34 per cent. of the capitula being attacked. 19. The areas in England where some degree of control has been effected, are described for the purpose of comparison with conditions prevailing in the ragwort areas of New Zealand. The importance of eliminating factors, such as rabbit activity and overgrazing by farm animals, which predispose to open soil conditions, is strongly emphasised. 20. Indications of the future progress of the work are given, but it is too early yet to foretell the ultimate results of the researches. 21. Other methods of control--cultural, grazing, and chemical are described. 22. Details of collecting and shipping the controlling insects are given. In all 633,289 individuals were exported to New Zealand. 23. An extensive bibliography on weed control is appended.

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