A Herbivory-Induced Volatile Disrupts Host Selection by Chilli Thrips, Scirtothrips dorsalis, on Strawberry

Abstract

The chilli thrips, Scirtothrips dorsalis Hood (Thysanoptera: Thripidae), is an invasive and polyphagous pest that exhibits a broad host range. S. dorsalis relies on both visual and olfactory cues to locate and recognize its host plants and exhibits preference for young foliage. However, little is known about the specific cues mediating host selection by S. dorsalis. The objectives of this study were to: (1) identify herbivore-induced plant volatiles (HIPVs) produced by strawberry plants in response to S. dorsalis infestation versus mechanical damage; and (2) assess the behavioral response of S. dorsalis to uninfested versus infested strawberry plants. Laboratory olfactometer experiments revealed that S. dorsalis preferentially oriented to the volatiles from uninfested strawberry plants over those released by plants infested with conspecifics. In addition to the observed preference of S. dorsalis for uninfested strawberry plants, gas chromatographic-mass spectrometry revealed significant differences between the volatile compounds produced by uninfested strawberry plants and those produced by plants that had been infested with S. dorsalis in both the ‘Florida Brilliance’ and ‘Florida Sensation’ cultivars. Seventeen volatiles were identified from strawberry plants that were released in response to either mechanical or S. dorsalis feeding-induced damage; however, only three [(Z)-3-hexenyl acetate, cymene, and α-farnesene] were significantly elevated in S. dorsalis-infested plants relative to undamaged plants. One of these HIPVs, (Z)-3-hexenyl acetate, eliminated natural attraction of S. dorsalis to uninfested strawberry even though it did not act as a repellent when presented alone. Our results suggest a possible practical application of semiochemicals for managing S. dorsalis in strawberry.

Introduction

The chilli thrips, Scirtothrips dorsalis Hood (Thysanoptera: Thripidae), is a relatively recent invasive pest in the southeastern United States. It was first detected in Florida strawberry in 2015. This pest is notable for its wide host range, with a recorded 225 plant species affected, including vegetables, ornamental plants, and fruit crops such as strawberry (Kumar et al. 2013). The potential economic impact of a widespread S. dorsalis infestation in the United States is substantial, with estimated annual agricultural losses ranging from $3.6 to $6.0 billion (Holtz 2006; Seal and Kumar 2010). In contrast to other thrips pests of strawberries, S. dorsalis targets newly emerged strawberry seedlings. This early-stage infestation allows them to establish populations at an early stage of plant growth, producing multiple generations within a single season (Kumar et al. 2013). As a result, their life cycle renders management challenging and costly, as growers must begin insecticide applications from the outset of the season. A recent study has highlighted a concerning trend: late-season S. dorsalis populations in strawberries may develop reduced susceptibility to conventional insecticides (Kaur et al. 2023). Nonetheless, growers have increasingly relied on insecticide applications to manage this pest. Therefore, there is a critical need for alternative and more sustainable management strategies, and further investigation in this area is warranted.

S. dorsalis likely relies on both visual and olfactory cues to locate and recognize its host plants. Preference for young foliage may be linked to specific visual and olfactory cues that guide their feeding behavior. Exploitation of chemical cues is a common trait among insects, which use them to navigate their environment and locate suitable hosts. The interaction between insect herbivores and host plants also triggers the production of secondary metabolites and volatile chemicals, which can serve as cues for natural enemies and influence subsequent herbivory (Karban and Baldwin 2007; De Moraes et al. 1998; Dicke 1999; Pare and Tumlinson 1999; Delphia et al. 2007). Identification of semiochemicals employed by S. dorsalis for host searching presents opportunities for pest management, such as the identification of attractants or repellents for monitoring or population management.

In some cases, semiochemicals mediating interactions between thrips and their hosts have been identified. For example, ethyl iso-nicotinate attracts onion thrips (Thrips tabaci Lindeman) in onions. Similarly, the combination of an alternative host plant and a specific plant volatile [(E)-β-farnesene] was used to control Western flower thrips (Frankliniella occidentalis (Pergande) in chrysanthemum (Van Tol et al. 2007; Bennison et al. 2002). In addition, certain plant volatiles have been identified as attractants for F. occidentalis. These include p-anisaldehyde, o-anisaldehyde, benzaldehyde, and salicylaldehyde (Teulon et al. 1993; Manjunatha et al. 1998; Koschier et al. 2000). These results with other thrips species led us to postulate that semiochemicals could be identified with potential practical application for managing S. dorsalis.

