Wopke van der Werf

Dose dependency of time to death in single and mixed infections with a wildtype and egt deletion strain of Helicoverpa armigera nucleopolyhedrovirus

Recombinant insect nucleopolyhedroviruses lacking the egt gene generally kill their hosts faster than wild-type strains, but the response of insects to mixtures of virus genotypes is less well known. Here, we compared the survival time, lethal dose and occlusion body yield in third instar larvae of Helicoverpa armigera (Hübner) after challenge with wild-type H. armigera SNPV (HaSNPV-wt), a strain with a deletion of the egt gene, HaSNPV-LM2, and a 1:1 mixture of these two virus strains. A range of doses was used to determine whether the total number of OBs influenced the response to challenge with a mixture of virus strains versus single strains. At high virus doses, HaSNPV-LM2 killed H. armigera larvae significantly faster (ca. 20 h) than HaSNPV-wt, but at low doses, there was no significant difference in survival time between the viruses. The survival time after challenge with mixed virus inoculum was significantly different from and intermediate between that of the single viruses at high doses, and not different from that of the single viruses at low doses. No differences in lethal dose were found between single and mixed infections or between virus genotypes. The number of occlusion bodies produced per larva increased with time to death and decreased with virus dose, but no significant differences among virus types were found.

Spatial distribution of populations of solitarious adult desert locust (Schistocerca gregaria Forsk.) on the coastal plain of Sudan

Abstractu2003 1u2002Densities of solitarious adult desert locusts were measured on regular grids of up to 126 sample sites in the southern part of the coastal plain of Sudan during the winters of 1999/2000 and 2000/2001. Geostatistical procedures were used to characterize spatial dependence of locust density, to evaluate the possibility of estimating locust densities at unvisited sites, based on information obtained at surveyed sites, and to create density maps.

A system identification approach for developing and parameterising an agroforestry system model under constrained availability of data

This paper introduces a system identification approach to overcome the problem of insufficient data when developing and parameterising an agroforestry system model. Typically, for these complex systems the number of available data points from actual systems is less than the number of parameters in a (process-based) model. In this paper, we follow a constrained parameter optimization approach, in which the constraints are found from literature or are given by experts. Given the limited a priori systems knowledge and very limited data sets, after decomposition of the parameter estimation problem and after model adaptation, we were able to produce an acceptable correspondence with validation data from a real-world agroforestry experiment.

System Identification in Production Ecology: from theory to agroforestry practice

Abstract This paper introduces a system identification approach to agricultural ecosystems. In particular, the identification of an agroforestry system, combining trees with crops, is subject of study. Typically, for these systems N p , where N is the number of data points and p the number of parameters in a (process-based) model. In this paper, we follow a constrained optimization approach, in which the constraints are found from literature or are given by experts. Given the limited a priori systems knowledge and very limited data sets, after decomposition of the parameter estimation problem and after model adaptation, we were able to produce an acceptable fit to validation data from a real-world agroforestry experiment.