Browsing by Subject "Population dynamics"

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Bromus-Arten in Winterweizen: Verbreitung, Bedeutung und Populationsdynamik
(2005) Moray, Ralph; Hurle, Karl
The increase of reduced tillage in agriculture and winter cereal dominated crop rotations has lead to a rise of annual and perennial grass weeds in the last decades. One of these are Bromus species that occur worldwide in cereal growing areas. In Germany they also attract more and more interest because of their increasing distribution. Therefore an inves-tigation was conducted to assess the current situation of Bromus species in Germany, competition, population biology and control. Additionally, control strategies were designed on the basis of analyses of population-dynamical parameters. A survey carried out in Germany, showed that Bromus sterilis and B. secalinus were found to be the dominant species. The problems that were predominantly mentioned were yield loss and difficulties at harvest which mostly appear in winter wheat. Their occurrence is closely connected with reduced tillage, early seeding and winter cereal dominated crop rotations whereas Bromus densities can be reduced effectively with a mouldboard plough or the increase of spring crops in crop rotation. In dicot. crops the chemical control was regarded as very good, whereas in cereals between good to sufficient. All of the survey participants expected a further distribution of Bromus species on arable land. Field trials were conducted on three locations over a period of three years to investigate the competition of B. sterilis, B. secalinus, B. tectorum and B. japonicus in winter wheat. Weed density and seeding date of winter wheat were considered for the species competition studies. The trials showed that distinct yield losses could be recorded in winter wheat depending on the different Bromus species. Thresholds where calculated based on the relationship between weed density and yield loss. The thresholds for the Bromus species in winter wheat were below the values given in literature for other grass weeds in winter cereals. This highlights the strong competitiveness of the species. Additionally the results showed that with late seeding the weed density decreases distinctively and thereby the yield loss can be reduced by half. Greenhouse trials on the efficiency of the three in Germany registered herbicides for the control of Bromus species showed, that a sufficient control of B. sterilis and B. secalinus could not always be achieved with the recommended dose. The simulation of the population dynamics showed clearly that with reduced tillage and direct seeding in a winter wheat monoculture without control measures a very fast increase of densities within two growing seasons up to a density-dependent level where high yield losses can be expected. In contrast mouldboard ploughing can control the species to 100 % as a depth of seeds of 10 cm already reduces emergence by half and seeds in the trials showed to have longevity in soil of no longer that 12 months. Moreover, it was demonstrated that the soil cultivation has a higher influence on the development of the weed population than the different seeding dates of winter wheat. The simulation under the influ-ence of herbicide application suggested, that it is possible to practice a winter cereal dominated crop rotation without yield losses. From the results of the investigations it can be concluded that Bromus as a problem in win-ter cereal dominated crop rotation in our latitudes will increase. In fact the control with available herbicides is not always sufficient, but for economical reasons integrated control strategies can not always be considered although quite a good control is given. Therefore one can presume that in future the control of Bromus in winter cereals still will be based on the use of herbicides and that these weed species will be as common in win-ter cereals as it became Alopecurus myosuroides and Apera spica-venti.
Density dependence of seed dispersal and fecundity profoundly alters the spread dynamics of plant populations
(2023) Zhu, Jinlei; Lukić, Nataša; Pagel, Jörn; Schurr, Frank M.
Plant population spread has fundamental ecological and evolutionary importance. Both determinants of plant population spread, fecundity and dispersal, can be density‐dependent, which should cause feedback between population densities and spread dynamics. Yet it is poorly understood how density‐dependence affects key characteristics of spread: spread rate at which the location of the furthest forward individual moves, edge depth (the geographical area over which individuals contribute to spread) and population continuity (occupancy of the spreading population). We present a general modelling framework for analysing the effects of density‐dependent fecundity and dispersal on population spread and parameterize this framework with experimental data from a common‐garden experiment using five wind‐dispersed plant species grown at different densities. Our model shows that density‐dependent fecundity and dispersal strongly affect all three population spread characteristics for both exponential and lognormal dispersal kernels. Spread rate and edge depth are strongly correlated but show weaker correlations with population continuity. Positive density‐dependence of fecundity increases all three spread characteristics. Increasingly positive density‐dependence of dispersal increases spread rate and edge depth but generally decreases population continuity. Density‐dependent fecundity and dispersal are largely additive in their effect on spread characteristics. For population continuity, the joint effects of density‐dependent fecundity and dispersal are somewhat contingent on the dispersal kernel. The common‐garden experiment and the experimentally parameterized mechanistic dispersal model revealed density‐dependent fecundity and dispersal across study species. All study species exhibited negatively density‐dependent fecundity, but they differed qualitatively in the density‐dependence of dispersal distance and probability of long‐distance dispersal. The negative density‐dependence of fecundity and dispersal found for three species reinforced each other in reducing spread rate and edge depth. The positively density‐dependent dispersal found for two species markedly increased spread rate and edge depth. Population continuity was hardly affected by population density in all study species except Crepis sancta in which it was strongly reduced by negatively density‐dependent fecundity. Synthesis. Density‐dependent fecundity and seed dispersal profoundly alter population spread. In particular, positively density‐dependent dispersal should promote the spread and genetic diversity of plant populations migrating under climate change but also complicate the control of invasive species.
