Source or sink? the ole of residential host plants in Asian citrus psyllid infestation of commercial citrus groves

Authors: SETAMOU, MAMOUDOU - Texas A&M University; Patt, Joseph - Joe; Tarshis Moreno, Aleena

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2021
Publication Date: 1/20/2022
Citation: Setamou, M., Patt, J.M., Tarshis Moreno, A.M. 2022. Source or sink? the ole of residential host plants in Asian citrus psyllid infestation of commercial citrus groves. Journal of Economic Entomology. 115:438-445. https://doi.org/10.1093/jee/toab249.
DOI: https://doi.org/10.1093/jee/toab249

Interpretive Summary: From the air, areas in the U.S. that produce citrus typically appear as landscape mosaics of commercial citrus groves interspersed with residential areas where most households have citrus trees on their property. Citrus greening disease (aka Huanglongbing or twisted dragon disease in Chinese) is a deadly disease that has severely reduced citrus production in most parts of the world. The bacterium that causes citrus greening is transmitted from tree to tree by a tiny insect called the Asian citrus psyllid, which feeds only on the sap of citrus trees and a few of their close relatives. Both the psyllid and bacterium originated in Asia and are now found in almost every citrus production area in the world. Asian citrus psyllid is a good flyer and feeds on citrus trees in both commercial groves and residential areas. Because citrus trees grown in residential areas are largely unmanaged, they tend to harbor higher psyllid populations than trees in commercial groves, which are frequently treated with insecticides. Therefore, we postulated that citrus trees in residential areas function as a ‘source’ habitat that provide psyllids which fly to and infest nearby commercial groves. The commercial groves, in turn, function as ‘sink’ habitats because frequent insecticide applications eliminate most psyllids in the groves. Abandoned and poorly managed citrus groves have been shown to function as source habitats for psyllid that colonize nearby operational groves; however, the level of psyllid movement between commercial groves and nearby residential areas has not been elucidated. To determine this, we conducted a study where we marked psyllids with brightly colored fluorescent powders and released them either in: 1) a neighborhood where many residents had citrus trees in their yards; and, 2) commercial citrus groves growing on three sides of the neighborhood site. The psyllids were recaptured on yellow sticky card traps hung on the branches of citrus trees in both the neighborhood site and the adjacent commercial citrus groves. By marking psyllids with different colored-powders, this ‘Mark-Release-Recapture’ study enabled us to determine whether the recaptured psyllids were released in the neighborhood or in one of the adjacent commercial groves. The color coding permitted us to determine the original release date of recaptured psyllids as well. After the recapture period was over, the traps were returned to lab and examined with a microscope. Psyllid stuck to the trap surface were illuminated with UV light, which caused any remaining colored dust particles on the psyllid’s body to fluoresce, and allowed us to determine the color of individual dust particles. A combined total of 15, 300 marked psyllids were released during the study. Although the recapture rate of these marked psyllids was very low (n=35), the results were instructive. Most of the marked psyllids recaptured in residential trees were released within the neighborhood while only 15.4% were released in the commercial groves. In contrast, approximately half of the marked psyllids recovered in commercial groves were released in residential trees. Of all the recaptured psyllids, about 40% changed habitats, but the change was skewed toward movement from the residential to commercial grove habitat. These results strongly suggest that there is a constant exchange of psyllids between the two habitats, with residential citrus trees functioning as a source habitat of psyllids that emigrate into adjacent commercial groves. The results also indicate that the further residential trees are located from commercial groves, the less likely they will be immediate sources of Asian citrus psyllid. We suggest that one way to reduce the risks of citrus grove colonization by Asian citrus psyllid, would be to establish new commercial groves away from residential habitats, when possible. Additionally,

Technical Abstract: Landscapes supporting citriculture in the of the U.S. are typically habitat mosaics of commercial citrus groves interspersed with residential areas supporting a variety of unmanaged citrus. Asian citrus psyllid (Diaphorina citri) the vector of Candidatus Liberibacter asiaticus, the putative causal agent of Huanglongbing or citrus greening disease, feeds on citrus in both habitats. Because citrus trees grown in residential areas are largely unmanaged, they tend to harbor higher D. citri populations than trees in commercial groves, which are frequently treated with insecticides. We postulated that residential areas supporting citrus trees function as a ‘source’ habitat that provide individuals that infest commercial groves. These, in turn, function as ‘sink’ habitats where chemical controls result in local extirpation of D. citri populations. Abandoned and poorly managed groves have been shown to function as source habitats for psyllid colonization of commercial groves; however, the level of psyllid exchange between commercial groves and nearby residential areas has not been elucidated. Here we report on an experimental mark-release-recapture study conducted at the interface of a residential neighborhood with abundant citrus trees with three commercial citrus groves. Adult D. citri marked with colored fluorescent powders were released in both habitats and their movement within and between milieus monitored. A combined total of ca.15,300 marked psyllids were released during the study. Although the recapture rate of marked psyllids was very low (n= 35), the results were instructive. Most of the marked psyllids recaptured in residential trees were released within that habitat, while only 15.4% were released in the grove habitats. In contrast, approximately half of the marked psyllids recovered in groves were released in residential areas. Of all the recaptured psyllids, about 40% changed habitats, but the change was skewed toward movement from residential to grove habitat. These data strongly suggest that there is a constant exchange of D. citri adults between the two habitats, with residential citrus trees functioning as a source habitat of psyllids that emigrate into adjacent commercial groves. The further the residential trees are located from commercial groves, the less likely they will be immediate sources of D. citri. Hence, to reduce the risks of citrus grove colonization by D. citri, new groves should be established away from residential habitats where possible. Additionally, effective pest management practices must also be implemented in residential habitats to reduce source populations of D. citri.