Abstract
Antennal olfactory receptor neurons (ORNs) for pheromone-related and plant volatile compounds were identified and characterized in the lucerne weevil, Sitona discoideus (Gyllenhal), using the single sensillum recording technique. Our study using five pheromone-related compounds and 42 plant volatile compounds indicates that S. discoideus have highly specialized ORNs for pheromone and plant volatile compounds. Different groups of ORNs present in both males and females of S. discoideus were highly sensitive to 4-methylheptane-3,5-dione (diketone) and four isomers (RR, RS, SR and SS) of 5-hydroxy-4-methylheptan-3-one, respectively. Our results also indicate that male S. discoideus, using the sensory input from antennal ORNs, can distinguish both diketone and the RR-isomer from others, and RS- and SS-isomers from others, although it was unclear if they can distinguish between RS-isomer and SS-isomer, or between diketone and the SR-isomer. It also appeared that female S. discoideus could distinguish between RS-isomer and SS-isomers. The antennae of S. discoideus thus contain sex-specific sets of ORNs for host- and non-host plant volatile compounds. Both sexes of S. discoideus have highly sensitive and selective ORNs for some green-leaf volatiles, such as (Z)-3-hexenol and (E)-2-hexenal. In contrast, male antennae of S. discoideus house three distinct groups of ORNs specialized for myrcene and (E)-β-ocimene, 2-phenylethanol, and phenylacetaldehyde, respectively, whereas female antennae contain three groups of ORNs specialized for (±)-linalool and (±)-α-terpineol, myrcene and (E)-β-ocimene, (±)-1-octen-3-ol, and 3-octanone. Our results suggest that S. discoideus use a multi-component pheromone communication system, and a sex-specific set of ORNs with a narrow range of response spectra for host-plant location.
Highlights
Sitona discoideus (Gyllenhål) (Coleoptera: Curculionidae), commonly called the lucerne weevil, was first reported in New Zealand in 1974 when larvae were found feeding on burr medic (Medicago polymorpha L.) near Napier in the North Island (Esson 1975)
Overall Response Profile of Antennal Olfactory Sensilla in S. discoideus The olfactory receptor neurons (ORNs) observed in S. discoideus showed spontaneous firing of action potentials at 15.62 ± 0.83 spikes/s and 13.24 ± 1.59 spikes/s in pheromone-related ORNs of males and females, respectively, and at 3.44 ± 1.21 spikes/s and 2.95 ± 1.12 spikes/s in plant volatile-related ORNs of males and females, respectively
Our results showed that highly sensitive and selective ORNs for non-greenleaf volatile (GLV) plant volatile compounds are present in the antennae of S. discoideus, and the corresponding olfactory active compounds for these ORNs were clearly different between males and females
Summary
Sitona discoideus (Gyllenhål) (Coleoptera: Curculionidae), commonly called the lucerne weevil, was first reported in New Zealand in 1974 when larvae were found feeding on burr medic (Medicago polymorpha L.) near Napier in the North Island (Esson 1975). From Morocco, it was Sitona discoideus has a univoltine life cycle in New Zealand (Goldson et al 1984; Sue et al 1980), with adult emergence from the soil in December–January (summer), followed by a period of vigorous feeding by the adults, during which time adults reach flight competency and fly to aestivation sites located along hedge rows and fence posts (Frampton 1987; Goldson et al 1984). The return to the lucerne is marked by reproductive maturation between the months of March and May (Goldson et al 1984). The biology in New Zealand is similar to that observed in the Mediterranean habitat which is considered to be the species center of origin (Aeschlimann 1979)
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