Effects of reduced pesticide use on flora and fauna in agricultural fields
1 Introduction
(Esbjerg, P. & Petersen, B.S.)
1.1 Background
The present project is a derivative of the governmental Pesticide Action Plan I of 1986. The goal of this plan was a reduction in two steps from 1987 to 1997 of pesticide use both in terms of amount of active ingredients used and in terms of treatment intensity expressed as treatment intensity index.
The treatment intensity index is the theoretical number of pesticide treatments per hectare, as calculated by dividing the amount of pesticides used on the land of current interest with the dosage indicated on the approved label of the particular products (Danmarks Statistik 1992). The Pesticide Action Plan used the treatment intensity index for all arable land in Denmark as a practical measure. Within the first five years of the plan, the first step of a 25% reduction in sold amount and in treatment intensity index should be achieved, and finally in 1997 via the second step a total of 50% reduction should be achieved. The overall aim of the reductions was to protect the groundwater against pollution and the flora and fauna against further degradation.
A few years before the end of the 10 years period of the plan, however, it became increasingly clear that the reduction in treatment intensity index was unlikely to be reached. Parallel with this it was debated which effects the reductions aimed at would have in practice. The so-called Bichel Committee was appointed with the task of evaluating the consequences of a wholly or partly phasing out of agricultural pesticide use in Denmark. Based on their report, a Pesticide Action Plan II was approved in 2000. In general terms, the goal of this action plan is to reduce the use of pesticides as much as possible without significant economic losses. As an interim goal, the treatment intensity index (summed for all pesticide classes) shall be less than 2.0 by the end of 2002.
The public debate, and later on the work within the Bichel Committee, disclosed that scientific evidence of the effects of different levels of pesticide use on flora and fauna in general was sparse. In earlier investigations of pesticide free field margins (Hald et al. 1988) some positive effects on flora and fauna had been demonstrated, and Braae et al. (1988) had found higher densities of birds at organic farms than at conventional farms. An experimental documentation of effects on flora and fauna by reducing the pesticide dosages in general was, however, lacking. Furthermore, there was a need for a project at such a scale that its results might be regarded of general value and immediately relevant for practical farming.
Large-scale studies had previously been carried out in the English Boxworth Project (Greig-Smith et al. 1992). In this project, running from 1982 to 1988, the effects of three different pesticide regimes (Full insurance, Supervised and Integrated) on flora, invertebrates, mammals and birds were investigated. However, the low-input regimes at Boxworth by and large correspond to normal farming practice in Denmark today, rendering the results of limited value in the present context.
1.2 Overall project design
1.2.1 Aim and conditions
In accordance with the tender documents the aim of the project has been to demonstrate possible effects on flora and fauna of a reduction of pesticide dosages. The effects in this context should be and have been investigated in a broad sense with changes in occurrence of a wide range of plants and animals as the measure. It follows from this that the project aim has in no respect included investigations of dose-response relationships of specific organisms to specific chemicals or groups of chemicals.
Throughout the report, the term "biodiversity" is used in a rather inexact sense, an increased biodiversity referring to an increased species richness or to increased frequencies/densities of one or more species of plants or animals. This is in line with current terminology of the Danish Environmental Protection Agency.
From the beginning of the project a series of conditions had pronounced influence on the selection of study areas, pesticides and possible technologies.
 | An economical frame, which despite wide did not allow the inclusion of all groups of pesticides or the inclusion of zero treatment plots. |
| A demand for large areas to enable bird observations and to demonstrate possible consequences for flora and fauna at a scale which was relevant to agricultural practice and economy. |
| A request for, if possible, including a minimum of three dosage levels and observing both direct and indirect affects across trophic levels. |
| A request for working with widely cultivated crops with high levels of pesticide use as well as to problems anticipated in case of reductions in pesticide use. |
| A request for using current farming technologies or, if deemed necessary, to introduce only changes in farm technology which could be brought into practice almost immediately. |
Besides the above points it has been an obvious desire to use methodologies which would ease comparison with earlier results of related investigations on flora and fauna of the arable land.
In order to respect the management by the farmers and to minimize interference with normal farming practice, the experimental fields were in all aspects, except those closely related to pesticide dosages, treated in accordance with normal practice at the site. Hence all the variation which would otherwise be minimized as much as possible to ensure transparency and unambiguousness of scientific results has been included. Apart from practical considerations, the advantage of this approach is that the general value of the results is probably increased.
