Browsing by Subject "Struktur"

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Leg attachment and egg adhesion of the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae) to different surfaces.
(2014) Al Bitar, Loris; Zebitz, Claus P. W.
Adults of Cydia pomonella live on host plant surfaces, differing considerably in their structural, chemical, and physicochemical characteristics according to host plant species, cultivar, plant organ, phenological stage, environmental conditions, and orchard management practices. This variable world provided by plant surfaces can profoundly affect many aspects of insect–plant interactions, such as attachment, locomotion, oviposition site selection, egg adhesion, and also survival of adults and their offsprings. Despite their importance, little attention has been given to the structural and wetting properties of the codling moth’s host plant surface and their effect on insect–plant interactions of this important pest. Therefore, studies in this thesis were undertaken to investigate the effect of structural and physicochemical characteristics of the substrate on two main codling moth-plant interactions: (1) the attachment ability of adults, and (2) the adhesion of their eggs. The first part of this thesis was performed to (1) analyze tarsal morphology of male and female C. pomonella to know more about their pretarsal attachment devices, and (2) to investigate their attachment ability on a variety of smooth and rough substrates, using a centrifugal force device. On all smooth artificial substrates tested, both sexes of C. pomonella adults achieved excellent attachment ability, by means of their smooth, flexible and well developed arolia. Hydrophobicity of the substrate had no considerable effect on friction forces. Cydia pomonella females showed a very good attachment ability to the smooth Plexiglas substrate in both horizontal and vertical positions. Thus, it can be concluded that the attachment system of C. pomonella is rather robust against physicochemical properties of the substrate and is able to achieve a very good attachment on vertical and horizontal plant surfaces. Results on the epoxy resin substrates, differing only in surface asperity size ranging from 0-12 µm revealed that the attachment ability of both sexes was significantly affected by surface roughness. Maximal friction force was measured on the smooth substrate whereas minimal friction force was assessed on microrough substrates with 0.3 µm and 1.0 µm size of asperities. On the remaining rough substrates, friction forces were significantly higher but still lower than on the smooth substrate. Both sexes generated similar friction forces on the same substrate, in spite of the considerable difference in their body mass, suggesting that both sexes attach effectively to variable rough plant surfaces in their habitat. However, since smooth surfaces have been reported previously to be the most favorable substrates for ovipositing females of C. pomonella, it is possible that they use their attachment system to sense the substrate texture and prefer those substrates to which their arolia attach the best. A better survival of the codling moth offspring is assumed to be ensured by the selection of suitable oviposition sites by females, as well as by a proper adhesion of deposited eggs to these sites. In apple orchards, eggs of the first generation of the codling moth are laid on leaf surfaces in the vicinity to small fruits, later in the growing season, most eggs are deposited directly on fruits. In the second part of this thesis, egg adhesion of the codling moth to different leaf and fruit surfaces of the domestic apple was investigated by measuring the pull-off force required to detach the eggs from the plant surface. Morphology, wettability, and free surface energy of the tested plant surfaces were analyzed to evaluate their role in egg adhesion. Furthermore, eggs and their adhesives covering leaf or fruit surfaces were visualized. Eggs on the smooth upper leaf sides of the tested cultivars were easily detached, requiring similar pull-off forces (total average of 6.0 mN). Up to 2-3 times stronger pull-off forces had to be applied to detach eggs from the trichome-covered lower leaf side, and these forces differed significantly between cultivars, owing mainly to different trichome covered areas. Whereas on the waxy fruit surface of all apple cultivars tested, eggs were very tightly adhered, and required 4-10 fold stronger pull-off forces than those previously measured on upper and lower leaf surfaces of the identical apple cultivars. Cydia pomonella eggs adhered stronger on the upper and middle fruit sections of all cultivars tested, than on the lower section. The influence of plant surface properties on egg adhesion, as well as the mechanisms used by the moth to overcome the presumable anti-adhesive properties of apple fruit surfaces, were discussed. Additionally, the results were debated in the context of the oviposition site selection, female attachment, as well as offspring survival of the codling moth.
