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Faster-acting recombinant baculoviruses have shown potential for improved suppression of insect pests, but their ecological impact on target and nontarget hosts and naturally occurring pathogens needs to be assessed. Previous studies have focused on the fitness of recombinants at the between-hosts level. However, the population structure of the transmission stages will also be decided by within-host selection. Here we have experimentally quantified the within-host competitive fitness of a fast-acting recombinant Autographa californica multicapsid nucleopolyhedrovirus missing the endogenous egt gene (vEGTDEL), by means of direct competition in single- and serial-passage experiments with its parental virus. Quantitative real-time PCR was employed to determine the ratio of these two viruses in passaged mixtures. We found that vEGTDEL had reduced within-host fitness: per passage the ratio of wild type to vEGTDEL was on average enhanced by a factor of 1.53 (single passage) and 1.68 (serial passage). There is also frequency-dependence: the higher the frequency of vEGTDEL, the stronger the selection against it is. Additionally, the virus ratio is a predictor of time to host death and virus yield. Our results show that egt is important to within-host fitness and allow for a more complete assessment of the ecological impact of recombinant baculovirus release.

Invasive plant species can be controlled by introducing one or more of their natural enemies (herbivores) from their native range; however such introduction entails the risk that the introduced natural enemy will attack indigenous plant species in the area of introduction. The effect of spillover of a natural enemy from a managed ecosystem compartment (agriculture) in the area of introduction to a natural compartment (non-managed) in which an indigenous plant species is attacked by the introduced natural enemy, whereas another indigenous plant species, which competes with the first, is not attacked, has been studied. The combination of competition and herbivory may result in extinction of the attacked wild plant species. Using a modelling approach, the authors have determined model parameters that characterize the risk of extinction. The findings point to the importance of spillover and the relative attack rates (specificity) of introduced natural enemies with respect to target and non-target plant species

Optimal allocation of available resources to minimize quarantine risks related to international trade is a problem facing plant protection agencies worldwide. In this paper a model of budget allocation to minimize quarantine risks is developed. Theoretical conditions that budget allocation should satisfy are derived. These conditions imply that optimal allocation of resources is achieved when the marginal pest risks are equalized across risky pathways. Furthermore, an empirical model of budget distribution is developed. In the empirical model, the protecting agency wants to minimize the expected number of infested ornamental plants imported in a given country. The model is parameterized using data on import of ornamental commodities, the associated quarantine risks and costs of import phytosanitary inspections pertaining to the Netherlands. The results of the empirical model suggest that under specific assumptions (such as constant risk) greater risk reduction can be achieved by allocating larger funds to inspection of riskier pathways, and less or no funds to less risky pathways. The protecting agency has to trade off the risks from pathways that vary in terms of risk. Keywords: optimal inspection; quarantine pest; ornamental plants

Crop weeds are patchily distributed. In weed management it is important to be able to estimate the weed-free fraction of the total field area, because this fraction determines the potential saving on herbicides that may be achieved by site-specific application (and not spraying those patches with no weeds). In this chapter, we model the weed-free fraction by combining Taylor¿s power law (TPL) for the variance-mean relationship with a prediction of the zero class frequency according to the negative binomial distribution. The resulting predictions of occupancy were compared to observations on weed density and occupancy in 32 data sets on occurrence of agricultural weeds in The Netherlands. The results using weed species specific parameters for TPL provided strong validation for the approach, with R2 prediction varying between 0.735 and 0.998 for 13 of the 14 species groups. Estimates of the slope parameter b of TPL varied substantially between weeds (from 0.78 for volunteer potatoes to 1.95 for Echinochloa crus-galli), but only slightly between data sets. Predictions based on a common slope parameter still had high coefficients of prediction for most weed species. Based upon a spatially explicit data set collected using counts in contiguous quadrats, the effect of scale of the sample unit was analysed. At levels of scale relevant to decision making in weed management, the effect of scale on occupancy was minor. We conclude that the relationship between density and occupancy for arable weeds is strong, and that there is scope for prediction of the weed-free area and prediction-based weed management