To date, the effects of strawberry plant volatiles on the behavior of S. dorsalis have remained unknown. Therefore, our objectives were to: (1) identify and compare herbivore-induced plant volatiles (HIPVs) produced by strawberry plants in response to S. dorsalis infestation or mechanical damage; and (2) assess the behavioral response of S. dorsalis to uninfested and S. dorsalis-infested strawberry plants. Our results indicate that thrips preferentially oriented to volatiles released from uninfested strawberry hosts over those from plants infested with conspecifics. Moreover, we identified specific induced volatiles that differentiated uninfested versus S. dorsalis-infested strawberry hosts. One of these HIPVs, (Z)-3-hexenyl acetate, eliminated natural attraction of S. dorsalis to uninfested strawberry even though it did not act as a repellent when presented alone. By gaining insights into the volatile cues that influence S. dorsalis host selection, our investigation identified an HIPV with potential practical application to develop more targeted and effective pest management tools for S. dorsalis in strawberry.

Materials and Methods

Scirtothrips dorsalis Colony

A colony of S. dorsalis was maintained on cotton plants, which were grown in 10 × 9 cm pots filled with a commercial potting mix (BWI Pro-Mix 2.8 cf.). The rearing conditions were identical to those described by Kaur et al. (2023). Briefly, infested plants were kept in Bug dorm-2120 F insect tents (Taichung, Taiwan) at 27 ± 1 °C and 60 ± 5% relative humidity (R.H.) under a 12 h photophase. The plants were watered three times per week and fertilized once per week with a general-purpose fertilizer (J R Peter’s Jacks General All-Purpose fertilizer, Allentown, PA, USA).

Experimental Plants

Two strawberry cultivars, ‘Florida Brilliance’ (Chandler et al. 2009; PP20,363) and ‘Florida Sensation’ (Whitaker et al. 2015; PP25,574), were sourced from Crown Nursery (Red Bluff, CA) for this study. Plants were grown in a pest-free environment and received experimental treatments in a greenhouse setting, without the application of any pesticides. The environmental conditions in the greenhouse were maintained as described for insect rearing, ensuring consistency across the experiment.

Volatile Collection and Analysis

To collect plant volatiles, strawberry plants of each cultivar (‘Florida Brilliance’ and ‘Florida Sensation’) were subjected to three different treatments: 1) Uninfested (control) plants were grown in the greenhouse without any stress; 2) Infested plant were inoculated with 10 adults female S. dorsalis 48 hours prior to volatile collection. The plants had developed 4–5 young expanded trifoliates at this stage; 3) Mechanically damaged plants received injury by pricking their young trifoliates with insect pins (0 gauge) for 10 seconds, once daily, starting on the day of S. dorsalis inoculation. Each treatment was replicated 6–7 times. Before the headspace collections, the following plant characteristics were recorded for each treatment: plant age (days after planting), canopy size, total leaf number, and leaf age (young and old leaves). The plants used for each treatment were grown in individual cages to prevent cross-contamination and maintained in the greenhouse under the conditions described above.

Plant headspace volatiles were collected using a dynamic sampling method. Individual plants were covered with oven bags (Reynolds Consumer Products Inc., Louisville, KY) with two slits cut on each corner to accommodate plastic tubes. The tubes were secured with zip-ties to prevent loss or contamination. To prevent filter contamination, the oven bags and HayeSepQ filters (Volatile Assay Systems/Vassays, Rensselaer, NY) were preconditioned by heating them at 57 °C for 48 h prior to the experiment. The sampling system was then connected to an air compressor, which was set to a flow rate of 1.3 L/min. Volatiles from each plant were collected for 24 h, after which the filters were carefully removed and wrapped in aluminum foil for storage until further processing and analysis.

