On damage, uncertainty and risk in supervised control : aphids and brown rust in winter wheat as an example

In supervised control of pests and diseases real-time information on pest and disease intensity and cost-benefit analysis of decision alternatives are used to support farmers' decisions on pesticide application. This thesis focuses on improving supervised control in the pathosystem aphids - brown rust - winter wheat. First, effects of the grain aphid, Sitobion avenae , on crop physiology are studied to arrive at simple relations between aphid density and damage. Second, the consequences of uncertainty about the costs of alternative strategies for optimal timing of chemical control of aphids and/or brown rust ( Pucciniarecondita ) are evaluated.Uptake of assimilates by S. avenae and reduction of the rate of net leaf photosynthesis by honeydew were quantified and introduced into a model which simulates growth and development of winter wheat from flowering till ripeness. Attainable yields from 4000 to 10000 kg ha -1and damage at the higher yield levels were simulated satisfactorily. Honeydew caused more damage than assimilate uptake at crop development stages until milky ripeness. Attainable yield, crop development stage, and timing and intensity of infestation were of major importance in explaining variation in damage. The results of the simulation model were summarized in regression models which predicted damage at given infestation levels as good as the best published empirical model.Sources of uncertainty in a decision model for evaluating costs of chemical control strategies were quantified using field data. Assessment of the contribution of various sources of uncertainty to model output uncertainty showed white noise in the relative growth rates of both aphids and brown rust to be the major cause of uncertainty. Risk-neutral damage thresholds representing pest intensities at which expected costs of control just exceed expected costs of no control, were considerably lower than the deterministic damage thresholds based on average input values. At the risk-neutral damage thresholds only 30% of chemical applications was found to be cost-effective. To support pest control under uncertainty a framework is proposed in which alternative strategies are evaluated in terms of 'profitability' and 'risk', operational measures of the economic objectives 'return on investment' and 'insurance', respectively

Spatial variation in biodiversity, soil degradation and productivity in agricultural landscapes in the highlands of Tigray, northern Ethiopia

There is a growing concern about food security and sustainability of agricultural production in developing countries. However, there are limited attempts to quantify agro-biodiversity losses and relate these losses to soil degradation and crop productivity, particularly in Tigray, Ethiopia. In this study, spatial variation in agro-biodiversity and soil degradation was assessed in 2000 and 2005 at 151 farms in relation to farm, productivity, wealth, social, developmental and topographic characteristics in Tigray, northern Ethiopia. A significant decrease in agro-biodiversity was documented between 2000 and 2005, mainly associated with inorganic fertilizer use, number of credit sources and proximity to towns and major roads. Agro-biodiversity was higher at farms with higher soil fertility (available P and total N) and higher productivity (total caloric crop yield). Low soil organic matter, few crop selection criteria and steep slopes contributed to soil erosion. Sparsely and intensively cultivated land use types, as determined from satellite images, were associated with high and low agro-biodiversity classes, respectively, as determined during on-farm surveys in 2005. This study gives insight into the recent changes in and current status of agro-biodiversity and soil degradation at different spatial scales, which can help to improve food security through the maintenance of agro-biodiversity resource