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Effects of reduced pesticide use on flora and fauna in agricultural fields 8. Conclusions and perspectives
8.1 ConclusionsThe reader should kindly notice that this chapter is short and only presents the main conclusions and derived suggestions supported by very few discussing remarks. The broader underlying discussions have been presented in chapter 7. While investigations of target organism responses to particular pesticides in varying dosages are part of the systematic evaluation of these chemicals in Denmark, no project has previously elucidated the effects of several dosage levels of several different pesticides on organisms from several trophic layers within the same fields. Viewed in this light the most important result of the present project is the finding that by reducing the dosages of herbicides and insecticides higher densities or abundances are obtained at all the three major trophic levels represented by plants, insects and birds. Taking into account the amount of disturbing variation, the above result is very promising. Thus the three nominated dosage levels include a considerable variation due to local geographic and year to year differences. Also the farmers' choices of normal dosage have caused dosage variation. As the main result has come out despite these conditions the following general conclusion can be drawn: dosage reductions to at least half of normal will result in a richer nature in the agricultural fields. To this statement should, however, be added that the best guarantee for reaching such an improvement of the nature content in arable fields will be a dosage reduction to one quarter of the normal level used in the present investigation. By this reduction all the remarkable improvements linked to plants, insects and birds were safely obtained. It should, however, be noticed that the dosages and their effect are merely a sort of ruler. Thus the choice of, for example, a more potent herbicide might not lead to the same improvement at quarter dosage because of more powerful effects. Therefore the really important aspect is the acceptance from farmers and advisory people of a certain level of plants and insects as non-detrimental to the production economy. For improvement of the nature content of the arable land the half dosage level is also of interest despite the less clear effects. At this dosage level cultivation problems are non-existent or negligible, at least on short term (< 4 years), while interestingly the biologically more rewarding quarter dosage level is also the level representing a zone of emerging agricultural problems. While the overall picture is that even quarter dosage is often sufficient from a grower's angle there are cases of evident agricultural problems connected mainly with particular weeds and their local occurrence. For instance the lack of effect of reduced dosages of glyphosate on quackgrass (Elymus repens) is confirmed. Also the patchwise accumulated weed problems within certain fields at the end of the third growing season calls for particular attention in case of a more wide spread use of the quarter dosage level for longer periods. However, this sort of a problem can probably easily be solved when use of Global Position Systems in field practice during pesticide application becomes more common. In a few cases of aphid control the renewed occurrence of aphids 10 days after control indicates that quarter dosage seems to be close to the required minimum. 8.2 Perspectives for the futureThrough the present investigations we have identified a problem, which needs further attendance: consequences for biodiversity in agricultural fields of mechanical methods for pest control. Reduction or even abandonment of pesticide use in future agricultural practice may strongly increase the use of various mechanical methods to control weeds in particular and maybe also some insects. For instance the possible negative influence on flora and fauna of mechanical hoeing, with different methods and at different intensities and timings, remains an important unanswered question. This question deserves attention in connection with the growing interest for organic farming. The present results also call for a better understanding of the mechanism behind the effects of reduced pesticide dosages, particularly on animal populations. This aspect might be elucidated by more specific investigations targeted towards population dynamics of a few carefully selected species. In the interface between agriculture and protection of natural flora and fauna the results of this project indicate room for potential changes. The positive effects on flora and fauna of a pesticide reduction to quarter dosage already in the first year calls for an immediate use. The risk of accumulating weed problems may be avoided basically by identifying "high risk spots" which should be kept under more strict control. If such an idea is brought a step further, then creation of a dynamic field patchwork (the spatial dimension) with different dosage levels between zero and normal being applied over time (the temporal dimension) could be envisaged. E.g. this practice would establish an escape route for animals to a neighbouring new low dosage field to counteract the negative effect when full dosage follows low dosage in a particular field. For plants the existence of at least a certain area with quarter or zero treatment levels will be a significant improvement compared to present practice. Such an approach would be one among several steps towards a much-improved nature content in the agricultural fields. It is, however, questionable whether the resources of the farmers and the advisory service are sufficient for such a step towards complication of planning and management. It can also be debated which incitement may be necessary to promote such a step.
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