Integrated weed management in intensive cropping systems : towards reduction of herbicide input
(2016) Jäck, Ortrud; Gerhards, Roland
Weed control in conventional cropping is commonly done using herbicides. Those, however, can have negative side-effects on the environment. The objective of this thesis is to investigate the potential of reducing herbicide input into cropping systems with a focus on two of the most important staple crops, maize and winter wheat. The thesis is divided into five main sections, dealing with different topics, i.e.: 1. Determination of herbicide efficacy 2. Efficacy of reduced herbicide dosages in maize 3. Efficacy of reduced herbicide dosages in winter wheat 4. Integrated weed management 5. Long-term effect of reduced herbicide dosages To examine herbicide efficacy at reduced dosages it is necessary to conduct a wide range of dose-reponse experiments. They are usually time and labour consuming and the assessment of herbicide efficacy is often not objective. To overcome these issues, a novel method for assessing herbicide efficacy using bi-spectral imaging was tested. The results show at the example of Bromus japonicus that weed coverage assessed with the bi-spectral camera system can serve as a non-destructive, rapid and objective method to assess herbicide efficacy. In field experiments the potential for reducing herbicide dosages in summer maize was investigated with several common herbicides and herbicide mixtures. These experiments show that it is possible to reduce herbicide dosages for weed control in maize without influencing weed control efficacy and crop yield. However, the extent to which dosages can be reduced without loss in efficacy is dependent on the herbicide used and the weed growth stage. One of the herbicide tested in the field (topramezone) is usually applied together with a methylated seed oil (MSO) adjuvant. Adjuvants play an important role for herbicide efficacy, but the mechanism for enhancing herbicide efficacy is often not clear, yet. To examine the mechanism of MSO in enhancing efficacy of topramezone, several experiments were conducted. Efficacy of topramzone was significantly increased by addition of MSO. The results show that MSO enhances the uptake and translocation of topramezone in the two tested weed species. Furthermore, physical properties of the spray solution were altered. More precisely, surface tension and thus the contact angle on the leaf was decreased. These results can explain why MSO adjuvant enhances topramezone efficacy. To investigate the potential of reducing herbicide dosages in winter wheat and its impact on weed seed production, experiments were conducted using A. fatua as weed. The results show, that dosages of the four tested herbicides could be tremendously reduced without loss in efficacy. However, A. fatua seed production was influenced by winter wheat competitiveness and herbicide mode of action and did not necessarily follow herbicide efficacy. These results on the one hand highlight the potential of herbicide dosage reduction for controlling A. fatua in winter wheat. But on the other hand, the results point out that decision on herbicide dosage reduction should not only be made on basis of herbicide efficacy data but also on its influence on weed seed production. Reduction of herbicide input cannot only be achieved by reducing herbicide dosages, but also by applying non-chemical weed control methods. These methods may not be able to fully replace herbicide usage, but they can serve as tool to reduce weed pressure and competitiveness. Field experiments were carried out to investigate the effect of adjusted winter wheat seeding rate and nitrogen fertilization on Calystegia hederacea abundance and herbicide control efficacy. The results show that increased seeding rate reduces density of this weed while enhancing herbicide efficacy. Lowering nitrogen fertilization rate towards Nmin based fertilization increased density of C. hederacea. This study points out how adjustment of agronomic parameters can influence weed competitiveness and can serve in enhancing weed control efficacy by herbicides. It had been shown in several studies that cover crops or undersown crops can effectively suppress weed growth. This suppression may not only be due to competition for resources, but also by chemical interaction via allelopathic active compounds. To examine the allelopathic effect of several cover crops, pot and laboratory experiments were carried out. These experiments show, that effects of the tested cover crops can be growth promoting or inhibiting, and are dependent on the species and extract concentration. Furthermore, cover crops can also affect growth of the crop. To discuss the long-term effect of reduced herbicide dosages on weed population development, a simulation model was set-up at the example of winter wheat and A. fatua. Different strategies for herbicide input reduction were simulated. This paper highlights the potential of reducing total herbicide input without population increase, while keeping grain yield and net return at high level. The presented articles work out the risks and potential for herbicide dosage reduction and point out the possibilities of using integrated weed management options for enhancing weed suppression.
The host parasite relation of the parasitic mite Varroa destructor (Anderson and Trueman) and the honeybee races A. m. syriaca (Skorikov) and A. m. carnica (Pollmann) in Jordan.
(2006) Al-Attal, Yehya Zaki Khalid; Zebitz, Claus P. W.