1.2.2 Selection of pesticides
In the setting of priorities concerning pesticide types and their effects, herbicides were the first to be included. The reasons were their pronounced dominance in practice and their known and presumed effects: directly on plants (1st trophic level) and indirectly on food availability and behaviour of insects (2nd and higher trophic levels) and on birds (2nd and higher trophic levels). Insecticides were included because of their status as the generally most toxic plant protection chemicals to animal life (Candolfi et al. 1999, Samsøe-Petersen 1993, 1995a,b) and because of their wide-ranging presumed effects: directly on target organisms, on other herbivorous insects (2nd trophic level), on predators and parasitoids (3rd and higher trophic levels), and indirectly on parasitic and predatory insects and birds (3rd and higher trophic levels).
Fungicides were left out because major pesticide effects of principal interest could be demonstrated through the use of herbicides and insecticides. Furthermore, the number of potential direct and indirect effects would be high, but also very difficult to demonstrate at field level. Interesting topics include effects on insect pathogenic fungi on both pests and beneficials as well as effects on fungal food sources of arthropods serving as a food reserve for many predatory insects. Also effects on fungi antagonistic to plant pathogens could be of interest but very difficult to demonstrate in the field. The economic burden of producing unambiguous results of this nature, however, proved prohibitive already in the planning phase.
1.2.3 Selection of crops and farms
Winter wheat and spring sown barley together account for the vast majority of the Danish cereal area (82%) and in addition for a remarkably large proportion (46%) of the total arable land in Denmark (Landboforeningerne 2001). As regards pesticide applications winter wheat is a relatively insecticide intensive cereal crop because of its sensitivity to cereal aphids. In addition to this, winter wheat is often treated with herbicides in both autumn and spring.
Sugar beets cover a relatively small area but can act as a key representative for row crops with their inherent weed problems due to a limited vegetation cover until late in the season. Furthermore, sugar beet for contract-based delivery is a row crop of significantly higher economic value than cereals, but still not with the same room for more costly operations as vegetable row crops.
Fig. 1.1.
The location of the five study sites. A: Gjorslev Estate, B: Oremandsgård
Estate, C: Lekkende Estate, D: Nøbøllegård, E: Nordfeld Estate ( ã
Kort og Matrikelstyrelsen (A. 42-02)).
In order to meet the area requirements of the ornithological studies it proved necessary to work with experimental plots of a minimum of 6 hectares. With three dosages this implied a need for fields of at least 18 hectares for every crop included at every farm. Because of the additional demand for a 3-years crop rotation of spring barley, winter wheat and sugar beet, the number of suitable farms was limited, as mentioned in the preface. Already in the planning phase it became clear that travelling time would be a major constraint because of the number of operations to be performed. Taking this into account it was decided to work on the five estates / large farms listed in the preface and shown in Fig. 1.1. Despite considerable soil variation at the local level, all study sites are placed on relatively heavy soils with a high fraction of clay.
1.2.4 Pesticide dosages
In accordance with the requests mentioned earlier it was decided to work with a defined normal dosage, and with 50% and 25% of this. Zero treatment plots, however attractive from a scientific point of view, were not included for practical and economical reasons. In terms of standardization and scientific practice it would seem desirable to use cropwise exactly the same pesticides and dosage levels at all five farms. This would, however, have been of limited sense in practice, primarily because of the occurrence of particular weed problems and accordingly special herbicide requirements at four of the five farms. Furthermore, it does not make sense to use the same insecticides if aphids are the main problem in one season but caterpillars in the next season.
With these complications and the demand for a close connection to practice, it was decided to request each farmer to make at every instance his own decision, based on local experience, about which chemical(s) to use, when to apply, and at which dosage(s). By definition, the farmer’s choice in the particular instance was then the normal dosage, despite all the scientific trouble implied. The amount of liquid applied had to be the same at all dosage levels of a particular treatment, with the exception of sugar beet (cf. below).
It might appear attractive to use the politically oriented measure: the treatment intensity index, calculated separately for herbicides and insecticides, as a common yardstick for dosage levels. However, it should be noticed that because, e.g., different herbicides contain different active ingredients with different effects on plants, this measure is of limited scientific value.
