Quantitative Ecological Dynamics Lab @ USF

Huanglongbing (HLB)

© A. Barra, 2004
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HLB, or citrus greening, is a devastating citrus disease native to Asia. In the last 10 years it invaded the Western Hemisphere, primarily Brazil and Florida, where it has spread rapidly. HLB is undermining the viability of an important international industry, and possibly endangering the long-term persistence of multiple species of citrus. Further, nowhere in the world is it under adequate control. Improving our understanding of the disease, its transmission, and developing effective control and management strategies is vitally important to managing the disease and mitigating the impact on commercial production. Further, because of the speed of spread and the economic and ecological costs of the disease, acting quickly and efficiently is key.

HLB is caused by three bacteria: Candidatus Liberibacter asiaticus (CLas), Candidatus Liberibacter africanus, and Candidatus Liberibacter americanus. The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is the primary vector. The disease causes chlorosis of leaves, dieback, and, in severe cases, tree death. Infected trees develop fruit that is of poor quality and drops early, reducing yields of edible and marketable fruit from diseased trees.

It has been widely recognized that controlling the spread of HLB requires better understanding of the bacteria/psyllid/tree system and the abiotic factors that impact spread and transmission. Empirical work has helped to elucidate details of many of components of the HLB pathosystem.

We are developing mathematical models of the spread of HLB in space and time, incorporating known details of the ACP life-cycle, within tree dynamics, and transmission propensities and the impact of various abiotic factors on all of the above. Further, we are incorporating possible control/mitigation strategies. Mathematical models can provide insights into the cost effectiveness of intervention strategies faster than almost any other approach. Importantly, they can consider the cost effectiveness of many possible combinations of viable interventions much more quickly and effectively than large-scale field manipulations. In addition to guiding intervention strategies, models can provide feedback on data collection and experimental efforts. Sensitivity analyses can tell us which variables likely have the biggest effects on outcomes, such as citrus yields and HLB burdens, and which contribute the most to uncertainty in outcomes.

Preliminary Models

We developed a preliminary model for HLB that we presented at the International Research Conference on HLB (IRCHLB). It is based on the classic Ross-McDonald compartment model for the spread a vector-borne disease.

Graphical representation of a simple model for HLB transmission. RED diamond → vectors, BLUE squares → hosts. S/E/I/R → Susceptible/Exposed/Infected/Removed. Solid lines → individuals moving between compartments; dashed lines → route of infection; dotted lines → demographic processes in the vectors.
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Further, we include:

Interactive Simulations:

Below we link to two simple, interactive examples of our preliminary model in action. Each incorporates a single type of intervention that the user can adjust to see the impact on epidemic dynamics and on expected economic output, based on very simple assumptions. You will need to install and allow the Wolfram CDF player Plug-in. Note - the simulations can take a few minutes to run when settings are changed.

THESE MODELS ARE STILL IN DEVELOPMENT and are not intended to provide intervention guidance at this time.

We are very keen to receive feedback from HLB community stakeholders on the details of bacteria/psyllid/tree system and the possible interventions (current and in development) that you would most like to see incorporated and tested in these types of models. So try your hand at controlling electronic HLB, and feel free to e-mail us (lrjohnson AT usf DOT edu) with your comments and suggestions.






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