For gas chromatography-mass spectrometry (GC-MS) analysis, the collected volatiles were prepared and processed as described in Griesheimer et al. (2023). Specifically, the filters were washed with 150 µL of dichloromethane (Sigma-Aldrich, St. Louis, MO) and 1 µg of nonyl acetate (TCI America, Portland, OR) was added as an internal standard (IS). The samples were then stored at -20 °C until further analysis.

One µL of each sample was injected into GC-MS (Thermo Scientific ISQ) using an autosampler. Helium was used for the carrier gas at a linear flow velocity of 2 mL/min. All samples were analyzed on a TG-5MS column (5% phenyl methyl polysiloxane) (Thermo Fisher Scientific, Waltham MA) 30 min × 0.25 mm ID. The column oven temperature was maintained at 40 °C for 1 min and increased at a rate of 7 °C/min to a final temperature of 300 °C and maintained at 300 °C for 6 min. The injector temperature was set at 270 °C with the detector set at 200 °C. Compounds were tentatively identified by comparison of mass spectra with available mass spectra libraries. Compounds were confirmed by comparison with external standards, when available.

Effect of Plant Damage on Response of S. dorsalis to Strawberry Odors

Seven-day-old strawberry (‘Florida Brilliance’) plants were used in this experiment. The plants were maintained as previously described and were not exposed to insect feeding prior to the commencement of testing. Adult S. dorsalis tested in bioassays were obtained from the culture described above and acclimated to the bioassay conditions under starvation for 3–4 h prior to testing. These were approximately 4–5 days post eclosion at a 1:1 male: female sex ratio and of unknown mating status.

A glass Y-tube olfactometer with a stem length of 200 mm, arm length of 230 mm (angled at 60°), and internal diameter of 23 mm, was positioned vertically for the behavioral bioassays. Prior to use, all glass tubes associated with the olfactometer were thoroughly rinsed with acetone and distilled water and then subjected to an overnight oven drying process at 100 °C. The arms of the Y-tube olfactometer were securely attached to two separate glass jars, each containing distinct plant treatments, as illustrated in Fig. 1 of Mann et al. (2011). The olfactometer was illuminated from above with a fluorescent bulb and surrounded by black walls to minimize visual cues. A constant airflow of 900 ml/min was drawn into the arms of the olfactometer. Each bioassay consisted of two glass arms, each containing two different source treatments originating from the attached glass jar chambers, thus allowing for direct comparison of behavioral responses from two sources of odor.

Fig. 1.

A) Bliplot and B) Scatterplot of a principal component analysis on volatiles releasedfrom ‘Florida Brilliance’ var. strawberry plants that were: undamaged (red); receivedmechanical damage (green); or were infested by Scirtothrips dorsalis (blue). Ellipses inpanel A indicate 95% confidence intervals, while numbers in panel B refer to the chemical compounds listed in Table 1

Four dual-choice sets of treatments were compared with S. dorsalis using the Y-tube: (1) clean air vs. clean air; (2) clean air vs. uninfested plant; (3) clean air vs. plant infested by S. dorsalis; and (4) uninfested plant vs. plant infested by S. dorsalis. S. dorsalis-infested plants were inoculated with 10 adult thrips 48 h before testing according to the procedure described above prior to volatile collections. One female S. dorsalis adult was released in the olfactometer, and the movement of this female was recorded after 5 min. Thrips that did not make a choice after 5 min were considered non-responders and excluded from statistical analysis. The treatments were replicated 40 times in this olfactory bioassay.

Effect of (Z)-3-hexenyl Acetate on Response of S. dorsalis to Strawberry Odors

Strawberry plants damaged by S. dorsalis feeding responded with increased release of several volatiles (See Results). Furthermore, S. dorsalis preferentially responded to the odor of uninfested strawberry plants as compared to infested plants (See Results). Since release of (Z)-3-hexenyl acetate was induced in both cultivars in response to S. dorsalis feeding, we chose this HIPV to investigate the possible effects on S. dorsalis behavior. Our hypothesis was that (Z)-3-hexenyl acetate could at least partially explain the variation in behavioral response observed in S. dorsalis to uninfested versus S. dorsalis-infested strawberry plants.

