D MORGAN, I BARKER and KFA WALTERS

Central Science Laboratory, Sand Hutton, York, YO4 1LZ, UK

Background and objectives
Barley yellow dwarf virus (BYDV) is a important disease of cereals and grasses throughout the world causing significant losses to crops. In the UK it is a particular problem of early-sown winter cereals with infection of newly-emerging crops occurring in September and October. Introduction of the disease into the crop (primary infection) is the result of viruliferous winged (alate) aphids migrating into fields and subsequent spread of the disease (secondary spread) results from the dispersal of their infectious wingless (apterous) offspring.

Traditionally management of the virus has depended upon the application of pesticides to control aphid vector populations. However pesticides tend to be applied routinely and irrespective of the risk of virus infection. Although an empirical infectivity index has been devised in the UK, based on a combination of alate cereal aphids caught in suction traps and the proportion carrying virus, it has proven unreliable for effective management of the disease as it does not take into account secondary spread of virus.

A reliable forecasting system is needed based upon thorough understanding of the biological processes involved. However, until recently little was known about the factors which determine the introduction and spread of BYDV [1] and no consideration had been given to modelling the spatial dynamics of vector dispersal and subsequent transmission of virus. Thus the aim of the project was to develop a computer model which predicted the spread of BYDV and which would underpin a rational Decision Support System (DSS) for both vectors and virus alike.

Results and conclusions
A stochastic individual-based simulation model has been developed integrating sub-models describing the population dynamics and behaviour of aphid vectors, and epidemiology of virus [2]. The system utilises a cellular automata approach so that each individual aphid and plant within the field is monitored by the model, which utilises routines to simulate aphid development, reproduction and mortality, and the dispersion of vectors and virus between plants.

The predicted spatial dynamics and temporal incidence of both aphids and virus are similar to those observed in cereal fields. Furthermore the model has provided useful insights into understanding the complex interactions between biological processes involved in BYDV epidemiolgy.

References
1. Mann JA, Harrington R, Morgan D et al., 1996. In Proceedings 1996 British Crop Protection Conference - Pests and Diseases 1, 179-184.
2. Morgan D, Morse DR, 1996. Aspects of Applied Biology 46, 257-262.