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Genetically modified (GM) plants may show unintended differences compared to the original varieties, due to the modification process. Such differences might in some cases affect non-target organisms linked to the crop into an agro-ecosystem. In this paper, we aimed to study interactions of two blight-resistant GM potato clones with the aphid species Macrosiphum euphorbiae Thomas, a non-target arthropod frequently feeding on potato plants and one of the major pests of that crop. One of the potato events used in our experiments caused an increased fertility of the aphids in the first generation, and consequently, a positive effect on the growth of the aphid population was estimated. When a second generation of the aphid was reared on potato leaves of the same GM event, differences in aphid fertility were no longer observed. Behavioural studies conducted in a wind tunnel using the aphid parasitoid Aphidius ervi Haliday indicated that neither of the two tested GM varieties had a significant effect on the attractiveness of potato plants towards the parasitoid. In planta tests proved to be sensitive protocols to detect unintended effects on a non-target arthropod; experimental results, however, indicate that these effects are not expected to be biologically relevant in this tritrophic system, if these GM events become available for commercial use in the future.

An attempt was made to relate variation in life-history characteristics within a population of Plantago major ssp. pleiosperma to small-scale environmental variability. At a beach plain, embanked in 1966, a mosaic environment was distinguished with spatial variability in vegetation structure as well as in nutrient availability and water content of the soil. Differences between three subsites in contemporary selection were demonstrated, e.g. in shoot morphology and allocation to reproductive tissue. The effects of nutrient supply and waterlogging on morphology and life history were studied on lines from the three subsites in a greenhouse. For most of the traits high levels of phenotypic plasticity were observed, covering almost entirely the observed phenotypic variability at the beach plain. In all treatments lines from the shrubs had, however, a higher leaf-area ratio as well as delayed flowering when compared to lines from more open subsites. In addition, in a reciprocal transplant experiment it was demonstrated that lines from the shrubs had larger shoots with e.g. broader leaves in the shady environment of the shrubs than other lines. From the experiments no indications were obtained that lines from any subsite were especially adapted to specific levels of nutrient supply or water content of the soil. With respect to these environmental factors P. major ssp. pleisoperma might occur and reproduce at all subsites by means of phenotypic plasticity, e.g. in plant form. However, it is suggested that spatial variability in vegetation structure caused a population subdivision in allocation patterns, leaf form and life history at the beach plain, over distances of about 15-25 m. This differentation occurred during primary succession over a period of twenty years.

As part of a research project on the variation in life-history characteristics within a population of Plantago major L. ssp. pleiosperma, seasonal and spatial variability in the availability of macronutrients (N, P, and K) were examined on a small scale in the 0–25 cm soil depth at a primary beach plain site, embanked since 1966. On the basis of distinct differences, among other things, in plant biomass, an a priori division into three different types of microhabitat occurring in a mosaic distribution pattern was made: an overall low-lying area (subsite 1) with slightly elevated patches of 0.5 to 1.5 m in diameter (subsite 2) and rather large patches, 20 to 40 m in diameter, of sea buckthorn shrubs, with small and relatively open spots (subsite 3) in the transitional zone from lower area into scrub. All three subsite types were studied within a total area of approximately 2000 m2. Three methods of analysis were applied: an inventory survey (sampling once at the start of the growing season), an analysis of the seasonal variation (sampling at approximately monthly intervals during the period April-November), and an assessment of nitrogen mineralization potentials in the laboratory (sampling once at the beginning of the growing season). All three procedures clearly demonstrated the occurrence of differences in the availability of nutrients over very short distances, i.e. a pronounced spatial variability among subsites. Particularly the availability of N and P appeared to have increased at the subsites 2 and 3, when compared to subsite 1. This small-scale differentiation in soil properties has occurred in an essentially homogeneous parent material (e.g. in texture and carbonate content) over a period of about 20 years. Besides a spatial variability, statistically significant temporal fluctuations were observed in the availability of N, P, and K. Relative fluctuations of mineral N (as indicated by the range/mean ratio) were especially large at the subsites 2 and 3.

Highlights of a multidisciplinary research program on innovative weed control are presented and discussed in this paper. The program was carried out from 1999 to 2002, and dovetailed most fundamental-strategic and applied research aspects in The Netherlands in that period with respect to weed management. The program was focused on both developing and implementing sustainable weed control strategies for agricultural and non-agricultural areas. Some projects in the program were on (1) developing and improving weed preventive and non-chemical methods, (2) methods that allow the farmer to apply lower dosage of herbicides than indicated on the label, e.g. the so-called "Minimum Lethal Herbicide Dose method" (MLHD), and (3) rational weed control on hard surfaces. The main results of these projects are presented. A successful development and implementation of new methods and systems of weed control that use considerably less herbicides, is determined by many factors. The role that these success factors played in the aforementioned projects is shortly discussed.