All chemicals applied, and the normal dosage of each, are listed in Appendix A. It should be carefully reminded that in accordance with the above-mentioned constraints, the normal dosage of a certain chemical may differ between farms and years. In Table 1.1 the farmers’ use of herbicides and insecticides during the study is summed up as treatment intensity indices. The table shows that the treatment intensity indices varied a lot from farm to farm and from year to year (field to field), reflecting the very different products and dosages chosen. For each of the three crops the farmers involved in this study had, on average, a higher treatment intensity index of both herbicides and insecticides than the average treatment intensity index in Denmark for the same crops.
Table 1.1.
Summed treatment intensity index for herbicide and insecticide applications
at normal dosage. Herbicides are divided in products against broad-leaved species and
products against grasses (Elymus repens and Avena fatua). "Target
2002" indicates the goals set up in Pesticide Action Plan II.
The use of herbicides in the crops was 24% higher than the average use in Denmark, while the use of insecticides was 119% above the average. These higher treatment intensity indices may be due to the location of the farms on high quality soil, where the average treatment intensity index normally is higher than on low quality soil (Jensen 2001). This is supported by the fact that the smallest deviations are seen in sugar beets, which are grown much more often on high quality than on low quality soils. Also the location of the farms in eastern Denmark might increase the use of insecticides, since aphid attacks are more frequent in eastern than in western Denmark (Poul Henning Petersen pers. com.). In addition, large farms/estates (as the experimental farms in this study) in general have a higher treatment intensity index than small farms (Jensen 2001). Due attention must be paid to these differences if the results of this study are extrapolated.
In sugar beets a further peculiarity proved necessary in order to combine the scientific project aims and the applied aspects. During the pilot phase in 1996 at Gjorslev, the sugar beet plots were treated with broad swath at all dosage levels. This practice, however, left a cover of weeds at half and quarter dosages which, irrespective of surprisingly small immediate yield effects, was unacceptable to farmers. The problem had to be solved if the project was to carry on for the subsequent three years, and again practicability influenced the solution despite some scientific shortcomings.
The method chosen was broad swath application at normal dosage, and band spraying (with normal dosage) in the half and quarter dosage plots, so that only 50% and 25-28% (minimum band width), respectively, of the area were treated in these plots. In addition to this, supplementary mechanical hoeing was carried out in all half and quarter dosage plots with sugar beets and also in the normal dosage plots depending on the desire of the individual farmer. (At Gjorslev the combination of broad swath spraying and mechanical hoeing had already been practiced for a number of years).
A table of field operations performed at each farm in each study year is enclosed as Appendix 1.5.
At all farms, the fields were included in a crop rotation scheme as illustrated on Fig. 1.2. In practice, there could be some distance between the fields, and in rare cases even between dosage plots on fields that might be considerably larger than 18 hectares.
Fig. 1.2.
Schematic example of crop rotation between the three experimental fields at
one farm.
Each field was subdivided into three plots, preferably in a regular fashion with the longest axes along the longest side of the field. The plot size of minimum 6 hectares should ensure that a sufficient number of birds were observed, while the rectangular shape allowed cultivation operations to be performed without substantial inconvenience.
Dosages were allocated to plots in a random way with one crucial exception: if the field was surrounded by hedgerows, the middle plot was always used for half dosage while normal and quarter dosage were randomly allocated to the outer plots. The reasons for this decision were as follows: Hedgerows have profound effects on the distribution of most species of birds and some species of arthropods within the fields (cf. section 4.1.1). Thus, plots that are bordering on hedgerows are not fully comparable with plots that are not. One solution to this problem is to randomize the assignment of treatments to plots; this avoids introducing any systematic bias but increases the variation. However, because the dosage-related differences in bird and arthropod densities might well be small, it was feared that such an increase in random variation would prevent the detection of any differences. Therefore, to maximize the chance of detecting any differences at all, it was decided to give priority to a comparison of normal and quarter dosage by allocating the dosages as described. Although the interpretation of the ornithological (and to some extent also the entomological) results from half dosage might be difficult, the results of the botanical investigations (the primary production level) should still be fully valid and thus provide a basis for some general conclusions about the effect of a halving of pesticide dosages.
The selected dosage plots were fixed throughout the project period in order to allow for possible accumulative effects of the reduction of pesticide dosages.
Maps of the five study sites are shown in figs. 1.3-1.7, and a short description of the field surroundings is given in each figure caption. Further field data are presented in Appendix B.
