Novel synthetic compounds enhance the attractiveness of host-plant volatiles: An opportunity to boost detection and monitoring of Asian citrus psyllid?

Authors: Patt, Joseph - Joe; WOODS, DANIEL - Inscent, Inc; DIMTRATOS, SPIROS - Inscent, Inc; Meikle, William; Stockton, Dara; Lapointe, Stephen; MAFRA-NETO, AGENOR - Isca Technologies, Inc

Submitted to: International Research Conference on Huanglongbing
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2012
Publication Date: 1/2/2014
Citation: Patt, J.M., Woods, D., Dimtratos, S., Meikle, W.G., Stockton, D.G., Lapointe, S.L., Mafra-Neto, A. 2014. Novel synthetic compounds enhance the attractiveness of host-plant volatiles: An opportunity to boost detection and monitoring of Asian citrus psyllid? International Research Conference on Huanglongbing. Available: http:escholarship.org/uc/iocv_journalcitruspathology/1/1

Interpretive Summary: The Asian citrus psyllid is a tiny insect that transmits the bacterium that is thought to be responsible for causing Citrus Greening disease, otherwise known as Huanglongbing. There is currently no cure for Citrus Greening, which is either fatal or permanently debilitating to all species of commercial citrus. Yellow sticky card traps are placed in trees to detection and monitor the psyllid. Although the psyllids are attracted to the yellow color of the traps, these traps do always reliably catch the psyllid. This makes it difficult to detect low numbers of psyllids or accurately estimate their population size in a given location. To improve trap captures, scientists have tried to develop scent lures that would increase the attractiveness of the traps. Scent baits made from the aroma compounds from citrus leaves offer the most likely means of improving capture levels of Asian citrus psyllid with sticky cards and other types of visual traps. However, developing scent lures that can compete with the attractiveness of actual citrus trees, especially when they are producing new shoots, is challenging. We are developing a new class of synthetic scent lures that may enhance the attractiveness of naturally occurring aroma compounds. These compounds are synthetic ligands (ligand = a chemical compound that binds to another) that bind to chemosensory proteins found in the sense organs that detect odors in target insects. When a ligand binds to a chemosensory protein, it sends a signal to the insect’s brain that it has perceived an aroma compound. These ligands may mimic naturally occurring aroma compounds and be highly attractive because of their strong affinity to chemosensory proteins. In our study, chemosensory proteins from the psyllid’s antennae were identified based on their reactivity to petitgrain oil (an essential oil extracted from sour orange leaves), which is also attracts the psyllid. Two behavioral assays were used to measure the psyllid’s behavioral responses to several candidate ligands. One assay measured how many times a psyllid probed into a line of emulsified wax (SPLAT®, ISCA Technologies) containing a test ligand, the other assay measured the amount of time of psyllids remained in an airstream carrying the ligand. One ligand, nicknamed ‘Titan’, was more stimulatory than limonene, a common citrus leaf aroma compound, while a mixture of Titan and limonene was significantly more stimulatory than either alone. Subsequent assays showed that Titan was as stimulatory to the psyllid as the odor emitted by growing sprigs of orange jasmine, one of the psyllid’s favorite host-plants. These results indicate that these ligands may synergize the attractiveness of naturally-occurring citrus volatiles and boost their effectiveness as scent lures for Asian citrus psyllid.

Technical Abstract: In the absence of pheromone attractants, host-plant volatiles offer the most likely means of improving capture levels of Asian citrus psyllid (ACP) with sticky cards and other types of visual traps. However, developing scent lures that can compete with the attractiveness of actual host-plants, especially those in flush, is challenging. We are developing a new class of synthetic scent lures that may enhance the attractiveness of naturally-occurring host-plant volatiles. These compounds are synthetic ligands that bind to chemosensory proteins (CSPs) found in the olfactory sensilla of target insects. These ligands may mimic naturally-occurring odorants and function as super-stimuli because of their strong affinity to CSPs. In our study, CSPs from ACP antennae were identified based on their reactivity to petitgrain oil (an essential oil extracted from sour orange leaves), an ACP attractant. Two behavioral assays were used to assess the biological activity of several candidate ligands. One assay measured ACP probing frequency into a line of emulsified wax (SPLAT®, ISCA Technologies) containing a test ligand, the other assay measured the retention time of psyllids in an airstream carrying the ligand. One ligand, nicknamed ‘Titan’, was more stimulatory than limonene, a common citrus volatile, while a mixture of Titan and limonene was significantly more stimulatory than either alone. Subsequent assays showed that Titan was as stimulatory to ACP as the odor emitted by flushing sprigs of orange jasmine, a favored host-plant. These results indicate that CSP ligands may synergize the attractiveness of naturally-occurring citrus volatiles and boost their effectiveness as scent lures for ACP.