Chenopodium album L. seedlings at the 4- and 8-leaf stage were exposed to low concentrations metribuzin [4-amino-6-(1,1-dimethyl)-3-(methylthio)-1,2,4-triazin (4H)-one] in nutrient solution to study herbicide uptake and the effects of low-dose rates. Chlorophyll fluorescence was measured to relate the inhibition of photosynthesis to herbicide dose. The minimum rate at which metribuzin fully inhibited photosynthesis was less than 1 μM for seedlings at the 4-leaf stage of development, and between 1 and 5 μM for the 8-leaf stage seedlings. With isolated chloroplasts, experiments were conducted to establish the relationship between the amount of herbicide molecules bound to each chloroplast and the inhibition of photosynthesis. From the dose-response curves obtained it was calculated that photosynthesis was fully inhibited when 7.5 10 5 molecules metribuzin were bound to each chloroplast. This amount of binding was used to estimate minimum-lethal dose rates of metribuzin required for seedlings differing in fresh weight of leaves and amounts of chloroplasts present. It is suggested that prediction of a low dose herbicide effect from studies on binding of photosystem-II inhibitors in combination with chlorophyll fluorescence measurements may lead to the development of a new weed management strategy.

SUMMARY The recently introduced empirical model for early prediction of crop loss by weed competition based on the relative leaf area of the weeds shortly after crop emergence (Kropff & Spitters, 1991), assumes a maximum yield loss of 100% at high weed infestations. This is biologically not realistic. If weeds have a shorter life cycle than the crop, when they emerge much later than the crop or when they are unable to overtop the crop, maximum yield loss at high weed infestations is expected to be less than 100%. A two parameter model is introduced in this paper that accounts for a variable maximum crop yield loss by weed competition. The parameters are the “relative damage coefficient”q, and a parameter that describes the maximum yield loss caused by the weeds (m). The one and two parameter models were evaluated with data on the effects of weeds on rice, sugarbeet and tomato. Both models were able to describe the data on the effect of different weed densities and periods between crop and weed emergence fairly well. The introduction of the second parameter resulted in a significantly better description of observed data in five out of nine datasets. The value of the parameter q depends on the time of observation of the relative leaf area. The change in the value of q in time could be estimated from the difference in relative growth rate of the species. The accuracy of this estimation is strongly determined by the variability in the relative growth rate of the leaf area of single plants. Possibilities for scientific and practical application of the model are discussed.

During two consecutive years the effects of intercropping fresh market white cabbage with two species of clover on pest populations and yield were studied. White cabbage cv. Minicole was intercropped with Trifolium repens (white clover) and Trifolium subterraneum (subterranean clover) as compared to the monocrop. During the season observations were made on pest population developments, especially of Mamestra brassicae L. (cabbage moth), Brevicoryne brassicae L. (cabbage aphid), Delia brassicae L. (cabbage root fly), and evaluation of caterpillar feeding injury. At harvest the yield in quantity and quality was determined to be able to assess the gross financial result. Intercropping effects in terms of suppression of oviposition and larval populations of various pests were found. Although no pesticides were used and competition reduced the weight, the quality of the intercropped cabbages lead to a better financial result compared to the monocropped cabbage crop. The results are discussed in the perspective of the practical implications in the context of IPM.

Plantago maritima L. is a halophytic species occurring on beach plains, mud flats, salt marshes, sea dikes, coastal grasslands and sea cliffs. A great variety in morphological characteristics can be found; based on differences in leaf characters, Heukels (1910) distinguished three types.

Survival, age and size at first flowering and seed production of five populations of Plantago major were studied in a reciprocal-transplant experiment to investigate if differences in flowering phenology can be explained by different selection regimes. Survival in the pre-reproductive phase was relatively low on a river bank, a beach plain and a path, compared to that in a sea-shore meadow and a dune grassland. Age at first flowering depended on the site as well as on the population from which plants originated (...)

Most grasslands and other areas with a short herbaceous vegetation in The Netherlands will, without grazing or cutting, follow a succession towards woodland. This fact is of crucial importance to all managers of conservation areas who wish to maintain or create diversity in short vegetation, and is now usually recognised in the management of nature reserves. Most of the grasslands currently managed by conservation agencies in The Netherlands were previously grazed or mown for hay. Under these practices, sometimes continued for centuries, a semi-natural landscape developed with a great variety of habitats, rich in plant and animal species.