Synthetic ligands of olfactory binding proteins modulate aggregation response of Asian citrus psyllid in the presence of host-plant volatiles

Authors: Patt, Joseph - Joe; Meikle, William; Niedz, Randall; WOODS, DANIEL - Inscent, Inc

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2018
Publication Date: 12/20/2018
Citation: Patt, J.M., Meikle, W.G., Niedz, R.P., Woods, D. 2018. Synthetic ligands of olfactory binding proteins modulate aggregation response of Asian citrus psyllid in the presence of host-plant volatiles. Frontiers in Plant Science. 9:1891. https://doi.org/10.3389/fpls.2018.01891.
DOI: https://doi.org/10.3389/fpls.2018.01891

Interpretive Summary: The Asian citrus psyllid (ACP) transmits citrus greening disease. Citrus greening is the most devastating disease of citrus trees in the world today. There is no cure for it: this has resulted in the loss of hundreds of thousands of acres of citrus groves and billions of dollars in productivity. There is a great need for reliable monitoring methods to effectively detect psyllid infiltration into citrus groves, monitor their population size and determine the effectiveness of control measures. ACP uses a combination of leaf color and odors and acoustic signals and pheromones from other ACPs to locate citrus foliage. Identifying these signals is necessary for developing effective attractants for luring psyllids to traps. Compounds called ligands may increase the psyllid’s responsiveness to citrus scent lures. In an earlier study, we found that ACPs tried to feed more actively when they were exposed to a natural citrus odor compound plus a ligand. In this study, we examined whether the presence of ligands would influence the number of psyllids that settled on citrus sprigs, both when the shoots were presented to the psyllids alone or in combination with a citrus scent lure named ‘Cmac scent’. Seven ligands were tested. In the presence of four of them, psyllid settling and aggregation levels were higher on sprigs with Cmac scent than on sprigs only. Presentation of one of the ligands resulted in an opposite effect in which psyllid settling and aggregation levels were lower on sprigs with Cmac scent than on sprigs only. Two of the test ligands produced results that were not as highly pronounced as with the other test ligands. Increased settling and aggregation levels of the psyllids did not occur largely on sprigs with the test ligands, and there was little effect of ligand concentration on psyllid response levels. This suggests that the test ligands themselves did not attract the psyllids but rather that they influenced the psyllid’s response to the Cmac scent. The results here suggest that ligands can increase the effectiveness of citrus scent lures.

Technical Abstract: There is interest in using ligands of chemosensory binding proteins (CBP) to augment an insect’s responsiveness to chemosensory cues. We showed previously that combining a synthetic ligand of a CBP with limonene, a common citrus volatile, enhanced the probing response of Asian citrus psyllid (Diaphorina citri). Here, we determined whether synthetic compounds, which were ligands of Diaphorina citri(D. citri) olfactory binding protein DCSAP4, influenced the settling and aggregation levels of psyllids on young citrus shoots. The shoots were presented either alone, with a seven-component mixture of synthetic citrus volatiles (‘Cmac scent’), or with different concentrations of test ligands. The test ligands and Cmac scent were dispensed from a droplet of an emulsified wax product (SPLAT). For each test, 200 D. citri were released in the test area and the numbers of psyllids on each sprig were counted 24 hours later. Sprigs with = 7 psyllids were considered to be an aggregation. A total of seven ligands were tested individually. Four of the ligands (654, 717, 784, and 861) modulated the psyllid settling and aggregation response causing greater settling and aggregation to a Cmac scent that otherwise did not induce responses greater than those with blank SPLAT. Presentation of one of the ligands (019) resulted in an opposite effect in which psyllid settling and aggregation levels were lower on sprigs with Cmac scent than on those with blank SPLAT. There were no differences in settling levels in the different treatment vials in the Ligand 905 experiment. In the Ligand 937 experiment, settling levels did not vary significantly between treatment vials although settling levels were relatively high in all treatment vials and there was a significant treatment effect. Increased settling and aggregation levels were largely not observed with in the vials with only the test ligands, and there was little effect of ligand concentration on psyllid response levels. This suggests that the test ligands themselves did not attract the psyllids but rather modulated the psyllid’s response to the Cmac scent. The results suggest that synthetic ligands of D. citri CBPs can be use to increase the effectiveness of citrus scent lures used to attract psyllids to monitoring traps and attract and kill devices.