Host Selection, Oviposition and Feeding by a Phytopathogen Vector, Diaphorina citri (Hemiptera: Liviidae), Modulated by Plant Exposure to Formic Acid

Abstract

Citrus volatiles and their breakdown products, such as formic and acetic acids, guide host finding by Asian citrus psyllids (Diaphorina citri). Formic acid elicits psyllid antennal responses and stimulates probing. This study investigated how exposure to vapor phase formic acid induced changes in plant volatile emissions, expression of plant defense genes, and modified psyllid host finding and feeding behaviors on susceptible and resistant citrus cultivars. Chemical analyses of headspace volatiles and behavioral assays were performed before and after exposure of D. citri susceptible and resistant plants to vapor phase formic acid. RT-qPCR elucidated changes in plant defense-associated gene expression following exposure. Electrical Penetration Graph (EPG) measured D. citri feeding behavior on citrus with and without exposure to formic acid. Volatilized formic acid caused a 200-500% increase in release of citrus volatiles 1-3 h after exposure, increased attraction of D. citri to Poncirus trifoliata (resistant) and increased oviposition compared with controls. RT-qPCR showed significant increases in expression of WRKY22, GST1, RAR1, and EDS1 genes following exposure that also reduced phloem, but not xylem, ingestion by adult D. citri. Formic acid exposure caused increased citrus volatile release that modified host finding and feeding behaviors of an important phytopathogen vector.

Feeding Behavior of Asian Citrus Psyllid [Diaphorina citri (Hemiptera: Liviidae)] Nymphs and Adults on Common Weeds Occurring in Cultivated Citrus Described Using Electrical Penetration Graph Recordings

“…Overall, the waveforms produced by both D. citri nymphs and adults during the 18 h recording periods were generally similar to those reported previously for them on citrus [25,27]. Examples of typical EPG recordings for adult psyllids on each of the four plant species tested are provided in Figure 1.…”

“…Phloem is more nutrient rich than xylem sap, and is important for nymphal development. Our previous studies showed that adults fed more extensively on xylem than phloem on citrus, compared to nymphs, to maintain water balance [24][25][26]. In the current study, the frequency of xylem feeding bouts on C. macrophylla was lower than that which occurred on weed species; this type of feeding was otherwise similar in duration per bout and total duration on both non-host weeds and citrus hosts.…”

“…Even though the weed species we chose to investigate here were characterized by softer leaf tissues as compared with new citrus flush growth, the characteristic physical barriers and plant secondary metabolites associated with weeds may have rendered them inferior host plants to D. citri by affecting xylem and phloem feeding behaviors. Previously, we showed that the fibrous ring associated with trifoliate citrus, Poncirus trifoliata (L.), prevents D. citri adults from accessing phloem tissues [24] and contributes to host plant resistance against psyllids that characterizes this citrus genotype [25]. In the current study, psyllids spent the majority of their time performing stylet pathway activities (C), xylem ingestion (G) and non-probing activities on weed species.…”

“…To complete the electrical circuit, a reference copper electrode (10 cm × 2 mm) was inserted into the soil medium near the base of each plant. Psyllid nymphs and adults were restricted to the abaxial surface of fully expanded leaves of weeds or citrus, the preferred feeding site for nymphs [24][25][26][27]. The feeding behaviors of individual D. citri adults and nymphs were monitored on Spanish needle (n = 22), Mexican primrose (n = 18), Dogfennel (n = 18), and C. macrophylla (n = 22) for a continuous period of 18 h.…”

Behavioral responses of Diaphorina citri to host plant volatiles in multiple-choice olfactometers are affected in interpretable ways by effects of background colors and airflows

“…Regarding the emitted light, Paris et al [19] observed that ACP adults were attracted to UV (390 nm), green (525 nm), and yellow (590 nm) light. Additionally, chemical volatile cues from Rutaceae are important for ACP hostplant location [20][21][22].…”