A simulation model for vegetable － insect pest － insect nucleopolyhedrovirus epidemic system 

 

 Nucleopolyhedroviruses ( NPVs) are highly specific insect pathogens that are infectious to major pest insect species，including the armyworms Spodoptera exigua and Spodoptera litura． Armyworms often have a vast host range including major high value cash crops  in large portions of the world，such as cotton，field vegetables and ornamentals，and all are difficult to control due to the development of insecticide resistance． Therefore，the development of nucleopolyhedroviruses as biocontrol agents for these insect pests is urgent． The practical implementation of nucleopolyhedroviruses is hampered by the long incubation time of NPV － induced disease． Insects continue feeding for 5 － 7 days after infection． A large worldwide genetic engineering effort has been going on in the last decade to develop nucleopolyhedroviruses with improved speed of kill ( Bianchi et al． ，2001，2002) ． Many such viruses have been successfully constructed by the deletion of the egt gene，which codes for an enzyme that protracts the survival and feeding of infected insects and hence increases virus yield as well as damage to the crop． Improved viruses have been also constructed by the insertion of a toxin gene． The field evaluation of these improved insect viruses is in its infancy，due to regulatory constraints，and a modeling framework is desirable to assist in this evaluation． Computer simulation may be used to screen the effectiveness of nucleopolyhedroviruses，to set up field experiments and reduce the number of field experiments when model simulations are in line with field experiments ( Bianchi et al． ，2001) ． Furthermore，simulation models can be used to gain insight in the relative importance of viral characteristics for crop protection． The purpose of this study is to build a simulation model for the epidemiology of genetically improved nucleopolyhedroviruses in field and glasshouse crops，and to assist in the evaluation of the efficacy，persistence and biosafety of these viruses in agroecosystems． Of course，such evaluation can not be made without experimentation，but Modelling helps to focus the questions that should be asked in experimental research． Some questions can be answered with Modelling，e. g． on virus persistence in agro － ecosystems，when main aspects of virus behavior in insects and crops are known． A detailed simulation model for evaluating the efficacy of wild type and genetically modified nucleopolyhedroviruses against S． exigua in glasshouse chrysanthemums was recently developed by Bianchi et al． ( 2001，2002) ． An earlier generalized simulation model developed by Zhang et al． ( 1997) was dedicated to evaluate the epidemiology of polyhedroviruses． Building on the works accomplished by Bianchi et al． and Zhang et al． ，we develop here a model that can be more easily adapted to different crops and cropping systems than the detailed and system － specific model of Bianchi et al． Thus，the model should fulfill the following criteria: ( 1) simple; ( 2) general; ( 3) transparent; ( 4) predictive; and ( 5) easy to use． The model is in principle applicable to various pest insects． Some realism and specificity to the modeled system is inevitably lost in the process of generalization，but the wider applicability of the generalized model will hopefully more than compensate for these drawbacks． The new model will be general enough to be readily adaptable to new cropping systems，yet realistic enough to produce credible outcomes． This will enhance practical application． The model is based on general principles governing the dynamics of insect populations，crop growth characteristics，temperature dynamics，nucleopolyhedrovirus infection cycle，and transmission routes，etc． The model can be used to simulate the dynamics of uninfected and infected insect populations，crop injury， active baculovirus dynamics，etc． The model is applicable to various crops，pest insects，and variable temperature，and the efficacy of genetically modified nucleopolyhedroviruses may be evaluated by using this model． The major objective of this report is to present users with a detailed documentation in which model concept，model description，parameterization，software description，and program codes are included． In the chapters 2 and chapter 3 the model concept and description are described，respectively． The parameterization of the model is described in chapter 4． Chapter 5 describes the software features． Program codes are included in appendices．

The model is programmed in Java，to enable interactive running on the World Wide Web，to allow easy access to users worldwide and for use in teaching programs and online researches and courses． In insect － NPV systems，the nucleopolyhedrovirus can infect and kill the host insect． The elementary infectious units of nucleopolyhedroviruses are nucleocapsids which are embedded in a polyhedron ( Vlak， 1993) ． The larvae of insects ingest leaf material together with the active polyhedra on the leaf． The polyhedra dissolve into infectious nucleocapsids and the latter propagate in the body． After an incubation time which is defined as the time between the moments of ingestion of polyhedra and death，and which is affected by temperature ( Stairs，1978) ，the larvae die of infection and the bodies disintegrate to release the infectious polyhedra． Polyhedra that are ingested by uninfected larva can cause new infections． The susceptibility to virus infection decreases in later instars． Polyhedra are inactivated by the ultra violet light ( Bianchi et al． ，2000，2001) ，which is the major factor responsible for inactivation of polyhedra by direct sunlight in the field ( Ignoffo et al． ，1997; Zhang et al． ，1997) ． Three routes are available for the infection of larvae． The first is the direct ingestion of sprayed polyhedra by uninfected larvae． The second，horizontal transmission，is the ingestion of polyhedra，produced by virus victims within the system，by other larvae． The third route，vertical transmission，is the direct transfer of virus from adults to their eggs． Vertical transmission results when later larval instars ingest polyhedra but do not die off and carry viral infection sublethally into the adult stage ( Smits and Vlak，1988b; Bianchi et al． ， 2001，2002) ．