Novel synthetic ligands enhance the behavioral responses of Asian citrus psyllid to naturally occurring host-plant volatiles

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

Submitted to: Natural Products for Pest Management
Publication Type: Book / Chapter
Publication Acceptance Date: 5/27/2013
Publication Date: N/A
Citation: N/A

Interpretive Summary: Huanglongbing, a devastating disease of citrus, is spread by a tiny sap-sucking insect called the Asian citrus psyllid (abbreviated as ‘ACP’). Management plans aimed at reducing the incidence and spread of Huanglongbing in citrus growing areas rely upon sampling ACP with yellow sticky cards, which are not always reliable, to detect the insect and track its spread. The development of highly effective scent attractants for the psyllid may improve trapping efficacy, which, in turn, is needed to increase the reliability of detecting and monitoring ACP and to reduce costs associated with these activities. Certain proteins, called ‘olfactory binding proteins’ (abbreviated as ‘OBPs’), that are present in the insect antennae can alter insect behavior; thus, these proteins are suitable targets for the development of novel control products using an approach called ‘rational design’, a method developed by the pharmaceutical industry for drug discovery. Psyllid OBPs that bound to petitgrain oil, an essential oil extracted from sour orange leaves, were isolated and then screened themselves with a chemical library comprising 30,000 small molecules in order to isolate synthetic compounds that bound to the proteins. The compounds, called ‘ligands’, were subsequently evaluated for their behavioral effects on ACP. A synthetic ligand named ‘Titan’ was found to be behaviorally active in preliminary tests; Titan was tested further with respect to its abilities to stimulate two key foraging behaviors of ACP (probing and searching) and to interact with limonene, a scent compound emitted by the leaves of many kinds of citrus trees. We found that the addition of Titan to limonene increased the amount of probing activity by the psyllid relative to the amount of probing elicited by limonene alone. In another test, the psyllids’ searching response to Titan was similar to that achieved when it was exposed to the foliage of orange jasmine, one its favorite host-plants. ACP searching response was also higher to Titan than to limonene alone. These results demonstrated that Titan is highly stimulatory and modifies psyllid response to limonene. Further tests will determine whether Titan can be used in concert with limonene and other naturally occurring scent compounds to develop potent scent attractants that will improve our capacity for detecting and monitoring this pest in orchards and backyards.

Technical Abstract: Huanglongbing, a devastating disease of citrus, is spread by the Asian citrus psyllid (ACP). Area wide management plans aimed at reducing the incidence and spread of Huanglongbing rely upon sampling ACP with yellow sticky cards, which are not always reliable. The development of highly effective scent attractants may improve trapping efficacy, which, in turn, is needed to increase the reliability of vector detection and monitoring and to reduce costs associated with these activities. The protein components of the insect olfactory system can control and alter insect behavior; thus, these proteins are suitable targets for the development of novel control products using rational design, as has been accomplished for drug discovery in the pharmaceutical industry. Odorant binding proteins (OBPs) from ACP were recombinantly expressed in vitro and screened for their ability to bind to petitgrain oil, an essential oil from sour orange foliage known to attract ACP, in order to identify OBP(s) responsible for attraction behavior. The OBP that bound petitgrain oil was then screened with a combinatorial chemical library comprising 30,000 small molecules in order to isolate synthetic ligands; these ligands were subsequently evaluated for their behavioral effects on ACP. A synthetic ligand named ‘Titan’ was found to be behaviorally active in preliminary tests; Titan was tested further with respect to its abilities to stimulate two foraging behaviors of ACP and to interact with limonene, a monoterpene emitted by many ACP host plant species. The addition of Titan to limonene increased the amount of probing by ACP into an artificial midrib composed of an emulsified wax relative to the probing level elicited by limonene alone. Probing level to Titan alone was influenced by the concentration of Titan in the wax midrib, with the lower concentration being more stimulatory than the higher concentration. In olfactometer tests, response to Titan was similar to the odor of young orange jasmine foliage. Olfactory response was also higher to a mixture of limonene and Titan than to limonene alone. These results demonstrated that Titan is highly stimulatory and modifies psyllid response to limonene. Further tests will determine whether Titan can be used in concert with naturally occurring terpenes to develop potent scent attractants for ACP.