Fig. 1.3.
Aerial view of the experimental fields at Gjorslev with the dosage plots
indicated. All fields surrounded by dense hedges with scattered trees. A single marl-pit
with reeds in Field 1 and three small marl-pits, surrounded by trees, in both Fields 2 and
3. Farm buildings and park immediately N and NW of Field 1. Fields 2 and 3 situated
between the park and the lake Gjorslev Møllesø. Deciduous forest on the other side of
the lake.
Photo by courtesy of Kampsax.
Fig. 1.4.
Aerial view of the experimental fields at Oremandsgård with the dosage plots
indicated. Large fields in a fairly open landscape. Single marl-pits, surrounded by trees
and scrub, in Fields 2 and 3. Deciduous hedgerows, partly quite open, N and W of Field 1,
between Fields 2 and 3, and N of Field 3. Alley with old, broad-leaved trees S of Fields 2
and 3. Old, deciduous wood E of Field 2 and farm buildings towards the SE. Photo by
courtesy of Kampsax.
Fig. 1.5.
Aerial view of the experimental fields at Lekkende with the dosage plots
indicated. Fields located in an open, rolling landscape, sloping towards the SE and
towards the strip of meadow and lakes to the NE. Field 1 with hedgerows along the NW and
parts of the SW border and scattered trees towards the SE. Hedgerow between Fields 2 and 3
and W of Field 3. Covert with deciduous scrub in Field 1 and a small marl-pit, surrounded
by trees, in Field 2. Photo by courtesy of Kampsax.
Fig. 1.6.
Aerial view of the experimental fields at Nøbøllegård with the dosage
plots indicated. Fields located in an open landscape along the coastline. Field1 bordered
by a fairly open hedgerow towards W and with a small village towards SE. Field 2 divided
into two parts, c. 250 m apart. Reduced dosage plots with well-developed hedgerow towards
the sea and less dense hedgerows towards E and W; normal dosage plot with hedgerow towards
Field 3. Field 3 also bordered by hedgerow towards W and partly towards N where a fairly
steep slope leads to the seashore. Single marl-pits, surrounded by trees or scrub, in
Fields 1 and 3; farmsteads in Fields 2 and 3. Photo by courtesy of Kampsax.
Fig. 1.7.
Aerial view of the experimental fields at Nordfeld with the dosage plots
indicated. Field 1 undulating, lying between two deciduous woods and surrounded by
hedgerows on three sides. Farm buildings towards NW. Fields 2 and 3 situated W of the
wood, close to the beach, with well-developed hedgerows on all sides. Both fields divided
into two parts by gravel road bordered by scattered trees (Field 2) or hedgerows (Field
3). Single, small marl-pits, surrounded by trees and scrub, in all fields. Photo by
courtesy of Kampsax.
The project has comprised a botanical part, an entomological part and an ornithological part. These parts aim at demonstrating effects at different trophic levels and thereby allow for the demonstration of indirect as well as direct effects. This is indicated by the arrows on Fig. 1.8, which also in principle illustrates why the work should focus more on some arthropods than on others. Thus a larva of a particular species which is linked to a particular weed and also is an attractive food item to birds is an insect of focus interest. The population density of such an insect may be affected by herbicide caused food limitation but may also be reduced by insecticide treatments. A further interesting element is added to this web if the herbivorous larva in focus is also prey for some ground beetles which are themselves food items for birds. Hence the entomological part of the project has been oriented as much as possible towards connections to the botanical and ornithological parts.
Very few direct effects are seen in birds, but birds may be affected indirectly by herbicide effects (via plant and arthropod food depletion) and by insecticide effects (via arthropod food depletion). Whereas plant and arthropod responses to reduced pesticide dosages were assessed by density measures, bird response could only be assessed in terms of occurrence (aggregation of birds in plots with reduced dosages). Population density studies would of course have been desirable, in particular if studies of productivity and survival in response to food availability could have been included as well. However, the appropriate scale demands for such studies would be rather different for plants, arthropods and birds. Also, detailed population studies would have to be limited to one crop and/or one site, and to one or two model species at each trophic level.
Fig. 1.8.
Diagrammatic presentation of the role of the organisms included in the study.
The arrows indicate which types of animals or plants are consumed by which other types.
The detailed methodologies of the three parts are described within the specific sections in chapters 2 to 4.
