Electroantennogram and machine learning reveal a volatile blend mediating avoidance behavior by Tuta absoluta females to a wild tomato plant

Abstract

Tomato cultivation is threatened by the infestation of the nocturnal invasive tomato pinworm, Tuta absoluta. This study was based on field observations that a wild tomato plant, Solanum lycopersicum var. cerasiforme, grown in the Mount Kenya region, Kenya, is less attacked by T. absoluta, unlike the cultivated tomato plants like S. lycopersicum (var. Rambo F1). We hypothesized that the wild tomato plant may be actively avoided by gravid T. absoluta females because of the emission of repellent allelochemical constituents. Therefore, we compared infestation levels by the pest in field monocrops and intercrops of the two tomato genotypes, characterized the headspace volatiles, then determined the compounds detectable by the insect through gas chromatography-linked electroantennography (GC-EAG), and finally performed bioassays using a blend of four EAG-active compounds unique to the wild tomato. We found significant reductions in infestation levels in the monocrop of the wild tomato, and intercrops of wild and cultivated tomato plants compared to the monocrop of the cultivated tomato plant. Quantitative and qualitative differences were noted between volatiles of the wild and cultivated tomato plants, and between day and night volatile collections. The most discriminating compounds between the volatile treatments varied with the variable selection or machine learning methods used. In GC-EAG recordings, 16 compounds including hexanal, (Z)-3-hexenol, α-pinene, β-myrcene, α-phellandrene, β-phellandrene, (E)-β-ocimene, terpinolene, limonene oxide, camphor, citronellal, methyl salicylate, (E)-β-caryophyllene, and others tentatively identified as 3,7,7-Trimethyl-1,3,5-cycloheptatriene, germacrene D and cis-carvenone oxide were detected by antennae of T. absoluta females. Among these EAG-active compounds, (Z)-3-hexenol, α-pinene, α-phellandrene, limonene oxide, camphor, citronellal, (E)-β-caryophyllene and β-phellandrene are in the top 5 discriminating compounds highlighted by the machine learning methods. A blend of (Z)-3-hexenol, camphor, citronellal and limonene oxide detected only in the wild tomato showed dose-dependent repellence to T. absoluta females in wind tunnel. This study provides some groundwork for exploiting the allelochemicals of the wild tomato in the development of novel integrated pest management approaches against T. absoluta.

Differential responses of Bactrocera dorsalis and its parasitoids to headspaces of different varieties of tree-attached mango fruits and the associated chemical profiles

“…Dual-choice olfactometer assays were carried out under the field conditions at Mwea East Sub-county, Kirinyaga County, Kenya (00 • 41 39.8 S, 037 • 24 26.7 E, 1,158 m ASL) to evaluate the responses of fruit flies and parasitoids to mango fruit volatiles, in situ, following the methods described by Nyasembe et al (2012) and Miano et al (2022) with some modifications. The dual-choice olfactometer and the mango holders were made of Perspex glass (Figure 2).…”

“…For each bioassay, 10 mated adult females (10-15 days old for B. dorsalis or 8-14 days for parasitoids) were placed in a releasing vial and kept for 10 min for acclimatization. Thereafter, the group of insects was released through a hole at the center of the bottom of the wind tunnel (Figure 2) using a black coated falcon tube and they were allowed 20 min to choose, following the procedure of group release described with modifications by Nyasembe et al (2012), Biasazin et al (2014), Njuguna et al (2018), Binyameen et al (2021), andMiano et al (2022) with modifications. The base of the dual-choice olfactometer was marked from 0 to 60 cm on either side of the insect release point to allow scoring.…”

“…The collection of mango headspaces was carried out simultaneously during the bioassay experiments. Dynamic headspace trapping (DHT) system (Ormeño et al, 2011;Miano et al, 2022) was used with some modifications (Supplementary Figure 2). Ambient air was drawn into the system using portable vacuum field pumps and passed via air flow meters at a rate of 250 mL/min and drawn out at the same rate.…”

“…Headspace volatiles were collected for 11 h (7.00 a.m. to 6.00 p.m.). For preservation and transportation of the headspace volatiles trapped in HayeSep-Q adsorbents the procedure explained in Miano et al (2022) was used. The headspace volatiles trapped in HayeSep-Q were then eluted in 200 µL DCM into 250 µL conical point glass inserts contained in clear 1.5 mL glass vials (Supelco, Bellefonte, PA, USA) using high-purity nitrogen gas as the pressurizing gas and immediately stored in a freezer at −81 • C until use.…”

“…For relative quantification of the release rates of volatiles, a serial dilution (2.25-1,000 ηg µL −1 ) of the authentic standards α-pinene and α-humulene (98% purity, Sigma-Aldrich R Solutions, St. Luis, MO, USA) were analyzed by GC-MS in full scan mode to generate linear calibration curves (peak area vs. concentration, Supplementary Figure 4; Njuguna et al, 2018;Miano et al, 2022). The linear equations generated were y = 2036653.8x − 5127153.0; R 2 = 0.9963 for α-pinene and y = 1127808.7x − 5512234.2; R 2 = 0.9991 for α-humulene and were used to quantify volatile compounds that had retention times that were either below or above 16 min, respectively.…”