Report from the Sub-comittee on the environment and health

66
Theoretically, one can imagine an optimum year in which <strong>the</strong> use of
pesticides has no effect on skylarks because <strong>the</strong>y have ample food and a
catastrophic year in which pesticide treatment reduces <strong>the</strong> skylarks’
breeding success to zero. However, such extremes are likely to be very
rare.
In <strong>the</strong> four years of <strong>the</strong> skylark study, <strong>the</strong> number of young leaving <strong>the</strong>
nest fell on average by 38% on sprayed fields compared with fields that
were not sprayed with herbicides and insecticides. The difference was
due to <strong>the</strong> fact that <strong>the</strong> skylarks in treated fields had more unsuccessful
attempts at breeding and largely abandoned <strong>the</strong> attempt after <strong>the</strong>
insecticide treatment, whereas many pairs of skylarks continued breeding
in untreated fields. For two weeks after treatment with insecticides, <strong>the</strong>re
was on average three times less insect food in <strong>the</strong> treated fields.
Thereafter, <strong>the</strong> difference between treated and untreated fields decreased
considerably. Barley fields are often treated with insecticides at <strong>the</strong> time
<strong>the</strong> skylarks’ breeding activity culminates. A considerable part of <strong>the</strong>
nestlings’ food consisted of ground beetles (42%), which occurred
irrespective of <strong>the</strong> pesticide treatment of <strong>the</strong> fields. However, <strong>the</strong> amount
of food is not necessarily <strong>the</strong> only critical variable; <strong>the</strong> quality of <strong>the</strong> food
may also be of importance. An analysis of <strong>the</strong> nestlings’ faeces revealed
that <strong>the</strong> content differed considerably in treated and untreated fields. The
proportion of ground beetles was greatest in sprayed fields, while
butterfly larvae, plant bugs, leaf-beetle larvae and several o<strong>the</strong>r species
were more frequent in unsprayed fields, where <strong>the</strong> diet was more varied.
Precisely among butterflies, plant bugs and leaf-beetles, a large
proportion of <strong>the</strong> species live <strong>from</strong> wild plants. Many of <strong>the</strong> herbivorous
insect species occurred in greatly reduced numbers in treated fields
because <strong>the</strong>ir conditions were considerably impaired by both herbicides
and insecticides.
In British studies of <strong>the</strong> grey partridge, it has also been concluded that <strong>the</strong>
fauna living on cereals are an important part of <strong>the</strong> bird’s diet, both
quantitatively and qualitatively (Potts 1986). In Britain, both partridges
and pheasants have been found to have a higher survival rate in fields
with unsprayed headlands (Rands 1985). The headlands are important for
<strong>the</strong>se gallinaceans because <strong>the</strong>y often forage along <strong>the</strong> edges of fields.
The edges of fields are <strong>the</strong> part of <strong>the</strong> field that is richest in species of
both flora and fauna, while at <strong>the</strong> same time <strong>the</strong> crop yield is often
reduced (Hald, Elmegaard 1989; Hald et al. 1994; Wilson, Aebischer
1995). The edge zone is <strong>the</strong>refore <strong>the</strong> part of <strong>the</strong> field where one gains
most <strong>from</strong> not spraying.
In Denmark, <strong>the</strong> sizes of yellowhammer broods have been studied on
organically and conventionally cultivated fields and found to be about
15% bigger on organically cultivated fields (Petersen et al. 1995). In this
study, too, <strong>the</strong> effect is attributed to a larger, more varied and more stable
food resource on organically cultivated fields, although <strong>the</strong>
yellowhammer’s food was not analysed. At <strong>the</strong> same time, higher
densities of yellowhammer were recorded on organically cultivated fields
in <strong>the</strong> wintertime, which may be due to <strong>the</strong> more abundant flora and <strong>the</strong>
consequently larger seed pool (Petersen, Nøhr 1992).