Journal of Environmental Science and Management 14(2): 13-20 (December 2011) ISSN 0119-1144 Simulation Modeling of Bunchy Top Epidemics in a Changing Climate Avelino D. Raymundo 1 and Ireneo B. Pangga 2 ABSTRACT Epidemics of bunchy top, the most destructive viral disease of abaca and banana in the Philippines, were modeled using STELLA version 9.1, a modeling software appropriate for systems analysis of biological populations. A previously developed coupled model of the population dynamics of the insect vector Pentalonia nigronervosa and epidemic progress of the bunchy top disease was improved and modied by incorporating the effects of temperature. Both insect vector and bunchy top epidemic submodels followed the H-L-S-R (healthy (H) - latent (L) - infectious (S) – removed (R)) epidemic modeling approach where the diseased plants or insect population was partitioned into nonoverlapping compartments or states. The modied model satisfactorily simulated P. nigronervosa population dynamics and bunchy top epidemics. The effect of climate change on bunchy top epidemics was simulated through the addition of 1 and 2°C to the average monthly temperature from 1998-2007 in Davao City, Philippines. The in- crease in monthly average temperatures of 1 and 2°C reduced the simulated bunchy top epidemics as the rates of increase in the number of viruliferous aphids and disease incidence were reduced, and epidemic onset was delayed. Key words: bunchy top disease, abaca, banana, modeling, Pentalonia nigronervosa; simulated climate change INTRODUCTION Bunchy top disease is the most destructive and widespread viral disease of abaca and banana in the Philippines (Raymundo 2000; Raymundo et al. 2001). Epidemics of this disease have devastated abaca plantations in the Bicol region for more than half a century (Raymundo 2000). A nationwide survey showed that the Bicol and Eastern Visayas regions were “hotspots” or areas with the highest concentration of the bunchy top disease, but the disease was also rampant in the CARAGA (Agusan and Surigao) and Southern Mindanao regions (Raymundo et al. 2001). Estimated ber yield losses in 2002 due to abaca bunchy top and mosaic diseases were valued at PhP 18.3 M in the Bicol region and PhP 8.4 M in the Eastern Visayas region (Raymundo et al. 2002). The epidemiology of the bunchy top disease is inuenced by environmental factors which affect both disease incidence and behavior of its aphid vector, Pentalonia nigronervosa. Epidemics can be triggered by the presence of alternate host plants of the vector such as gabi, cania and caladium. Abaca or banana is a perennial crop that guarantees uninterrupted host availability for the vector and virus. The slow disease expression causes virus detection only after it has already successfully infected another plant (Raymundo 2000; Raymundo and Bajet 2000). Bunchy top can be controlled by eradication of diseased plants but this approach has failed to slow down disease spread due to an inadequate understanding of the epidemiology of the disease and population dynamics of the aphid vector. Epidemics continue to spread at a rapid rate as the virus persists and multiplies rapidly as diseased plants are simply cut down allowing the emergence of infected suckers (De la Cruz and Raymundo, 2007). These re-growths, usually not eliminated due to limited resources, serve as ready sources of inoculum. A systems approach, considering all aspects of the host-virus-vector relationship, appears to be needed in bunchy top disease management (Raymundo 2000; Raymundo and Bajet 2000). The complex nature of the host-pathogen relationship of the bunchy top disease justies the need for a holistic approach to manage the disease. Systems modeling allows an integrated view of the whole system behavior and provides a quantitative description and mechanistic understanding of the system (Thornley and Johnson 1990). This study aimed to develop a simulation model linking a bunchy top disease epidemic submodel and a population dynamics submodel of the aphid vector, P. nigronervosa. Outputs of the model can be used in developing integrated and sustainable disease management strategies in response to climate change. MATERIALS AND METHODS Model Development The linked models of bunchy top epidemic and P. nigronervosa population dynamics were developed in STELLA software version 9.1 (ISEE Systems 2008). The original linked models of bunchy top epidemics and P. nigronervosa population dynamics (Raymundo 2001) (Figure 1) were modied by incorporating the effects of temperature using data from the literature. The modied models followed the plant virus - insect vector model of Madden et al. (2000), which was a deterministic model characterizing plant virus disease epidemics in relation to the population dynamics of the insect vectors and four different 1 Professor, Crop Protection Cluster, College of Agriculture, University of the Philippines Los Baños, E-mail: adraymundo@yahoo.com (corresponding author) 2 Assistant Professor, Crop Protection Cluster, College of Agriculture, UPLB