Infestation of the gall midge Dasineura oleae provides first evidence of induced plant volatiles in olive leaves

Diel rhythm of volatile emissions from males and females of the olive fruit fly Bactrocera oleae using PTR-ToF and GC-MS

The olive fruit fly Bactrocera oleae, is the major key pest of olive groves worldwide. As an odor-driven species, its intraspecific communication has been thoroughly investigated, yielding a combination of spiroacetals, esters and hydrocarbons. However, its management with pheromone is still restricted to olean, the major pheromone component. Given the crucial role of circadian rhythm and pheromone blends in mediating flies reproductive behavior compared to single compounds, B. oleae headspace chemical profile was carefully examined, through the combination of Proton Transfer Reaction Time of Flight Mass Spectrometry (PTR-ToF) and Gas Chromatography coupled with Mass Spectrometry (GC-MS). This novel approach aimed at continuously investigating the temporal scale of volatilome profile of B. oleae individuals, as well as the determination of new candidate sex-borne compounds (particularly those emitted in traces or having low molecular weight), that may be relevant to the fly's chemical communication and were unreported due to limitations of frequently used analytical techniques. Our results describe the dynamics and diversity of B. oleae chemical profile, highlighting the emission of 90 compounds, with clear diel rhythm of release, of known pheromone components of B. oleae (e.g., olean, alpha-pinene and muscalure) and new candidates. In contrast to ammonia, acetaldehyde and muscalure, which were highly emitted during the afternoon by males and mixed groups, olean was mostly released by mature females and mixed groups, with a peak of emission during early-morning and afternoon. This emission of olean around dawn is reported for the first time, suggesting early-morning mating activity in B. oleae. Furthermore, esters, such as methyl tetradecanoate, which had been earlier identified as a pheromone for B. oleae, did not exhibit any discernible release patterns. These findings are the first to demonstrate the emission of chemicals, which are only produced when males and females are close to one another, with an emission peak during the afternoon (mating period), and that may have aphrodisiac properties for B. oleae males. These results emphasize the relevance of compounds with distinct diel rhythm and address their potential function as intraspecific messengers, according to their source and timing of release.

The Cuticular Hydrocarbons of Dasineura Oleae Show Differences Between Sex, Adult Age and Mating Status

In insects, cuticular lipids prevent water loss and act as semiochemicals. Because of their ecological function, the profile change across the insects' sex and development offers insight into insect biology and possible tools for pest management. Here, the first work on cecidomyiid cuticular extracts is proposed considering Dasineura oleae (Diptera: Cecidomyiidae) males and females at different adult ages (0-12 h, 12-24 h, 24-36 h) and distinct sexual conditions (virgin and mated). A set of 49 compounds were recorded (12 alkanes, 1 monomethyl alkane, 11 fatty acids, 4 esters, 1 aldehyde, 1 allylbenzene, 1 amine, 1 flavonoid, 1 ketone, 1 phenol, 1 steradiene, 1 sterol, 1 terpene, 1 triterpene and 11 unknown compounds), and 18 of them showed significant differences between groups. Among alkanes, hexacosane (nC26) exhibited a decreasing trend from the youngest to the oldest females, while pentacosane (nC25) and nonacosane (nC29) showed a decreasing trend from 0 to 12 h to 12-24 h virgin females. In addition, nonadecane (nC19) was significantly more abundant in the youngest males compared to older males and females. The alkanes nC25, nC26 and nC29 have been reported to be age-related also in other dipterans, while nC19 has been described as gender-specific chemical cue for platygastrid parasitoids. Further behavioural trials and analyses are required to assign the specific ecological roles to the characterized compounds. Our results may contribute to develop new low-impact control strategies relying on the manipulation of D. oleae's chemical communication (e.g. disruption of mating or species recognition). HIGHLIGHTS: • Cuticular hydrocarbons are often involved in dipteran intraspecific communication. • We explored the cuticular profile of D. oleae at different age, sex, mating condition. • Five alkanes and one mono-methyl alkane showed differences among groups. • Linoleic acid is the most abundant compound in virgins, absent in mated insects. • Eleven compounds disappear in mated insects, but were present in all virgins.