Plants and insects used for bioassays were maintained as described above. All chemicals were obtained from Sigma Aldrich (St. Louis, MO) with purities ranging between > 97% and 100%. The synthetic (Z)-3-hexenyl acetate was dissolved in 100 µl of dichloromethane and pipetted onto a 2 cm Richmond cotton wick (Petty John Packaging, Inc. Concord, NC) at a 10.0 µg dosage. This dosage was chosen based on known behavioral responses to synthetic attractants and repellents in this olfactometer with Hemiptera (Mann et al. 2012) and Coleoptera (Martini et al. 2015). The control treatment consisted of a cotton wick impregnated with solvent only. The solvent from both treatments was allowed to evaporate within a fume hood for 30 min prior to assays.

The bioassay procedures with synthetic chemicals were identical to those described earlier for plant samples. However, in treatments that included (Z)-3-hexenyl acetate or solvent controls as additional odor sources, cotton wick sources of treatment were placed into the glass jars containing either uninfested or S. dorsalis-infested strawberry plants which were connected to each arm of the Y-tube via Teflon-glass tube connectors. Four dual-choice sets of treatments were compared with S. dorsalis: (1) solvent blank vs. solvent blank; (2) solvent blank vs. uninfested plant; (3) solvent blank vs. (Z)-3-hexenyl acetate; (4) solvent blank vs. uninfested plant + (Z)-3-hexenyl acetate; (5) uninfested plant vs. uninfested plant + (Z)-3-hexenyl acetate; and (6) plant infested by S. dorsalis vs. uninfested plant + (Z)-3-hexenyl acetate. One female S. dorsalis adult insect was released in the olfactometer per assay, and the movement of this female was recorded after 5 min. The treatments were replicated 40 times in this olfactory bioassay.

Data Analysis

The olfactometer data collected from S. dorsalis orientation to uninfested versus infested strawberry and with synthetic (Z)-3-hexenyl acetate were subjected to a χ² test using the Statistical Analysis Software version 9.4 (SAS Institute, Cary, NC, U.S.A.) to evaluate the preference in choice tests. The amounts of volatiles released by strawberry plants were estimated by dividing the area of each peak compound by the area of the IS in the respective chromatogram. These volatile/IS ratios were analyzed using multivariate analysis of variance (MANOVA) followed by a post-hoc Tukey’s Honestly Significant Difference (HSD) test, where applicable. The induced compounds were further analyzed using Tukey’s HSD to determine the differences among treatments. Statistical significance was determined at P < 0.05. Volatile data were also visualized using principal component analysis (PCA) conducted in R (version 4.2.2).

Results

Identification of Plant Volatiles Through GC-MS

For the ‘Florida Brilliance’ cultivar, a total of 17 volatile compounds were identified (Table 1). Among those volatiles identified, (Z)-3-hexenyl acetate, (Z)-4-methyldecane, 2-ethylhexanol, undecane, 2,6,10-trimethyl-tetradecane, α-murolene, and (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene were present at significantly lower levels or were not detectable in the undamaged (control) or mechanically damaged plants compared with S. dorsalis-infested plants (Table 1). No significant differences (P > 0.05) were observed in the levels of β-ocimene, levomenthol, bornyl acetate, and cubebene among the three treatments. In contrast, seven volatiles were found to be significantly elevated in S. dorsalis-infested plants compared to control and mechanically damaged plants. These volatiles included (Z)-3-hexenyl acetate, (Z)-3-carene, undecane, methyl salicylate, 2,6,10-trimethyl-tetradecane, (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene, and α-murolene. Initially, (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene was not detected, but re-examination of the data revealed its presence at elevated levels in S. dorsalis-infested plants.

Table 1.

