The ethanolic extract of P. vulgaris pods proved attractive to A. suturalis and A. lucorum, and the volatile tetradecane yielded the strongest EAG response among other volatiles in the extract. The attractive effect on mirid bugs was confirmed both by field trapping experiments and olfactory tests in the laboratory. Furthermore, the study proved that other tetradecane analogues display a robust attractive effect on mirid bugs. As this compound is a common volatile among host plants, it is likely a key olfactory recognition cue to suitable host plants. But host recognition of A. suturalis and A. lucorum depends on more than olfaction.
Plants produce multiple secondary metabolite volatiles including hydrocarbons, alcohols, aldehydes, ketones, esters, organic acids, and terpenes, which are important cues for herbivorous insects in identifying and locating their hosts (Defagó et al. 2016, Lu et al. 2012a, Najar-Rodriguez et al. 2013). For example, it has been demonstrated that seven compounds, e.g. cis-formic acid-3-hexene ester, m-xylene, and 3-ethyl benzene, among the volatiles of eighteen host plants will trigger a strong EAG response of A. lucorum. Moreover xylene, butyl acrylate, acrylic acid butyl ester and butyl butyrate showed a more significant attraction to mirid bugs and played an important role in host conversion (Pan et al. 2015a, Pan et al. 2015b). P. vulgaris is an important host of A. suturalis and A. lucorum, regarded as important to the growth and development of mirid bugs (Xiao et al. 2013), and its extract has no less attractive effect, which may greatly facilitate practical applications. The present investigation relied on ethanolic extract to characterize the host-localization cues used by mirid bugs on P. vulgaris. Our data strongly indicated tetradecane is central for host attraction, as illustrated by electrophysiological and behavioral results.
Tetradecane seems widely distributed among the volatiles of host plants exploited by mirid bugs, making it surprising that the attraction activity of the compound was rarely reported. When A. suturalis and A. lucorum was only allowed to asses hosts by olfaction, their attractiveness was closely related to their relative amounts of tetradecane in volatiles, indicating it is an important olfactory clue for mirid bugs. This was further confirmed by the increased selected-response rates after supplementation with tetradecane to host plants presenting a low concentration of that compound. Further studies have shown that some tetradecane analogues can be more attractive to mirid bugs than tetradecane, which opens an opportunity for the development of attractants with similar chemical structure.
Our data also suggests that the selection of hosts by mirid bugs will vary depending on whether they physically assess plants, implying other sensorial organs play important roles in host recognition. For example, some insects have a complex visual perception. Physical and chemical signals can synergistically compound attractiveness to host plants. For instance, color seems to play an important role: black and red traps on pine woods will capture more Hylastesater (Coleoptera: Scolytidae) and Arhopalusferus (Coleoptera: Cerambycidae) adults among other traps of different colors, regardless of whether attractants like alpha-pinene are included. Still, the addition of attractants can be conducive to trapping more pests (Kerr et al. 2017). This phenomenon has also been demonstrated in mirid pests, A. lucorum was more attracted to cotton plants with green lights, and the selective response was significantly higher than that of two signals alone (Pan et al. 2015c). For this reason, yellow boards with attractants may have compounded a synergistic effect during the field experiment.
Gustation and tactile cuesmayalso be critical to host recognition by arthropods. This has been demonstrated for a number of species. For example, direct contact involving chemical, visual, and tactile cues triggered stronger responses of Tunicotheres moseri to the hosts (Ambrosio &Brooks 2011). Gustatory sensilla have even been described from the prothoracic legs of female adults of the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae) which respond to sucrose, glucose, fructose, maltose, inositol, and 20 different amino acids, thus enabling adults to evaluate nectar upon perching, triggering feeding behavior (Zhang et al. 2010). In short, host recognition by mirid bugs is described as a complex process that may be affected by different sensory organs in different spatial levels (Henze et al. 2018).Future studies should thus focus on defining the role of sensorial organs, especially gustation and tactile receptors,in host recognition.
In the current study, we observed a difference in the sensitivity to tetradecane between male and female mirid bugs. In fact, previous studies indicated that males usually respond more strongly to sex pheromones than females in locating mating partners, while host plant volatiles were more attractive to female adults. For example, male antennae of Heliothis virescens and H.subflexa were more responsive to the major sex pheromone compound, (Z)-11-hexadecenal, than were female antennae (Groot et al. 2005); female Apantelest aragamae responded significantly longer to the volatiles of host plants than to clean air, while male adults responded significantly longer to clean air rather than to host plants (Nurkomar et al. 2017). In this paper, female mirid bugs were found to be attracted by two concentrations of tetradecane; however, the compound elicited no attractiveness to male adults at lower concentrations. Therefore, we hypothesize that female mirid bugs will locate hosts using tetradecane, while male adults were more likely to be attracted to host plant fields by the sex pheromones of females, due to their low sensitivity to the volatiles. The differential olfactory response suggests there are differences between male and female mirid bugs in sensorial factors, like odor binding proteins (OBPs), chemosensory proteins (CSPs), and odor receptor protein (ORs), such as has been demonstrated with some other pests by transcriptome analyses (Große-Wilde et al. 2010, Li et al. 2015). Still, the molecular basis of host recognition in mirid bugs using tetradecane warrants further research.
Finally, we explored the recognition mechanism of mirid bugs towards multiple host plants and proposed controlling methods of mirid bugs for the cotton field. While the attractants surveyed in this study needed to be matched with yellow boards, it should be expected that they yield good attractiveness when used singly; nonetheless, combining with other attractants is usually a good strategy (Kendra et al. 2017, Tasin et al. 2018). Previous studies showed that sex pheromones have a significant attractive effect on male adults, making up for the poor attractiveness of tetradecane observed in males. Moreover, the volatiles of host plants can increase the EAG response of males to sex pheromones and promote mating behavior (Binyameen et al. 2013, Ian et al. 2017, Namiki et al. 2008). Therefore, we speculate that the combination of sex pheromones with tetradecane hold sample prospects for future research. In addition, olfaction recognition has been demonstrated as only a component of the host recognition, so that the shape and color of the traps also significantly impact attractiveness in the field. The commercial scope of attractants for mirid bugs remains quite limited compared with available options against Lepidoptera pests, although some products have already been pre-tested under field conditions. We hope that the current study will provide the possibility of developing more efficient and safe pest control measures againstmirid bugs in the future.