Volatile Characterization of Lychee Plant Tissues (Litchi chinensis) and the Effect of Key Compounds on the Behavior of the Lychee Erinose Mite (Aceria litchii)

Abstract

Herbivore-Induced Plant Volatiles (HIPVs) are volatile signals emitted by plants to deter herbivores and attract their natural enemies. To date, it is unknown how lychee plants, Litchi chinensis, respond to the induction of leaf galls (erinea) caused by the lychee erinose mite (LEM), Aceria litchii. Aiming to reveal the role of HIPVs in this plant-mite interaction, we investigated changes in the volatile profile of lychee plants infested by LEM and their role on LEM preferences. The volatile profile of uninfested (flower buds, fruit, leaves and new leaf shoots) and infested plant tissue were characterized under different levels of LEM infestation. Volatiles were collected using head-space-solid phase microextraction (HS-SPME) followed by gas chromatography-mass spectrometry (GC-MS) analyses. Fifty-eight volatiles, including terpenoids, alcohols, aldehydes, alkanes, esters, and ketones classes were identified. Using dual-choice bioassays, we investigated the preference of LEM to uninfested plant tissues and to the six most abundant plant volatiles identified. Uninfested new leaf shoots were the most attractive plant tissues to LEM and LEM attraction or repellence to volatiles were mostly influenced by compound concentration. We discuss possible applications of our findings in agricultural settings.

Keywords: behavioral bioassays; eriophyoid mites; gas chromatography-mass spectrometry (GC-MS); head-space-solid phase microextraction (HS-SPME); herbivore-induced plant volatiles (HIPVs).

Figure 1

Schematic drawing of the bioassay used for assessing LEM preferences. Panel (a) shows the experimental arena used in the dual-choice bioassay. The drawing represents a microscope slide (75 mm × 25 mm) divided into Sections 1–4 and TAPE. Panel (b) shows the white Styrofoam board (20 cm × 10 cm) used to hold four slides and two Eppendorf^® tubes (1.5 mL) per slide. Only LEM crossing half of the slide (Sections 3 and 4 and TAPE) were considered to make a choice between the two sources offered. The number of mites in each section was assessed after 24 h.

Figure 2

Abundance (%) of the chemical classes in volatiles from potted (infested) lychee plants and from field-collected (non-infested) lychee tissues. MH- monoterpene hydrocarbons, OM- oxygenated monoterpenes, SH- sesquiterpene hydrocarbons, alcohols, aldehydes, alkanes, esters, and ketones. Initial infestation (A1), intermediate infestation (A2), heavy infestation (A3), overexploitation (A4), and healthy, uninfested plant (A5). Floral buds (B1), open flowers (B2), leaf buds (B3), and new leaf shoots (B4).

Figure 3

Panel (a) shows the Biplot Analysis for Principal Component Analysis (PCA) of volatile compounds emitted from potted lychee plants under different levels of LEM infestation (A1 to A5). Panel (b) shows the Agglomerative Hierarchical Cluster (AHC) analysis for LEM infestation samples A1 to A5 based on selected volatile active compounds. Initial infestation (A1), intermediate infestation (A2), heavy infestation (A3), overexploitation (A4), and healthy, uninfested plant (A5).

Figure 4

Panel (a) shows the Biplot Analysis for Principal Component Analysis (PCA) of volatile compounds emitted from field-collected (non-infested) lychee tissues (B1 to B4). Panel (b) shows the Agglomerative Hierarchical Cluster (AHC) analysis for samples B1 to B4 based on selected volatile active compounds. Floral buds (B1), open flowers (B2), leaf buds (B3), and new leaf shoots (B4).

Figure 5

Preference of LEM when offered different field-collected (non-infested) lychee tissues. Panel shows LEM choice when offered different combinations of plant tissues (leaf vs. fruit, leaf vs. bud, leaf vs. leaf shoot, fruit vs. bud, fruit vs. leaf shoot, leaf shoot vs. leaf shoot, N = 8). * GLMM: p < 0.05; ns, not significant. Bars indicate proportion (±SE) of mites reaching the end section of the slide (Sections 3 and 4 and TAPE) towards each plant tissue in 24 h.

Figure 6

Preference of LEM to the volatile compound nonanal and its solvent ethanol (control), in a dual-choice experimental unit. Panel shows eight Nonanal concentrations tested: 3%, 5%, 7%, 10%, 25%, 50%, 75% and 100%. Numbers inside bars represent number of replicates; GLMM: * p < 0.05; ** p < 0.01; ns, not significant. Bars indicate the proportion (±SE) of mites reaching the end section of the slide towards nonanal and ethanol in 24 h.

Figure 7

Preference of LEM to the volatile compound decanal and its solvent ethanol (control), in a dual-choice experimental unit. Panel shows all eight Decanal concentrations tested: 3%, 5%, 7%, 10%, 25%, 50%, 75% and 100%. Numbers inside bars represent number of replicates; GLMM: * p < 0.05; ** p < 0.01, *** p < 0.001; ns, not significant. Bars indicate proportion (±SE) of mites reaching the end section of the slide towards decanal and ethanol in 24 h.

Figure 8

Preference of LEM to the volatile compound β-caryophyllene and its solvent ethanol (control), in a dual-choice experimental unit. Panel shows all eight β-caryophyllene concentrations tested: 3%, 5%, 7%, 10%, 25%, 50%, 75% and 100%. Numbers inside bars represent number of replicates; GLMM: * p < 0.05; ** p < 0.01, *** p < 0.001; ns, not significant. Bars indicate proportion (±SE) of mites reaching the end section of the slide towards β-caryophyllene and ethanol in 24 h.

Figure 9

Preference of LEM to the volatile compound ar-curcumene and its solvent ethanol (control), in a dual-choice experimental unit. Panel shows all eight ar-curcumene concentrations tested: 3%, 5%, 7%, 10%, 25%, 50%, 75% and 100%. Numbers inside bars represent number of replicates; GLMM: ** p < 0.01; ns, not significant. Bars indicate proportion (±SE) of mites reaching the end section of the slide towards ar-curcumene and ethanol in 24 h.

Figure 10

Preference of LEM to the volatile compound limonene and its solvent ethanol (control), in a dual-choice experimental unit. Panel shows all eight Limonene concentrations tested: 3%, 5%, 7%, 10%, 25%, 50%, 75% and 100%. Numbers inside bars represent number of replicates; GLMM: * p < 0.05; ** p < 0.01; ns, not significant. Bars indicate proportion (±SE) of mites reaching the end section of the slide towards limonene and ethanol in 24 h.

Figure 11

Preference of LEM to the volatile compound sabinene and its solvent ethanol (control), in a dual-choice experimental unit. Panel shows all eight Sabinene concentrations tested: 3%, 5%, 7%, 10%, 25%, 50%, 75% and 100%. Numbers inside bars represent number of replicates; GLMM: * p < 0.05; ** p < 0.01, *** p < 0.001; ns, not significant. Bars indicate proportion (±SE) of mites reaching the end section of the slide towards sabinene and ethanol in 24 h.