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Effects of reduced pesticide use on flora and fauna in agricultural fields Summary and conclusionsThis report presents the results of investigations of responses of wild flora, insects and birds in arable fields to reduced dosages of pesticides. The investigation was related to the Danish Pesticide Action Plan I (1987-96) and complied with requests from the financing Ministry of Environment and Energy as regards large scale, three dosage levels and technical readiness for practical implementation. The studies were carried out in 1996 (pilot study at one farm) and 1997-99 (main study at five farms). All farms were situated on clay soils in southeastern Denmark. Hosting was contracted with five large farms with spring barley, winter wheat and sugar beets as crop rotation. The two cereal crops differ in structure and cover the major part of Danish arable land, while sugar beet is a row crop with built-in problems due to lack of competitiveness. All study fields were sufficiently large to include three dosage plots of 6 hectares or more. In these plots, herbicides and insecticides were applied at normal, half and quarter dosages whereas fungicide dosages were not reduced. The pesticides used and the dosage level defined as normal were at each occasion decided upon by the farmer, based on his experience. From a scientific point of view this was inconvenient, but anything else would have been meaningless due to local variations of the weed problems. In this way also the request for practical implementation was met. In the pilot study, broad swath application of reduced dosages in sugar beets led to unacceptable amounts of weeds. Therefore band spraying was used to obtain the two reduction levels in the main study. As a supplementary weeding, mechanical hoeing was carried out on the farmer’s decision. The number of weed species present was determined before and after herbicide applications, and weed densities and phenology were recorded. Also the seed rain was studied. Sampling of insects was performed every 7-10 days from mid-May to mid-August using a 4WD-tractioned vacuum sampler and supplementary pitfall trapping in fenced sub-plots was carried out in wheat. Within the same period, birds were censused and their location within the fields recorded every five days. A combination of point counts and line transects was used. As a supplement to the biological investigations, crop yields were determined. Weed densities after spraying differed significantly between dosages, with 30 plants/m2 at normal, 48 plants/m2 at half and 55 plants/m2 at quarter dosage. Besides there was a considerable difference between crops: 84 plants/m2 in barley, 32/m2 in sugar beets and 28/m2 in wheat. Species richness tended to increase with decreasing dosage (significant in barley only) and rare and scarce species occurred more frequently at reduced dosages. The proportion of flowering species increased with decreasing dosage, and there were indications of increased seed production at quarter dosage. Insect abundance generally increased at reduced dosages. This was very clear in barley while the picture was slightly obscure for wheat and sugar beets if narrow taxonomic units were considered. An overall analysis of non-carnivores and carnivores in the three crops strongly supported the improvement at quarter dosage. More specifically, higher densities of beneficial insects were found at quarter. Aphids were also more numerous at this dosage level but did not occur in densities of economic importance. Combined analyses showed some correlation between plant and insect abundances. This was clear among others for selected herbivorous weevils and larvae of moths. The bird counts revealed a change from uniform distribution early in the season towards concentration in plots with reduced treatments. Skylarks, Whitethroats and "small seed-eaters" (Yellowhammer, Linnet etc.) all responded significantly to dosage reductions. The effect was most pronounced for species that breed in hedgerows and search their food in the fields. In July the number of Whitethroat records doubled at quarter dosage while small seed-eaters increased by 50% and Skylarks by 20-25% relative to normal dosage. The effect of half dosage was less clear, but the estimates indicate that half of the improvement at quarter dosage was also obtained at half dosage. The effect of reduced dosages was independent of crop and year. In cereals, yield was significantly reduced in 3 of 64 cases, always at quarter dosage. Sugar beet yields were reduced in 5 of 32 cases; three at quarter dosage and two at half. The average yield in sugar beet varied much more than in cereals and at one farm the revenue was impaired by 11-27%. Despite this, profitability analyses indicate that pesticide reductions as used here generally have a very limited economic impact, at least on short term. Effects not properly covered are the risk of accumulated weed problems by continuous use of reduced dosages and possible adjustment costs associated with new growing conditions. In conclusion, both reductions to half and quarter dosages of herbicides and insecticides improve the "nature element" of the fields. The gain at quarter dosage is much more marked than the gain at half. However, use of half dosage will only create negligible, if any, agricultural problems, especially if supplementary control of particular weed patches is carried out. General use of quarter dosage may be more problematic in this respect. Only longer time series can support a more conclusive picture, and the possible side effects of increased mechanical weeding also call for more attention.
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