Florida’s citrus groves have faced enormous challenges over the last two decades. The Asian citrus psyllid has spread citrus greening disease, known as Huanglongbing, or HLB, to trees across the state. The result is a decline in citrus production of more than 72 percent. To fight the pest, growers often rely on imidacloprid, or IDP, a systemic insecticide that trees take up through their roots. But Florida’s sandy soils are highly porous and low in organic matter. These conditions increase the risk that pesticides can move rapidly through the soil and contaminate shallow groundwater.
A new study from the University of Florida Institute of Food and Agricultural Sciences and collaborators in Spain examines this issue closely. Scientists looked at how water and IDP move through sandy soils under both saturated, or heavy rainfall and flooding, and unsaturated, or typical field, conditions. The research comes from Qudus Uthman’s 2023 doctoral dissertation. He earned his Ph.D. from the UF/IFAS Department of Soil, Water, and Ecosystem Sciences (SWES).
Why Sandy Soils Matter
Florida’s Entisols and Ultisols contain more than 90 percent sand and less than 0.5 percent organic matter. These conditions make it challenging for farmers to apply and control fertilizers and pesticides. Large rain events or too much irrigation can quickly saturate the citrus critical root zone (CCRZ). This zone extends 60 centimeters from the surface into the soil. Too much water will flush pesticides below the root zone and into groundwater.
“Understanding how IDP behaves in these soils is crucial,” said Uthman, the study’s lead author. “While we found that water moves one and a half to two times faster than IDP in sandy soils, the pesticide can still leach below the root zone under prolonged rainfall.”
The Research Approach
The team combined field experiments, laboratory column studies, and computer modeling to track water and pesticide transport. In the laboratory, they used saturated soil columns to test how IDP moved under fully soaked conditions. In the field, soil moisture was continuously monitored at multiple depths over nearly a year, including during major rainfall events such as Hurricane Ian. The WAVE computer model then simulated how water and IDP move through unsaturated sandy soils. (WAVE stands for Water and Agrochemicals in the Soil, Crop and Vadose Environment.)
“By pairing real-world measurements with advanced models, we captured both the complexity of Florida’s soils and the unpredictability of its climate,” Uthman said. “This dual approach provides a much stronger basis for recommending management practices to growers.”
To refine the models, the researchers ran more than 70,000 simulations. They used global search and optimization techniques to match field observations with predicted data.
Results and Impact
The study showed that IDP can move beyond the root zone after several hours of consistent rainfall. While water percolated faster, the pesticide still leached to depths of 45 to 60 centimeters within a week under field conditions. Topsoil layers with more organic carbon slowed pesticide movement. This suggests organic amendments, such as compost, could help keep IDP in the upper root zone, making it more available to trees. The WAVE model effectively reproduced both water flow and pesticide transport, offering a valuable decision-support tool for growers.
“Combining field data with advanced modeling helps us better understand when and how pesticides may move beyond the root zone,” said Davie Kadyampakeni, an associate professor in SWES and Uthman’s faculty adviser. “These insights can guide irrigation and soil management practices that protect both crop health and water quality.”
Moving forward, the researchers say more work is needed to refine pesticide modeling under real-world field conditions. That should give Florida citrus growers better guidelines to manage water and pesticide applications more sustainably.
The article, “Differential Water and Imidacloprid Transport Under Unsaturated Florida Citrus Field Conditions,” is published in the Vadose Zone Journal. The full article is available in the journal.
5
