The potential of developing HIPVs into IPM compatible products is well-recognized due to their good selectivity to pests, safety to non-target organisms and the environment (Isman 2000; Tewary et al. 2006b; Pickett and Khan 2016; Xu and Turlings 2018; Ali et al. 2022). For example, MeSA (Wang et al. 2011, 2019), EBF (Bowers et al. 1972; Li et al. 2019), eugenol (Dardouri et al. 2019) and Carvone (Pascual-Villalobos et al. 2017) have demonstrated efficacy in repelling aphids. However, the limited efficacy (James 2006; Wu et al. 2022) or structural stability (Holopainen and Blande 2013) of such natural products pose challenges for their practical applications. To optimize the utility of HIPVs, we rationally designed and synthesized compound 3e by splicing the active fragments of MeSA and EBF in our previous work (Qin et al. 2020). Notably, compound 3e exhibited remarkable effectiveness as an aphid repellent. However, both the repellent properties of compound 3e and its repellent efficacy against aphids in greenhouse or field settings remain to be determined.
To elucidate the repellent properties of 3e, behavioral bioassays were first conducted in the laboratory. The findings revealed that 5 µg and 15 minutes were identified as the optimal dosage and time, respectively, diverging from the dose-dependent effect properties of conventional insecticides, such as neonicotinoids that target nicotinic acetylcholine receptor (nAChR). It was speculated that this could be attributed to its unique mode of action, particularly binding to ApisOBP9 (Qin et al. 2020; Yang et al. 2022) and its volatile characteristics, which would be further verified in future studies.
For determining the aphid-repellent efficacy of 3e in a greenhouse setting, pot experiments were then implemented in 2021 and 2023. Results of both experiments indicated that the number of aphids on plants treated with 3e were significantly lower than the control, corroborating the significant repellent activity of 3e against aphids in greenhouse. Importantly, the relationship between the concentration of 3e and its repellent activity against aphids was non-linear, aligning with the findings of the laboratory bioassays. Specifically, the intermediate concentrations of 3e demonstrated a superior bioactivity compared to both higher and lower concentrations, suggesting a dose-independent behavioral response in aphids to 3e, similar to the bioactivity traits of volatiles described in the extant corpus of literature (Verheggen et al. 2007a; Kheloul et al. 2019; Cantó-Tejero et al. 2022). Moreover, while the quick-acting of 3e in controlling aphids was lower than beta-cypermethrin, it sustained its efficacy in aphid management for a duration of at least 14 days. The aforementioned results reaffirmed the unique mode of action of 3e, which was distinct from traditional aphicides that target the insect nervous system.
In accordance with the results of the greenhouse pot experiments in 2021 and 2023, a subsequent field trial was carried out to further investigate the repellent activity of 3e against aphids in situ. Results from the field trial robustly confirmed the significant aphid-repellent activity of 3e in field conditions, with better repellency at intermediate concentrations. Furthermore, the field trial also established that 3e maintained its repellency consistently for a duration extending to at least a fortnight. These results underlined the importance of both concentration and timing in the application of 3e to maximize its repellent capacity.
In the context of IPM, in addition to intercropping, the strategic deployment of HIPVs has been widely recognized (Sarles et al. 2015; Xu et al. 2018). Both laboratory and field studies have consistently demonstrated the potential of HIPVs releases to simultaneously repel pests and attract natural enemies (i.e., ‘push–pull’ plant protection strategy) (Ninkovic et al. 2003; Zhou et al. 2016). For instance, HIPVs such as MeSA and EBF have been tested for their efficacy in repelling aphids and attracting natural enemies (Zhu and Park 2005; Harmel et al. 2007; Mallinger et al. 2011; Rodriguez-Saona et al. 2011; Liu et al. 2021). Additionally, compound 3e, integrating active fragments of MeSA and EBF, has demonstrated repellent properties against aphids (Qin et al. 2020) and also acts as an attractant for ladybugs (Yang et al. 2022). However, the potential of compound 3e to attract other aphid natural enemies remains to be elucidated.
To investigate the attractant potential of 3e towards hoverflies, an EAG test was first performed. The results verified that adult male and female E. corollae were sensitive to both 3e and MeSA, with 3e eliciting a stronger response in the antennae of both sexes than MeSA. The findings implied that 3e may exhibit superior E. corollae luring activity compared with MeSA.
A three-year field trial was subsequently carried out to further confirm the attractive activity of 3e to hoverflies. During these trials, 3e consistently displayed excellent attractant activity, with the concentrations of 58 and 116 mg/L showing the highest attraction, which was superior to MeSA. In addition, the attractant effect of 3e in field environments can be maintained for at least 14 days. These results suggested the promise of 3e to be used as a tool for attracting hoverflies.
To elucidate the contribution of an additional EBF active fragment in 3e to its enhanced hoverfly attractant efficacy compared with MeSA, the possible molecular mechanism was investigated using the molecular docking method. The results indicated that the additional EBF active fragment endowed 3e with a stronger hydrophobic interaction with upstream target EcorOBP15 and a higher steric space effect with the downstream target EcorOR3 than MeSA, potentially clarifying the better hoverfly attraction activity observed in 3e.
Pollinators such as bees are essential for the pollination of a wide variety of crops and the majority of wild flowering plants (Stanley et al. 2015; Kleijn et al. 2016), but there is a growing evidence indicating that the exposure to field-realistic levels of pesticides can lead to sublethal effects on bees, affecting their foraging behaviour (Gill et al. 2012; Gill and Raine 2014), homing ability (Henry et al. 2012; Fischer et al. 2014) and reproductive success (Whitehorn et al. 2012; Rundlöf et al. 2015). To assess the toxicity of 3e on bees, the A. mellifera was selected as the test organism. And the results showed that 3e was a low toxicity HIPVs derivative toward honeybees.
Consequently, 3e can be considered as a potential candidate for push–pull strategies in sustainable agricultural practices, offering a novel approach to aphid control without detrimental effects on non-target organisms.