The pilot season 1996 was preceded by a relatively cold winter during which January was very dry. Also March and April were much drier than normal. During the growing season, June and July were rather chill and dry while august was warm (mean 2° C above normal) (Friis et al. 1996).
In the first year of the main study, 1997, February was warm (mean 3° C in contrast to a normal of 0° C). The weather in spring was close to normal, although with a fairly cool and wet May. July was rather warm and August very warm (mean 20.2° C in contrast to normally 15.6° C). The precipitation of the growing season was fairly normal. In the post harvest period, October was rather cold and the first snowfall occurred early (Jensen et al. 1997).
1998 started with a mild winter, with February temperatures as much as 5° C above normal and with a considerable amount of precipitation. The spring was somewhat peculiar, with almost twice as much precipitation in April as normal, followed by a warm and dry May. June was very much like Danish average while July was chill and wet (mean precipitation 92 mm, 40% more than normally). The weather situation in August was quite normal, but September had the lowest number of sunshine hours and October the highest amount of precipitation (171 mm) recorded for more than 100 years. At many locations the moist conditions troubled the autumn seed drilling (Hansen et al. 1998).
The early winter part of 1999 was mild and humid, and with a high precipitation in March plants in larger patches of autumn-sown fields were drowned regionally. This agriculturally problematic situation was eased by fairly normal conditions in April-May, but in June again very high amounts of precipitation occurred (mean 120 mm, normal mean 52 mm) resulting in weed problems at many locations. During July the weather gradually changed, with high temperatures (up to 30° C) occurring towards the end of the month. In August the weather was close to normal whereas September was the warmest for years with a mean of 16.2° C (normal mean 12.7° C) (Sørensen et al. 1999).
1.2.9 Yield effects and economy
As the focus of the project has been on the biological effects, less emphasis was given to effects on yield and agro-economy. However, the need for a scale to justify possible compensations because of reduced yields was met by establishing an array of very small treatment plots of 9 or 12 m x 2.5 m in the cereal fields, according to guidelines of The Danish Agricultural Advisory Centre. For practical reasons these field trials were placed in the normal dosage plots. Because of the small size of the plots it was not possible to assign a particular dosage level to the same area of ground every year. Therefore these plots have not been able to demonstrate the possible accumulation of weeds over the three project years. To some degree, however, the final assessments of accumulated weed problems carried out after the last season compensate for this.
In sugar beet, mini-plots were also used during the pilot phase but obvious difficulties in harvesting questioned the validity of results. Therefore a manual sampling procedure was developed in consultation with "The foundation for sugar beet research" which also took care of the final yield assessment.
Despite these shortcomings of the yield estimates they were also used to obtain a minimum of evaluation of the economic effects of reducing pesticide dosages. This evaluation also leant on the final assessment of weed problems carried out by the project and the corresponding economical compensations based on qualified estimations of the costs of weed removal.
1.2.10 Statistical analyses
Fundamentally, the whole experiment may be looked upon as a fully balanced Latin Square design with five replicates (farms), and three crops rotating between three fields in three years. Due to the cultivation history at each farm, the allocation of crops to fields in the first year was not random, but it is unlikely that this has introduced any notable bias to the results. Each field was divided into three dosage plots which constituted the basic experimental units (45). As described above, the allocation of dosages to plots was not fully random. The implications of this depend on the organisms studied and are discussed in the individual chapters on the botanical, entomological and ornithological studies.
The main focus of the analyses has been on the demonstration of differences between dosages, taking possible crop effects and the (random) variation between farms, fields and years into account. In some cases tests for dosage effects could be performed across all crops, in other cases each crop had to be tested separately. In general, the statistical analyses were based upon general linear models (anovas etc.) with the dependent variable (number of plants, density of birds etc.) being transformed as appropriate to achieve an approximately normal distribution. Sometimes, however, the nature or distribution of the response variable demanded that an analysis based on a Poisson distribution (a generalized linear model) or a non-parametric test was used.
Tests of effects of differences at one trophic level on another (higher) trophic level were performed by including covariates describing density/diversity at the lower trophic level(s) in the analyses of response variables related to the higher trophic level (e.g., arthropod biomass was included in analyses of the occurrence of insectivorous bird species).
The statistical methods are described in more detail in the appropriate sections of chapters 2 to 6.