Weather Driven Psyllid Movement Within and Between Citrus Orchards

Huanglongbin (HLB) is a disease that affects citrus plants that is thought to be caused by the bacterium Candidatus Liberibacter asiaticus. It is commonly referred to as “citrus greening”. Transmission of the bacteria occurs via grafting or a insect vector, one of two psyllid species. There are no symptoms exclusive to HLB, and not all symptoms may be seen when infection occurs, but common symptoms include yellow shoots, blotchy leaf mottle, lopsided fruits with colour inversion, and aborted seeds. HLB has had a devastating effect on citrus farming in affected areas, including Florida. We have developed a mathematical model for the movement of psyllids within and between citrus orchards, that includes the lifecycle of the psyllids and the infected status of the psyllids and trees.

Temperature, wind and precipitation all have an effect the life-cycle and movement of the vector and thus the location that HLB is found. Temperature has both a direct and indirect effect on psyllids; direct through alteration of their de- velopment cycle; and indirect by altering the temperature dependent flush cycles of citrus trees which they rely on. In particular, during the years where the average temperature is high it takes less time for the flush to harden, giving less time for the psyllids to feed. Subsequently, the number of psyllids would decrease.

We perform simulations using IBM’s Deep Thunder, a high spatial and temporal- resolution forecasting system, which is based, in part, on the ARW core of the Weather Research and Forecasting (WRF) model. We have combined high resolution historical simulations of weather with psyllid trap data to build and examine a mode of the psyllid lifecycle and movement. We have conducted a sensitivity analysis of the model to identify key parameters that need further clarification and refinement for mathematical models to be leveraged in identifying optimal control strategies.