Partners and International Organizations
(English)
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B, CH, CRO, D, DK, E, F, H, I, N, NL, S, SK, UK
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Abstract
(English)
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Within a European research programme the work carried out should provide basic information on potential biological control of pigweeds (Amaranthus spp.) in Europe. In Switzerland several Amaranthus species, mainly A. retroflexus LINNAEUS, A. powellii S.WATSON and A. bouchonii THELLUNG cause locally serious problems in agriculture. An inquiry turned out that crops having amaranth infestations are maize, vegetables, potatoes, sugar beets and beans. Resistance to several herbicides renders control of pigweeds increasingly difficult. Biological control could contribute to a satisfactory control of these weeds. A comprehensive literature revealed many organisms (insects, mites, nematodes, fungi, bacteria and viruses) being associated with Amaranthus spp. worldwide. In spite of the relatively large number of records, only few of the known species seem to have potential as biocontrol agents for Amaranthus spp.. Field surveys were carried out for insects (and pathogens) associated with Amaranthus species. Ten locations in Switzerland and in neighbouring countries were visited at monthly intervals. The surveys produced a total of 137 phytophagous insect species collected from A. retroflexus, A. bouchonii and A. powellii: 30 Coleoptera, 47 Homoptera, 34 Heteroptera, 14 Lepidoptera, 11 Thysanoptera, and 1 Orthoptera species. For most of the species found Amaranthus spp. were not known as host plants. It was found that all species collected are either polyphagous or possibly oligophagous, and none monophagous. This finding was supported by comparative insect collections on other plant species in the immediate neighbouring area which showed that many insect species collected on Amaranthus spp. occurred also on these. The surveys revealed also that no herbivore insect species from the area of origin has been accidentally introduced into Europe. Based on the extended field surveys it would seem that none of the species of phytophagous insects associated with target Amaranthus spp. in Europe has a potential as biological control agent. Therefore, it is proposed to carry out additional surveys in the southern part of North, as well as in Central and South America, the centres of origin of noxious Amaranthus species occurring in Europe, to locate potential insect control agents. An important part of the work carried out were botanical experiments which aimed - amongst others - to study some aspects of the biology of the main target weed. The fitness of triazine-susceptible and -resistant biotypes of A. retroflexus was studied, including susceptibility to insect and pathogen attack. Another experiment aimed to investigate the effect of Amaranthus plant density on survival, biomass and seed production. It was found that survival of individual plants was very high, even at high densities. Biomass production per area did increase up to a density of 256 plants/m², but reached already 75% of its maximum at a density of 16 plants/m². Maximum seed production was reached already at a density of 16 plants per m². Plant density had no influence on seed viability and germination behaviour of the seeds produced. Germination rates of seeds from all densities ranged between 94.5% and 100%. Ideally, the exact phenology of the target weed needs to be known before the introduction of potential biological control agents. It was observed that Amaranthus retroflexus can grow in three cohorts in a maize field, of which only the first cohort has a serious impact on maize. However, our own observations showed that Amaranthus spp. can grow throughout summer (mainly after soil disturbance), forming more cohorts than in a crop situation under given conditions. The goal of another experiment was to study differences in growth, biomass and seed production of artificially created cohorts. The survival rate was relatively high (87-100%) for all cohorts and did not show a decreasing tendency during the season. However, the average height of plants successively decreased during summer from cohort to cohort. Plant weight was stable during the first three cohorts and decreased rapidly afterwards. The latest emergence date of a cohort allowing development of fertile seeds was 16 July. Based on these findings it is concluded that potential biological control agents should be active from the beginning of May to the end of July. Plants from later germinating seeds do not produce any mature seeds. In further studies seed dispersal was investigated. The main factor for dispersal of Amaranthus spp. seem to be agricultural practices. It was found that seeds are not spread over greater distances by natural factors like wind. Some 95% of