Mean (± standard error) volatile peak area relative to the internal standard for volatile compounds released by the strawberry cultivar ‘Florida brilliance’ that were undamaged (Control), received mechanical damage (Mechanical), or were infested by Scirtothrips dorsalis (Thrips)

No.	Compound	RT	Control	Mechanical	Thrips	F	P
1	1,2,4-trimethylbenzene	12.001	0.10a ± 0.02	0.29b ± 0.09	0.28ab ± 0.07	3.11	0.08
2	(Z)-3-hexenyl acetate	12.453	0.23a ± 0.08	0.41a ± 0.15	4.71b ± 1.04	15.76	0.0003
3	4-methyldecane	12.915	0.04a ± 0.01	0.29b ± 0.09	0.45b ± 0.05	14.43	0.0004
4	cymene	12.973	0.03a ± 0.01	0.08b ± 0.02	0.07ab ± 0.03	2.48	0.12
5	2-ethylhexanol	13.126	0.27a ± 0.04	0.76ab ± 0.12	1.27b ± 0.29	7.18	0.007
6	β-ocimene	13.405	0.07a ± 0.02	0.01a ± 0.01	0.06a ± 0.04	0.97	0.401
7	(Z)-3-carene	13.725	0.04a ± 0.01	0.06a ± 0.06	1.18b ± 0.59	3.33	0.065
8	undecane	15.289	0.03a ± 0.01	0.13a ± 0.04	0.32b ± 0.07	10.66	0.0015
9	levomenthol	17.497	0.01a ± 0.01	0.19a ± 0.12	0.10a ± 0.05	1.64	0.229
10	methyl salicylate	18.136	0.11ab ± 0.04	0.01a ± 0.01	0.43b ± 0.19	3.28	0.06
11	2,6,10-trimethyl-tetradecane	20.503	0.00a	0.38b ± 0.11	0.18c ± 0.02	10.44	0.0017
12	bornyl acetate	20.738	0.27a ± 0.15	0.50a ± 0.18	0.45a ± 0.35	0.23	0.8007
13	nonyl acetate	21.34	1.00a	1.00a	1.00a	-	-
14	cubebene	25.88	0.03a ± 0.01	0.40a ± 0.37	2.24a ± 1.50	1.65	0.23
15	α-muurolene	26.316	0.27a ± 0.17	0.87a ± 0.39	8.53b ± 2.98	6.45	0.01
16	α-farnesene	26.411	0.15a ± 0.08	1.96ab ± 1.56	7.37b ± 3.49	2.84	0.092
17	(3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene	28.091	0.01a ± 0.01	0.00a	3.07b ± 1.24	5.49	0.017

Three volatile compounds (1,2,4-trimethylbenzene, cymene, and 2-ethylhexanol) were significantly elevated in both mechanically damaged and S. dorsalis-infested plants, relative to control plants (Table 1). Principal component analysis (PCA) was employed to explore the variability among the volatiles, revealing that the cumulative variability accounted for by the first two principal components (PCs) was 72%. The first principal component (PC1) explained 50% of the total variability, with strong positive loadings attributed to (Z)-3-hexenyl acetate, (Z)-3-carene, 2-ethylhexanol, undecane, methyl salicylate, cubebene, and (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene, indicating positive correlations among these volatiles. The second principal component (PC2) explained 22% of the variability, with positive loadings of 1,2,4-trimethylbenzene, 4-methyldecane, cymene, and 2,6,10-trimethyl-tetradecane, and negative loadings of β-ocimene, with no significant influence of other volatiles (Fig. 1A and B).

Further evaluation of the PCs revealed their ability to differentiate between treatment effects. The analysis of variance (ANOVA) model, incorporating PC1 and PC2, was significant, with P-values of 0.007 and 0.021, respectively. A contrast statement was set up to assess the differences between treatment groups, where the ‘thrips’ (infested) and ‘control’ (uninfested) coordinates were significantly different from 0 on the first component, and the ‘mechanical’ (damaged) and ‘control’ coordinates were significantly different from 0 on the second component (alpha = 0.05). This trend was also evident from the PCA biplot, where the ‘thrips’ plant volatiles did not overlap with the ‘control’ group, indicating significant differences between their volatile profiles (Fig. 1A).

Similar to the results observed with the ‘Florida Brilliance’ cultivar, a total of 17 volatile compounds were identified for the ‘Florida Sensation’ cultivar (Table 2). Notably, only three volatile compounds [(Z)-3-hexenyl acetate, cymene, and α-farnesene] were found to be significantly elevated in S. dorsalis-infested plants relative to control and mechanically damaged plants. For all other volatiles, no significant differences were observed among the treatments.

