Poster

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Weed, Arthropod and Bird Responses to Reduced
Pesticide Dosages and Conversion to Organic
Farming
Anne-Mette M. Jensen Ib Johnsen Biological
Institute University of Copenhagen Øster
Farimagsgade 2D DK - 1353 København K
Bo Svenning Petersen Orbicon Ringstedvej
20 DK-4000 Roskilde
Søren Navntoft Peter Esbjerg Department of
Ecology, KVL Thorvaldsensvej 40, DK-1871
Frederiksberg C
Effects of reduced pesticide use
Effects of conversion to organic farming
Background and aim The status of biodiversity in
agroecosystems is a major concern due to
pesticide depletion of flora and fauna.
Pesticides do not only have negative effects on
natural enemies of crop pests and weed biomass.
Several guilds of insects being important diet
components for birds and small mammals are also
reduced by pesticides together with
non-competitive weed species providing food for
arthropods and birds. The aim of the present
investigation was to assess the effects of
reduced herbicide and insecticide regimes on the
abundance of weeds, insects and birds. Such
information may provide guidelines for better
field management in order to conserve and enhance
the biodiversity.
Background and aim The investigations into the
effects of reduced pesticide applications were in
autumn 1999 extended to include a two-year study
of the effects of conversion in cereals from
conventional to organic farming. This latter
study became possible when one of the five farms
decided to convert to organic farming. The aim of
the study was to identify and quantify changes in
flora, insect fauna and bird fauna occurring
during this conversion.
Methods In 1997-99 a large-scale investigation
was carried out on the effects of reduced
herbicide and insecticide dosages on five farms
in Southern Zealand, Denmark. In barley, winter
wheat and sugar beets three levels of insecticide
and herbicide were used current farming practice
and half dosage and quarter dosage of the current
use. Fungicides were applied uniformly to all
experimental fields. The variables recorded
were Weed density/diversity/flowering, insect
density/biomass and bird abundance.
Methods The conversion effects were followed in
two cereal fields during two years, thus the
experimental period was three years before
(1997-99) and two years after conversion
(2000-2001). The variables recorded were Weed
density/diversity/biomass/flowering, insect
density/biomass and Skylark abundance.
Results The density of both competitive and
non-competitive weed species was at least 60
higher at lower dosages. Also the weed diversity
benefited from lower dosages especially in
barley, with 30 more species at quarter dosage.
Moreover a higher percentage of the plants
reached flowering at quarter dosage. Generally
higher abundances of arthropods occurred at
reduced pesticide dosages, particularly at
quarter dosage. Herbivores such as sawfly larvae
and aphid-specific predators such as hover flies
benefited from the reductions with density
increases between 20 and 40 at quarter dosage.
Whitethroat numbers were significantly
correlated to dry mass of arthropods indicating
that increased amounts of arthropod food attract
insect-feeding birds (Fig.1). Abundances of
Skylarks, Whitethroats and small seed-eaters
across crops were increased by 23, 100 and 49
respectively at quarter dosage. Yield
depressing weed densities only occurred very
locally and spot-wise at quarter dosage.
Results Weed density and diversity reached a
level after conversion similar to what was
obtained by a pesticide reduction to quarter
dosage. The percentage of flowering species more
than doubled in the conversion period. After
conversion the weed biomass was up to 10-fold
higher in spring barley than in winter wheat.
Conversion resulted in 60 higher arthropod dry
mass in spring barley. Densities of all examined
arthropod groups increased with the exception of
ground-dwelling beetles. Such differences were
not found in winter wheat, perhaps because of the
much lower weed biomass than in spring barley.
The absence of positive effects of conversion on
soil-dwelling arthropods might have been caused
by mechanical weeding. Conversion increased
Skylark numbers by 55.
The Skylark is the most common bird in Danish
arable land. (Model photo, J. Martin)
Conclusion Reduced herbicide dosages resulted in
higher weed density, diversity and flowering.
Reduced insecticide and herbicide dosages in
combination resulted in increased arthropod
abundances that again resulted in higher
abundances of insectivorous birds. Thus a
herbicide and insecticide reduction to one fourth
of the current usage would significantly enhance
the natural life in agricultural fields without
having short term economic implications. It may
be suggested for improvement of sustainability
wherever possible. The results also indicate that
a conversion to organic farming would immediately
benefit the wildlife.
The Danish Environmental Protection Agency
founded both projects. For more information see
www.mst.dk and search for Pesticides Research
no. 58 Pesticides Research no. 74
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