Phagostimulants for the Asian citrus psyllid also elicit volatile release from citrus leaves

Authors: Lapointe, Stephen; George, Justin

Submitted to: International Society of Chemical Ecology Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 7/1/2017
Publication Date: N/A
Citation: N/A

Interpretive Summary: Naturally occurring chemicals that affect the behavior of the Asian citrus psyllid are being sought for use in traps and other devices to control this major pest insect of citrus. We discovered a mixture of 3 chemical compounds that stimulate the psyllid to probe and attempt to feed. We used advanced statistical methods to identify the optimal blend of the three ingredients. A novel test was developed to distinguish between attraction (selective movement towards the source of an odor) and a phagostimulant response (repeated attempts to feed). We found no evidence that the psyllid is attracted to our blend of formic acid, acetic acid and para-cymene. However, the blend caused psyllids to probe and feed more from wax beads that contained the phagostimulants. We also found that plants respond to the presence of formic acid in the air by increasing their release of various organic volatile compounds. Plants that had been exposed to formic acid released more odors and were more colonized by the psyllid in cage tests in a greenhouse. Psyllids also preferentially fed on citrus leaf area painted with dilute formic acid compared with an untreated area of the same leaf. Continued examination of this phenomenon may result in an improved trap, an attract-and-kill product, or enhanced means of managing the psyllid and citrus greening disease.

Technical Abstract: Chemical cues that elicit orientation by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), are of great interest because it is the primary vector of the causal pathogen of citrus greening disease. We identified an optimal blend ratio of formic and acetic acids that stimulated substrate probing by D. citri [1]. We then applied geometric mixture designs and response surface modeling to identify and optimize a 3-component blend that further increased the number and size of salivary sheaths produced by D. citri on a wax substrate (SPLAT™) containing a 3.5:1.6:1 blend of formic acid, acetic acid and p-cymene, respectively [2]. A probing assay was developed that allows differentiation between attraction by olfactory cues and phagostimulatory effects produced by tastants. No evidence was found for remote orientation by D. citri adults by olfaction to the phagostimulant blends. Increased probing in response to the presence of phagostimulants in the wax matrix occurred only after contact with the substrate. Yellow wax beads, with or without odorants or tastants, attracted more D. citri adults and received more probes compared with white wax beads. Yellow beads containing the 3-component phagostimulant blend were probed by D. citri 2 to 3 times more often compared with yellow beads alone. The phagostimulant effect was also tested by covering wax beads containing the 3-component blend with a plastic film to minimize olfaction or contact chemoreception by antennation or tarsal chemoreceptors. The plastic film did not affect the probing response suggesting that chemosensation responsible for the phagostimulant response is associated with chemosensory receptors present on psyllid mouthparts. Salivary sheaths produced in wax beads containing the phagostimulant blend were 4.5 times longer than sheaths produced in beads without tastants. We also described the release of volatile organic compounds by citrus plants after brief (1 h) exposure to volatilized formic acid. Exposure dramatically altered the volatile profile released from citrus and resulted in modified settling of D. citri adults. A larger number of adult psyllids settled on the susceptible Citrus macrophylla in cages compared with a resistant accession of Poncirus trifoliata. After exposure of the P. trifoliata accession to formic acid, no such preference was observed. Psyllids also preferentially fed on citrus leaf area topically treated with dilute formic acid compared with an untreated area of the same leaf. Continued examination of this phenomenon may result in an improved trap, an attract-and-kill product, or enhanced means of managing D. citri and citrus greening disease. [1] George J, Robbins PS, Alessandro, RT, Stelinski, LL, Lapointe, SL (2016) Chem Senses 41:325-33 [2] Lapointe, SL, George J, Hall DG (2016) J. Chem Ecol 42:941-51