all germinated plants were found within a range of 1.5 m from the maternal plant, 86% even within a range of 0.5 m. In contrast, soil cultivation translocated seeds at least over a distance of about 50 meters. It was concluded that careful soil cultivation could reduce dispersal speed of pigweeds. Another experiment investigated the competitive effect of Trifolium subterraneum LINNAEUS on A. retroflexus. Undersown subterranean clover reduced the number of A. retroflexus plants by some 80%. Biomass and seed production were also significantly reduced by competing clover. At higher amaranth plant density the negative impact of T. subterraneum was however much less pronounced. Competition with T. subterraneum had no impact on seed viability of A. retroflexus. Allelopathy was considered as a potential reason for the competitive dominance of the clover. In the green house, an experiment was set up to investigate potential allelopathic effects of Trifolium subterraneum on germination and growth of Amaranthus retroflexus. Pots containing each 50 seeds of A. retroflexus were exposed to different treatments: extracts of Trifolium shoots and roots as well as whole plant extract, soil from pots in which Trifolium had been grown, dry mulch of Trifolium, and seeds of Trifolium. Similar treatments were made with Amaranthus extracts and Amaranthus mulch. The results showed that the soil from pots in which Trifolium had been grown had a negative impact on germination: Amaranthus seeds germinated later. However, extract of Trifolium and also of Amaranthus stimulated germination of A. retroflexus seeds. Root extract stimulated germination more than shoot extract. The mulched Trifolium delayed germination. A few other experiments aimed at clarifying possibilities for integration of undersowing T. subterraneum in already existing cropping techniques. In three field experiments, the suppressing effect of undersown Trifolium subterraneum and of a clover-grass mixture on emergence and growth of Amaranthus spp. and other weeds were tested. The results differed remarkably between the crops. Sugarbeets seem to be very sensitive to early competition by other plants. Unfortunately, the clover as well as the mixture decreased crop yield to an unacceptable level. Much less impact on sugarbeets was reached by mulching the undersown clover. In addition, mulched Trifolium produced sugarbeets with highest sugar content and allowed only a few weeds to grow. In cabbage, undersowing had only little impact on the crop. However, due to irrigation, the weed pressure was very high and the undersown clover and mixture could not prevent weed growth. The best result was obtained in an onion field, where the undersown cover crops, covering the soil surface, prevented the germination of the pigweeds. The clover-grass mixture had no negative impact on the crop and Trifolium subterraneum even significantly increased crop yield. To avoid too strong impact on crops by competing cover crops, mowing of the cover crop as an additional cultivation method was investigated. In an experiment, the effect of mechanical control by clipping and plant competition by undersown Trifolium subterraneum (subterranean clover) on Amaranthus retroflexus (redroot pigweed) was investigated. Redroot pigweed and subterranean clover were grown together in plots and clipped once or twice at different dates. The effect of clipping on these two plant species was completely different: it was negative for A. retroflexus and positive for T. subterraneum. Clipping reduced final plant height of A. retroflexus by 69% (72% in plots where growing together with Trifolium), dry weight by 88% (95%) and seed production by 99% (99%). After clipping, subterranean clover recovers very fast, covers the soil surface, and prevents regrowth and new germination of A. retroflexus. Within the framework of the European Cost-Action 816, a 5-year collaboration between scientists from 10 institutions in 7 European countries has made an important contribution to the biological control of Amaranthus spp. in Europe. The previously unstudied insect fauna associated with Amaranthus spp. in Europe is now known. This provides - in combination with the extensive knowledge of the target weed's biology - a basis for future introductions of a non-native biocontrol agent into Europe. In addition, two promising microbial herbicides, Alternaria alternata and Trematophoma lignicola have been discovered. Further work on their use in integrated farming systems is required. The use of microbial herbicides in conjunction with new cropping systems, such as green cover crops or living mulch using Trifolium subterraneum is an approach which offers much potential. In summer 1999 during the EWRS conference, in demonstration field plots the achievments of five year co-operative research were presented to a broad public.
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