Table 2.

Mean (± standard error) volatile peak area relative to the internal standard for volatile compounds released by the strawberry cultivar ‘Florida sensation’ that were undamaged (Control), received mechanical damage (Mechanical), or were infested by Scirtothrips dorsalis (Thrips)

No.	Compound	RT	Control	Mechanical	Thrips	F	P
1	1,2,4- trimethylbenzene	12.001	0.15a ± 0.04	0.39a ± 0.07	0.98a ± 0.5	2.13	0.162
2	(Z)-3-hexenyl acetate	12.453	0.26a ± 0.17	0.13a ± 0.08	3.34b ± 1.61	3.80	0.05
3	4-methyldecane	12.915	0.12a ± 0.08	0.32ab ± 0.05	0.47b ± 0.13	3.40	0.067
4	cymene	12.973	0.01a ± 0.01	0.09ab ± 0.02	0.30b ± 0.13	4.02	0.04
5	2-ethylhexanol	13.126	0.52a ± 0.15	0.98a ± 0.09	7.72a ± 6.26	1.25	0.32
6	β-ocimene	13.405	0.13a ± 0.05	0.03a ± 0.03	2.28a ± 1.94	1.28	0.31
7	(Z)-3-carene	13.725	0.02a ± 0.01	0.00a	1.05a ± 0.72	2.05	0.17
8	undecane	15.289	0.08a ± 0.05	0.16ab ± 0.04	0.46b ± 0.19	2.86	0.09
9	levomenthol	17.497	0.02a ± 0.01	0.33a ± 0.06	2.81a ± 2.46	1.15	0.35
10	methyl salicylate	18.136	0.11a ± 0.05	0.00a	2.79a ± 2.0	1.86	0.19
11	2,6,10-trimethyl-tetradecane	20.503	0.49a ± 0.45	0.42a±0.020	0.20a ± 0.07	0.28	0.76
12	bornyl acetate	20.738	0.28a ± 0.14	0.79a ± 0.27	3.22a ± 2.34	1.33	0.29
13	nonyl acetate	21.34	1.00a	1.00a	1.00a	-	-
14	cubebene	25.88	0.21ab ± 0.20	0.00a	0.75b ± 0.33	3.07	0.08
15	α-muurolene	26.316	0.02a ± 0.01	0.04a ± 0.03	3.91b ± 2.07	3.52	0.06
16	α-farnesene	26.411	0.06a ± 0.03	0.13a ± 0.11	2.81b ± 1.05	6.62	0.01
17	(3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene	28.091	0.01a ± 0	0.00a	3.98b ± 2.21	3.25	0.07

Principal component analysis (PCA) was performed to investigate the variability among the volatiles. The cumulative variability accounted for by the first two principal components (PCs) was 80.1%, indicating that these components describe the majority of the variation among the volatiles. The first principal component (PC1) accounted for 68.5% of the total variability, with a higher spread of the volatiles on this component. The second principal component (PC2) explained 11.6% of the variability, with positive loadings attributed to 1,2,4-trimethylbenzene and (Z)-3-hexenyl acetate and negative loadings of 4-methyldecane, with no significant influence of other volatiles (Fig. 2A and B). For the PCA analysis, the ANOVA model for both PCs was not significant at α = 0.05 (PC1 = 0.086 and PC2 = 0.554) (Fig. 2A and B).

Fig. 2.

A) Bliplot and B) Scatterplot of a principal component analysis on volatiles released from ‘Florida Sensation’ var. strawberry plants that were: undamaged (red); received mechanical damage (green); or were infested by Scirtothrips dorsalis (blue). Ellipses in panel A indicate 95% confidence intervals, while numbers in panel B refer to the chemical compounds listed in Table 2

Effect of Plant Damage on Response of S. dorsalis to Strawberry Odors

S. dorsalis exhibited a significant preference for uninfested strawberry plants over thrips-infested strawberry plants in a choice experiment (χ² = 13.5, P = 0.0002, df = 1) (Fig. 3). Specifically, 74% of tested S. dorsalis individuals were attracted towards uninfested strawberry plants, as compared to clean air (χ² = 11.52, P < 0.0007, df = 1). However, there was no significant preference observed between S. dorsalis-infested plants and clean air (χ² = 2.0, P = 0.1573, df = 1).