Seeing through two lenses: applying actor-centred and structural perspectives to understand farmer innovation and technological change
(2022) Pircher, Thomas; Knierim, Andrea
Science and technology can make a major contribution to ending hunger, achieving food security and improving nutrition. Developing and spreading of innovations in agriculture and nutrition therefore is a main objective of international agricultural research. Despite decades of research, understanding the complex processes around technological change by small-scale farmers remains a challenge for researchers. Whereas behavioural approaches often neglect the influence of the wider innovation systems, approaches that focus on larger systemic challenges tend to downplay human agency. Research approaches that analytically differentiate structural and actor-centred perspectives and their interplay hold potential for more nuanced understanding of farmer innovation and technological change. This thesis reviewed and explored the application of approaches that aim to understand farmer innovation and technological change through the interplay of two analytical lenses: actor-centred and structural. The three empirical research studies addressed different aspects of agriculture and nutrition, and included multiple study locations. Although each of the studies had its own objectives, they all relate to analytical dualism. In this regard, the study approaches separately focused on actor-centred and structural perspectives, and analysed how these influenced each other. Data collection and analysis in the empirical chapters followed these principles by developing and applying adapted conceptualisations of seed systems and agricultural innovation systems. The systematic literature review in Chapter 2 provided an overview on methods for studying farmers’ choices and demand for seed of roots, tuber and banana crops. The review identified 46 studies in which researchers studied various aspects and types of farmers’ demand for seed of five crops in 18 countries. The qualitative analysis and categorization of the identified studies have led into a classification scheme. In one type of studies farmers expressed their preferences and choices through surveys or engagements in trials, auctions, choice experiments and interviews (explicit demand articulation). In another types of studies, researchers characterized farmers’ use of varieties through determinants of adoption, current seed management practices or the functioning of their seed and farming systems (implicit demand articulation). The study of the cassava seed system in Chapter 3 developed and applied a research approach that recognizes the interplay between farmer’s demand for seed and the seed supply functions of the cassava seed system in Nigeria. The farmers maintained and gradually replaced a portfolio of varieties from multiple sources that reflected individual trait preferences. The national agriculture development program alone did not have the capacity to supply farmers with sufficient seed of desired varieties. Exchange between farmers and informal seed sellers contributed to the distribution of seed and new varieties. Informal seed sellers and decentralized seed multipliers have the potential to respond to farmers’ heterogeneous demands. However, they would need continuous support from formal seed system actors to reach underserved markets. The study of innovation and scaling in Kenya and Uganda in Chapter 4 analysed innovation processes in agriculture and nutrition through farmer-centred and structural perspectives. In an international research and development project, researchers introduced farmers to new agriculture and nutrition practices in action learning activities. The farmers selected, adapted and combined the promoted practices according to their individual preferences and needs. In addition to the researchers from the project, a wide range of innovation support providers encouraged farmers to develop innovations in farming, marketing, and nutrition. Promoting farmer innovation processes beyond the project sites and duration would require the engagement of multiple innovation support providers in creating an enabling environment for experimentation and demand articulation. Analysing the empirical chapters with the overarching theoretical framework of the thesis highlights how structural conditions of seed systems and agricultural innovation systems influenced farmer innovation or technological change processes. The cases also illustrate how farmers reacted upon these conditions through social interactions. As a major finding, the analysis points out that the process of structural elaboration – how the agency of farmers influenced structural conditions - remains limited across the empirical chapters. This indicates a need to empower actors in articulating their demands for research and extension services and shaping their institutional environments.
Vergleichende Betrachtung von Mikroklima, Struktur und aus dem Xylemsaftfluss von Bäumen hochskalierter Transpiration eines tropisch-montanen Regenwaldes und eines Wolkenwaldes in Südost-Ecuador
(2010) Ohlemacher, Christian; Küppers, Manfred
In a tropical montane rain forest 1990 m a.s.l. and a cloud forest 2240 m a.s.l. in the Andes of Southeast Ecuador, tree stand structure (tree height, tree basal area, tree sapwood area, tree crown ground projection area and leaf area index (LAI)) was registered and the sap flow rate of trees, the microclimate (photosynthetically active radiation, temperature, air moisture) inside and above the canopy, and short term stem radius changes were measured. The period of measurement was one year, and synchronous measurements exist from 7 months. To investigate, whether the stand conductance for water differs between the two sites, their stand structure, xylem sap flow and microclimate were compared. Annual stand transpiration was calculated by means of upscaling the xylem sap flow, under inclusion of the site specific microclimate, with the basal area as scaling parameter. Mean radial stem growth was derived from stem radius changes. Radial stem growth is higher in the montane rain forest than in the cloud forest by a factor 3-5. No clear hints were found for different conductivity of trees between the investigated stands. The lower annual stand transpiration of the higher sited cloud forest (238 mm/a^-1) than that of the lower sited montane rain forest (438 mm/a^-1) is caused by a lower stand and foliage density (LAI = 1,6 in the cloud forest vs. LAI = 3,7 in the montane rain forest) and the higher air moisture at the higher sited stand. The crown area-related sap flow rate of single trees is approximately equal at both stands.