Metabolic Responses in Leaves of 15 Italian Olive Cultivars in Correspondence to Variable Climatic Elements

This study aims to evaluate the metabolic changes that occurred in olive leaves as responses over time to variations in climatic elements. Rainfall, temperature, and solar radiation data were collected over 4 months (August-November) to assess the impact of different climatic trends on the metabolism of the leaves of 15 Italian olive cultivars, cultivated at the experimental farm of the University of Florence. The net photosynthetic rate (AN) and stomatal conductance (gs), measured as main indicators of primary metabolism, were mainly influenced by the "cultivar" effect compared to the "climate" effect. The lowest AN value was showed by "Bianchera", while "Ascolana" recorded the highest (8.6 and 13.6 µmol CO2 m-2s-1, respectively). On the other hand, the secondary metabolism indicators, volatile organic compound (VOC) and oleuropein (OL) content, were much more influenced by climate trends, especially rainfall. A phase of high rainfall caused a significant increase in the VOCs emission from leaves, even with different behaviors among the genotypes. The highest differences were observed between "Maiatica di Ferrandina", with the highest average values (~85,000 npcs), and "Frantoio", which showed the lowest (~22,700 npcs). The OL content underwent considerable fluctuations in relation to the rainfall but also appeared to be controlled by the genotype. "Coratina" always showed the highest OL concentration (reaching the maximum ~98 mg g-1), indicating the great potential of this cultivar for the industrial recovery of OL.

Electroantennographic and olfactory responses of Quadrastichus mendeli to eucalyptus volatiles induced by the gall-forming insect Leptocybe invasa

BACKGROUND

Although parasitoids can precisely locate hidden gall-inducing insects, the host location mechanism is unknown. In this study, our aim was to clarify the olfactory responses of the parasitoid Quadrastichus mendeli to eucalyptus volatiles induced by the gall wasp Leptocybe invasa.

RESULTS

Q. mendeli preferred volatiles from gall-damaged plants compared with those produced by mechanically damaged and undamaged plants. Coupled gas chromatographic-electroantennographic detection results demonstrated that 3-carene, decanal, d-limonene, ethanone,1-(4-ethylphenyl)-, p-cymene and benzene,1-methyl-4-(1-methylpropyl)- from DH 201-2 (Eucalyptus grandis × Eucalyptus tereticornis) elicited significant antennal responses in Q. mendeli in all treatments. Q. mendeli was repelled by decanal and d-limonene and was attracted to 3-carene, benzene,1-methyl-4-(1-methylpropyl)-, ethanone,1-(4-ethylphenyl) and p-cymene. Quaternary blends containing 3-carene, p-cymene, benzene,1-methyl-4-(1-methylpropyl)- and ethanone,1-(4-ethylphenyl)- at a ratio of 1:1:1:1 were attractive to Q. mendeli. However, quaternary blends with added decanal and d-limonene alone or both together induced significant repellence in Q. mendeli.

CONCLUSION

Our report is the first to demonstrate that volatiles produced by galls induced by L. invasa are attractive to Q. mendeli, which suggests that this parasitoid could utilize herbivore-induced plant volatiles to locate its host. The results are beneficial for understanding the function of plant volatiles in host searching by parasitoids of gall-forming insect pests. © 2022 Society of Chemical Industry.

Impact of Chronic Exposure to Two Neonicotinoids on Honey Bee Antennal Responses to Flower Volatiles and Pheromonal Compounds

Volatile compounds provide important olfactory cues for honey bees (Apis mellifera L.), which are essential for their ecology, behavior, and social communication. In the external environment bees locate food sources by the use of floral scents, while inside the hive, pheromones such as the queen mandibular pheromone (QMP) and alarm pheromones serve important functions in regulating colony life and inducing aggressive responses against intruders and parasites. Widely reported alterations of various behaviors in- and outside the hive following exposure to pesticides could therefore be associated with a disturbance of odor sensitivity. In the present study, we tested the effects of neonicotinoid pesticides at field concentrations on the ability of honey bees to perceive volatiles at the very periphery of the olfactory system. Bee colonies were subjected to treatments during the summer with either Imidacloprid or Thiacloprid at sublethal concentrations. Antennal responses to apple (Malus domestica L.) flower volatiles were studied by GC-coupled electro-antennographic detection (GC-EAD), and a range of volatiles, a substitute of the QMP, and the alarm pheromone 2-heptanone were tested by electroantennography (EAG). Short-term and long-term effects of the neonicotinoid treatments were investigated on bees collected in the autumn and again in the following spring. Treatment with Thiacloprid induced changes in antennal responses to specific flower VOCs, with differing short- and long-term effects. In the short term, increased antennal responses were observed for benzyl-alcohol and 1-hexanol, which are common flower volatiles but also constituents of the honey bee sting gland secretions. The treatment with Thiacloprid also affected antennal responses to the QMP and the mandibular alarm pheromone 2-heptanone. In the short term, a faster signal degeneration of the response signal to the positive control citral was recorded in the antennae of bees exposed to Thiacloprid or Imidacloprid. Finally, we observed season-related differences in the antennal responses to multiple VOCs. Altogether, our results suggest that volatile-specific alterations of antennal responses may contribute to explaining several behavioral changes previously observed in neonicotinoid-exposed bees. Treatment effects were generally more prominent in the short term, suggesting that adverse effects of neonicotinoid exposure may not persist across generations.