Since the honeybee mite Varroa destructor (Anderson and Trueman) succeeded to parasitize the Western honeybee Apis mellifera L. and accept this species as a new host it became the most serious threat to Apiculture worldwide. The very few stable relations between Varroa mites and the new host are either associated with honeybees of African origin, or with tropical and sub-tropical climates. This balanced relations seems to be due to reduced fertility of the female mites in worker brood cells, a shorter post-capping period of the worker brood and a set of highly differentiated active defense traits of the host bees. In this work I investigated several aspects of the host parasite relation between the honeybee and the Varroa mites in Jordan. The endemic honeybee race of Jordan is A. m. syriaca Skorikov, which join African and European bee traits. The Jordanian beekeepers use this ?local? bee as well as its hybrids with imported European bees. Therefore, Jordan provides the possibility to prove the influence of different honeybee races and different climatic conditions on this host-parasite system. For a better direct comparison, I evaluated the host-parasite relation of two honeybee races at the same study site: the ?local? honeybee (A. m. syriaca) and the imported ?carnica? honeybee (A.m. carnica Pollmann), which is susceptible to Varroa infestation under Central Europe conditions.To assess the current status of Varroa mite in Jordan, we surveyed the infestation rates of in capped brood cells and on adult worker bees in 180 honeybee colonies at six locations. All colonies were kept untreated for at least 8 month. The results revealed high infestation rates, which exceed, in part, the thresholds for colony damages. No significant differences between honeybee races or climatic condition were visible. The population dynamics of the host and its parasite represents the most important parameter of the honeybee-Varroa relation and was described in the ?local? and the ?carnica? (imported from Hohenheim) honeybees at Baqa (dry Mediterranean climate) and Yadodeh (wet Mediterranean climate) for a one-year period. In all colonies the number of adult bees and brood cells were evaluated every three weeks by the ?Liebefeld? method. During all evaluations, samples of adult bees and capped brood of all colonies were analyzed to determine Varroa infestation rates. The population dynamics of the honeybee colonies revealed a significantly higher population density of the ?local? honeybee race compared to the ?carnica? colonies. The average number of adult worker bees was 8,368 ± 2,724 in the ?local? colonies and 6,447 ± 2,338 in the ?carnica? colonies, while the average number of capped worker brood cells was 9,164 ± 3,336 in the ?local? and 7,628 ± 3,166 in the ?carnica? colonies. Compared to colonies in Central Europe, my results indicate a surprisingly shorter life span of adult worker bees. The corresponding population dynamics of Varroa mites revealed an exponential growth phase till the maximum infestation and a decreasing phase until the beginning of the next season. The maximum Varroa population density ranged between 2,614 ± 2,190 mites in the ?carnica? colonies and 4,397 ± 2,746 mites in the ?local? colonies. Using an exponential function, growth rate = eb, the average exponential growth rate of Varroa population per three weeks interval ranged between 1.33 and 1.46 and was significantly different between both locations. The subsequent decrease in the mite population was two folds higher than the decrease in the effective bee population (adult bees plus capped worker brood cells together). Therefore, a higher mortality rate of the parasites or its host activity must contribute to the observed drastic decrease of the Varroa population. No significant race-specific differences in the infestation rates could be observed. However, the mortality rate was higher in the ?carnica? colonies (? 40%) compared the local colonies (? 10%). This indicates a general higher fitness of the ?local? colonies independent from Varroa infestation rates. As a threshold for the survival of honeybee colonies, maximum infestation rates of 20% in adult worker bees and 40% in capped worker brood were determined. The evaluated resistance mechanisms, which are considered to contribute to stable host-parasite relations, did not reveal any pre-adaptation of the ?local? honeybee to Varroa mite. Neither in the fertility of female mites nor in the reproductive rate significant differences between the local and the ?carnica? honey bee race could be detected. The post-capping period of the worker brood was nearly the same in both bee races and it corresponds to data from Central Europe. Also no significant difference could be revealed in the daily mite mortality between both races, which ranged between 0.8% and 1.5% of the total mite population in the colonies. Nevertheless, in few individual ?local? colonies, the mite mortality comprises a surprisingly high percentage of the total estimated number of the phoretic mites within the colonies. By RFLP of the CO-I unit, only the V. destructor Korean haplotype was detected in all examined mite samples from Jordan. Additionally, mite genotyping based on the sequences of two genetic markers shows very low genetic variability among different mite populations, which confirm recent publications and makes the hypothesis that differences in mite virulence could be responsible for a stable host parasite relationship, less probable. Conclusions ? The ?local? honeybee of Jordan is not more resistant to Varrosis compared to the susceptible European honeybee races. ? The Mediterranean climate has no significant inhibition effect on Varroa population dynamics. ? Varroa infested ?local? honey bee colonies revealed a significant higher survival rate than imported ?carnica? colonies under the same conditions. ? Variation in the mite infectivity between different V. destructor population is less probable.
Vermehrungs- und Schadpotential der Grünen Gurkenlaus (Aphis gossypii Glover) an Gewächshausgurke (Cucumis sativus L.)
(2003) Bünger, Isa Brigitte Annemarie; Zebitz, Claus P. W.
The aim of this work was to collect basic data for the development of a model which could simulate the injury of greenhouse cucumbers caused by Aphis gossypii G.. This work mainly deals with two aspects: first it is the population dynamics of A. gossypii influenced by infestation density (initial infestation 2-10 individuals per plant) and host plant quality (age) and second it is the quantitative and qualitative assessment of damage by investigation of growth, yield, and chemical composition of the cucumber plants. The investigation took place under controlled, semicontrolld and practical conditions.