Quarantine diseases comprise a distinct class of plant diseases. In contrast to other plant diseases, direct losses through crop damage are often limited. Yet, quarantine diseases may have serious economic consequences for a country as they threaten the country¿s export of affected crops. In particular for this category of diseases, it is important to design a control strategy that is optimal from an epidemiological as well as an economic point of view. This chapter presents the development of a bioeconomic model to evaluate control strategies in terms of their cost-effectiveness, specified for brown rot in the Dutch potato production chain. The conceptual model consists of two modules: an epidemiological module, which is a stochastic, spatially explicit simulation model that simulates the spread of potato brown rot over all potato-growing farms and fields in the Netherlands, and an economic module, which calculates the total costs of brownrot prevalence, based on the results of the epidemiological model. The model is applied for two brown-rot policy scenarios, which differ in sampling frequency of harvested potato lots but are otherwise similar. The two scenarios are compared with respect to their effectiveness and efficiency. Concerning the costs of controlling brown rot, a low monitoring level appears to be more cost-efficient; however, when including expected export consequences, a high monitoring level may be preferable. The model presented here strongly facilitates the development of an optimal control strategy as it provides insight into the effectiveness of brown-rot control strategies in relation to their costs. Moreover, the introduced modelling concept can be a useful tool in analysing the epidemiological and economic effects of other (quarantine) diseases.

Contribution to Proceedings

Sinds 1995 is de Nederlandse aardappelketen getroffen door diverse uitbraken van bruinrot, een quarantaineziekte die veroorzaakt wordt door de bacterie Ralstonia solanacearum race 3, biovar 2. Dit artikel gaat over het voorkomen van bruinrot, omdat het economisch belang daarvan erg groot is. De overheid vraagt zich af of het beleid nog wel kosteneffectief is en of er zaken verbeterd kunnen worden. Dit was het onderwerp van het promotieonderzoek ‘bio-economische modellering van bruinrot in de Nederlandse aardappelproductieketen’. Een verslag over de conclusies uit dat onderzoek, en een overzicht van beheersstrategieën en economische gevolgen van bruinrot

Uit empirisch onderzoek wordt steeds duidelijker dat de inrichting van het agrarisch landschap de onderdrukking van plaagorganismen in de landbouw kan beïnvloeden. In dit artikel wordt geïllustreerd hoe simulatiemethoden van ruimtelijke dynamische interacties tussen natuurlijke vijanden en plaagorganismen op landschapsschaal gebruikt kunnen worden om meer inzicht te krijgen in de werking van biologische bestrijding in een landschapscontext. Dergelijke modellen kunnen goed helpen bij het zoeken naar een landschapsinrichting die biologische bestrijding maximaal ten goede komt

Phytophthora infestans is, vanuit populatiedynamisch oogpunt, misschien wel een van de meest sprekende voorbeelden van een opportunist (r-strategist) die we in de landbouw kennen. Grote populaties ontstaan tijdens het groeizeizoen en produceren enorme aantallen nakomelingen. Deze worden verspreid en in een enkel geval valt een nakomeling, in dit geval een sporangium, ín vruchtbare aarde' waarna infectie kan ontstaan. Wat daarvan het effect is in een regio met een hoge aardappeldichtheid, onder voor Phytophthora gunstige omstandigheden, laat het haardenkaartje van het MasterPlan Phytophthora voor het seizoen 2007 duidelijk zien