Fig. 3.

Response of Scirtothrips dorsalis to strawberry in the Y-tube olfactometer bioassays depending on whether plants were pre-infested for 48 h with 10 conspecific adults. Asterisk indicates significant difference (P<0.05)

Effect of (Z)-3-hexenyl Acetate on Response of S. dorsalis to Strawberry Odors

When presented with a choice between (Z)-3-hexenyl acetate versus a solvent blank, S. dorsalis exhibited neither (χ2 = 0.48, P = 0.4869, df = 1) attraction to, nor repellency from (Z)-3-hexenyl acetate (Fig. 4). Instead, the behavioral response was similar to that observed in the double blank negative control (χ2 = 0.00, P = 1.0000, df = 1), which indicated no positional bias (Fig. 4). While uninfested plants attracted S. dorsalis as compared with a solvent blank (χ2 = 11.24, P = 0.0008, df = 1), the odors from uninfested plants with concurrent release of synthetic (Z)-3-hexenyl acetate did not attract (χ2 = 0.13, P = 0.7201, df = 1) S. dorsalis when directly compared against a solvent blank (Fig. 4). Uninfested plants were significantly (χ2 = 5.07, P = 0.0243, df = 1) more attractive to S. dorsalis than uninfested plants with concurrent release of synthetic (Z)-3-hexenyl acetate (Fig. 4). There was no preference (χ2 = 0.03, P = 0.8601, df = 1) observed when S. dorsalis were presented with a choice between S. dorsalis-infested plants and uninfested plants with concurrent release of synthetic (Z)-3-hexenyl acetate (Fig. 4).

Fig. 4.

Response of Scirtothrips dorsalis to strawberry with or without concurrent release of (Z)-3-hexenyl acetate in the Y-tube olfactometer bioassays depending on whether plants were pre-infested for 48 h with 10 conspecific adults. Bars indicate the proportion of individuals choosing each treatment arm. Asterisks denote significant preferences (* p < 0.05, *** p < 0.001)

Discussion

We identified 17 plant volatiles produced by strawberry plants in response to mechanical damage and S. dorsalis feeding. We also determined that S. dorsalis shows preference for uninfested strawberry plants over plants infested by conspecifics. Similar behavior has been observed with F. occidentalis where untreated control tobacco, Nicotiana tabacum L. (Solanales: Solanaceae), plants were preferred over mechanically damaged or feeding-induced plants due to the release of high amounts of nicotine that repelled thrips (Delphia et al. 2007). Likewise, the release of floral volatiles by rose plants (City of Belfast cul.) repels F. occidentalis (Gaum et al. 1994). Also, salicylaldehyde and α-tomatine inhibit response and act as antifeedants against F. occidentalis and Thrips palmi Karny (Thysanoptera: Thripidae), respectively (Hirano et al. 1994).

In addition to the observed preference of S. dorsalis for uninfested strawberry plants, our headspace analysis revealed significant differences between the volatile compounds produced by uninfested and S. dorsalis-infested strawberry plants for both cultivars examined. Principal component analysis indicated that the volatiles from infested plants did not overlap with those from uninfested plants. These results let us to postulate that the differential responses of S. dorsalis to uninfested versus herbivore-induced strawberry could be attributed to the differing volatile bouquets characterizing these two treatments.

