Interactions between the chrysomelid beetle Gastrophysa viridula and the herbicide asulam in the control of broad-leaved dock Rumex obtusifolius; field trials, University of Lancaster, Lancashire, England
Published source details
Speight R.I. & Whittaker J.B. (1987) Interactions between the chrysomelid beetle Gastrophysa viridula, the weed Rumex obtusifolius and the herbicide Asulam. Journal of Applied Ecology, 24, 119-129
Published source details Speight R.I. & Whittaker J.B. (1987) Interactions between the chrysomelid beetle Gastrophysa viridula, the weed Rumex obtusifolius and the herbicide Asulam. Journal of Applied Ecology, 24, 119-129
In the UK, broad-leaved dock Rumex obtusifolius is a common and sometimes invasive weed of pastureland. Its pest status in Britain has resulted in the development of control measures, usually herbicide application, but occasionally biological control has been trialed. The commercial control of dock in Britain is often attempted by applying the systemic herbicide asulam. Its long-term effectiveness is, however, variable. The native chrysomelid beetle Gastrophysa viridula is known to feed on dock, and although it is unlikely that G.viridula acting alone, could effectively control R.obtusifolius in Britain, it is possible that, acting with asulam, more effective or less costly control may be achieved. In this field study, the effects of separate and simultaneous grazing by G. viridula and asulam treatment on R.obtusifolius were investigated.
Experimental design: Four blocks containing six 2 x 2 m plots were established on area of unmanaged grassland. Six treatments were applied: a high asulam concentration (5 ml/l i.e. manufacturer's recommended concentration) a low asulam concentration (1.25 ml/l); plus a water control, each in the presence or absence of beetle-grazing.
A 1 m mown strip acted as a barrier to beetle movements between plots. On 27 April 1981, 384 1-month-old R.obtusifolius plants were planted out at regular spacing of 16/plot. Dry weights were obtained from a further randomly selected 16 plants. The ungrazed plots were kept free of invertebrate using a systemic insecticide known not to influence plant growth.
Beetle release and monitoring: On 19 May 1981, 144 adult beetles (sex ratio approximately 50:50), 12/ plot, were released in randomly allocated 'grazing plots'. On 1 June 1981, all plants were growing vigorously so plots were sprayed with the allocated asulam solutions, or water only.
From 21 May to 21 October, the numbers of G.viridula eggs, larvae and adults were counted on each of the four inner plants on each grazed plot (i.e. 16 replicates/ treatment).
Dock harvesting: Dock plant material was harvested four times during the 1 year experiment.
August and October 1981 - three border plants from each plot (i.e.12/treatment). These harvests followed the end of the first G.viridula generation and the end of the field season, respectively.
February 1982 - three border plants from the grazed and ungrazed water-sprayed plots only, to quantify an apparent difference in shoot weight.
April 1982 – for three of the four inner plants in each plot, root, shoot and total plant dry weights were determined. Leaf areas were obtained for the February and April harvests for the grazed and ungrazed water-sprayed plants; but not asulam-treated ones as curling of the leaves made this impratical.
Influence of further-reduced asulam concentrations on R.obtusifolius and G.viridula: One experimental aim was to determine whether asulam concentrations could be reduced if beetle damage is present. The same experimental design and procedure was used, but asulam solutions were diluted further, i.e. 1.00 ml/l and 0.75 ml/l. The site was planted on 27 April 1982, beetles were released on 3 May and counts started on 5 May. Asulam was applied on 2 June. No plant harvests were made.
There were fewer second generation G.viridula on asulam-treated dock plants than on untreated ones; the total, shoot and root dry weights of asulam-treated plants were significantly lower than untreated ones. Beetle-grazing significantly reduced the dry weight of water-only sprayed plants but did not further reduce the dry weight of asulam treated plants.
Dock survival was unaffected by grazing or low herbicide levels. Even at high levels of asulam application (5 ml/l), only approximately 40% of plants were killed.
The stage of beetle development when the host plants were sprayed was important for the continued survival of G.viridula. Numbers of G.viridula on treated and untreated plants were similar during the first generation but the second generation on untreated plants during July and August was significantly higher than on treated plants. Second generation beetles on the asulam-free plots, laid on average four times as many eggs per adult female on asulam-sprayed plots. Survival from eggs to larvae on the asulam-free plots was c. 25%, it was 4-12% on the treated plots. These effects were probably largely caused by the defoliation of the food plant rather than by direct effects of asulam on the beetle.
However, some apparently sub-lethal effects have been noted in a laboratory trial (see www.conservationevidence.com/ViewEntry.asp?ID=834 for a summary).
Conclusions: The highest (i.e. recommended) concentration of asulam used resulted in 40% mortality of plants after 12 months. Grazing alone did not kill plants but did significantly reduce plant weight and regrowth at the start of the next growing season. If R.obtusifolius is subjected to asulam treatment, this may reduce the effect of beetle grazing either because the asulum is toxic to the beetle (directly or when ingested in plant tissue) or because the growth habit of the plant makes it less acceptable to the beetle. In these field trials observations suggest that asulam-treated plants were not a good environment for G.viridula, probably because of the lack of high quality foliage rather than a direct effect on the beetles themselves.
It is suggested that the density of beetles and the time of their release in relation to the spraying date of dock with asulam may be manipulated to improve the efficiency of any dock control programme. Spraying should take place during beetle pupation or adult emergence (i.e. a week or two after large, black third-instar larvae are observed on the underside of leaves).
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