1. The desert locust is a migratory pest whose population development in remote areas must be monitored to prevent outbreaks, upsurges and plagues. Monitoring would be very much facilitated if the area of search could be restricted to sites of likely population increase. 2. The spatial distribution of solitarious desert locusts on the Red Sea coastal plain of Sudan was determined over 3 years from November to March. Additional observations were made on habitat factors, such as plant community, soil texture, soil moisture and land use. 3. Locust densities varied according to the amount and distribution of rainfall and longevity of the annual green vegetation, with virtually no locusts being observed in the driest season. 4. Samples on a grid of 120 sites within a 120-km stretch of coastal plain showed that locusts were prevalent only in the millet-Heliotropium plant community, which is found at sites with a fine sandy soil texture and comparatively high and long-lasting soil moisture in wadi deltas. These sites constitute less than 5% of the area of this part of the plain. 5. An accessory study showed association between desert locust distribution and millet cropping in an area where no Heliotropium was found. Other samples confirmed the association between solitarious desert locust and millet agriculture. 6. Synthesis and applications. The results indicate that surveys for early detection and control of desert locust on the Red Sea coast of Sudan can focus on millet cropping areas. The results suggest that the efficiency of monitoring migratory pest outbreaks in remote areas could be enhanced by using associations between plant communities and herbivorous insects to predict risk areas and target survey efforts.

Samenvatting van de voordracht te houden op 30 november 2005 tijdens de Najaarsvergadering van de KNPV (Koninklijke Nederlandse Plantenziektekundige Vereniging). Resultaten uit een onderzoek naar het gedrag van onkruidpatronen in ruimte en tijd in de continue maisteelt

Samenvatting van de voordracht te houden op 30 november 2005 tijdens de Najaarsvergadering van de KNPV (Koninklijke Nederlandse Plantenziektekundige Vereniging)

Thema op de gewasbeschermingsdag was geïntegreerde gewasbescherming en het resulterend milieutechnisch resultaat

Auteurs en titels van de samenvattingen: 1) J.G.N. Wander, H.G. Spits, G.T.J. Kessel en H.T.A,M. Schepers. BOS [BeslissingsOndersteunendeSystemen] veldbeoordeling gefocust op resistentie verschillen tussen rassen; 2) Wopke van der Werf, Peter Skeksey, Diedert Spijkerboer, Walter A.H. Rossing en Geert J.T. Kessel. Van landschapsecologie naar ziektebeheersing: ruimtelijke epidemiologie van Phytophthora infestans; 3) L.P.N.M. Kroon en W.G. Flier. Waardplant-specificiteit in Phytophthora soorten; verbreding van de waardplantreeks door interspecifieke hybridisatie en adaptatie; 4)M. Hospers. Biologische aardappelteelt in Europa; inventarisatie van Phytophthora management in de praktijk

A research project is being undertaken to design a bio-economic model of potato brown rot (Ralstonia solanacearum) in the Netherlands. The first part of the project consists of an analysis of the past and current brown rot policy for its effectiveness and costs. Then, a stochastic, bio-economic model will be designed that shows the relationship between the costs of control measures and their effect on the probability of brown rot. This model will eventually be used to determine optimal control strategies and to make recommendations for improvement of the Dutch brown rot policy

The multicapsid nucleopolyhedrovirus, SeMNPV, is naturally maintained in Spodoptera exigua populations by vertical transmission from females to eggs and by horizontal transmission from cadavers of caterpillars to other caterpillars. Laboratory and greenhouse experiments were conducted to quantify transmission rates and provide baseline data to assess the potential of this virus to maintain itself naturally in pest populations after application in biocontrol programs. As to vertical transmission, 18% of the first instar larvae originating from egg batches produced by infected moths contracted SeMNPV, while 34% of the egg batches gave rise to one or more infected first instar larvae. As only few large egg batches were virus-free, 70% of all uninfescted larvae hatched in egg batches that contained one or more infected larvae. Substantial horizontal transmisson was observed under greenhouse conditions. Presence of 0, 1 or 10 infected first instar larvae in a batch of 100 first instar larvae resulted in survival to the fifth instar of 41, 30 and 20%, respectively. However, the effect on crop injury was marginal. The observed high transmission rates favour the maintenance of SeMNPV in S. exigua populations in greenhouses.