Among the HIPVs identified here, terpenoids, including α-muurolene, α-farnesene, and (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene, were significantly elevated in plants damaged by S. dorsalis compared to control treatments. Furthermore, terpenoids were strongly correlated with green leaf volatiles (GLVs) in the first principal component (PC1) for both the ‘Florida Brilliance’ and ‘Florida Sensation’ cultivars. Notably, terpenes exhibited a higher loading effect on PC1 compared to other volatiles in both cultivars. Terpenes are known to play important roles in plant defense against herbivory, both directly and indirectly (Sharma et al. 2017). For instance, terpenoids such as linalool (a monoterpenoid) and (E)-β-farnesene (a sesquiterpene) serve as repellents against herbivores (Aharoni et al. 2005; Maffei 2010). Additionally, terpenes play a key role in plant indirect defenses by attracting predators (Heil 2008). For example, production of terpenoids, such as (E)-β-ocimene and linalool, by lima bean plants (Phaseolus lunatus L.) in response to spider mite (Tetranychus urticae Koch) damage serves as an attractant for the predatory mites, Phytoseiulus persimilis Athias-Henriot (Takabayashi and Dicke 1996).

Selection pressure ‘tunes’ behavioral responses of insect herbivores to very specific blends of plant volatiles to differentiate optimal hosts from non-hosts or otherwise sub-optimal hosts (Bruce and Pickett 2011). For example, female Helicoverpa assulta (Guenee) moths are attracted to a specific blend of HIPVs characterizing their host (Sun et al. 2012). In a more detailed example, Webster et al. (2010) demonstrated that a complex blend of plant volatiles attracts the black bean aphid, Aphis fabae Scopoli, but individual volatiles comprising that blend repel this herbivore when presented alone (Webster et al. 2008). These data suggest a context-dependent modulation of volatile perception, whereby individual volatiles elicit non-host responses when presented in isolation, whereas their combination within a blend triggers a host-orienting response. In the current investigation, it is possible that newly sprouting and uninfested strawberry seedlings (most susceptible stage for thrips infestation) release a specific volatile blend that attracts S. dorsalis. However, plants damaged by S. dorsalis feeding release induced volatiles, such as (Z)-3-hexenyl acetate, that may indicate an already occupied, and thus sub-optimal host. In this manner, S. dorsalis may avoid competition in a monoculture of strawberry by ignoring plants infested with conspecifics and instead favoring to accept nearby uninfested alternatives.

Importantly, our results do not support the interpretation that odors from thrips-infested plants are repellent, nor that (Z)-3-hexenyl acetate functions as a stand-alone repellent at the dose tested. Rather, (Z)-3-hexenyl acetate was itself among the volatiles significantly induced by chilli thrips feeding injury and therefore represents part of the herbivory-associated odor signature of infested strawberry plants. When added to the volatile blend of uninfested plants, (Z)-3-hexenyl acetate eliminated thrips preference for that otherwise attractive odor source, consistent with disruption or masking of key host-location cues and/or signaling an ‘occupied’ or suboptimal host. Notably, our experimental design did not quantify whether exogenous exposure to (Z)-3-hexenyl acetate induces additional volatiles; thus, blend disruption by an herbivory-induced cue provides the most parsimonious explanation supported by the data.

Based on our current results, it is possible that certain HIPVs may have practical application for S. dorsalis management. When (Z)-3-hexenyl acetate was presented concurrently with the odors from uninfested strawberry plants, it eliminated normal attraction to strawberry. We speculate that treating strawberry seedlings with (Z)-3-hexenyl acetate may have practical utility by mimicking the odor of thrips-infestation and thereby disrupting normal host finding behavior by S. dorsalis. Field-based experiments are warranted to test this hypothesis. Given the polyphagous nature of S. dorsalis, it is also unclear whether (Z)-3-hexenyl acetate may affect the response of this generalist to other host plant species beyond strawberry. Furthermore, it remains to be investigated how the other S. dorsalis-induced volatiles identified here in strawberries may affect host finding behavior of this economically important thrips species.

Author Contributions

G.K. and S.L conceived and designed the research. G.K. and L.S. conducted the experiments and analyzed the data. X.M. assisted with GC-MS study, data analysis and provided feedback on the study design. All authors provided feedback on the manuscript. All authors read and approved the manuscript.

Funding

This research was supported by United States Department of Agriculture National.

Institute of Food and Agriculture Hatch Project No. FLA-GCR-005888 and FLA-GCR-006545.

Data Availability

The datasets are available from the corresponding author on request.

Declarations

Competing Interests

The authors declare no competing interests.