Identification of a Female-Produced Sex Attractant Pheromone of the Winter Firefly, Photinus corruscus Linnaeus (Coleoptera: Lampyridae)

Abstract

Firefly flashes are well-known visual signals used by these insects to find, identify, and choose mates. However, many firefly species have lost the ability to produce light as adults. These “unlighted” species generally lack developed adult light organs, are diurnal rather than nocturnal, and are believed to use volatile pheromones acting over a distance to locate mates. While cuticular hydrocarbons, which may function in mate recognition at close range, have been examined for a handful of the over 2000 extant firefly species, no volatile pheromone has ever been identified. In this study, using coupled gas chromatography - electroantennographic detection, we detected a single female-emitted compound that elicited antennal responses from wild-caught male winter fireflies, Photinus corruscus. The compound was identified as (1S)-exo-3-hydroxycamphor (hydroxycamphor). In field trials at two sites across the species’ eastern North American range, large numbers of male P. corruscus were attracted to synthesized hydroxycamphor, verifying its function as a volatile sex attractant pheromone. Males spent more time in contact with lures treated with synthesized hydroxycamphor than those treated with solvent only in laboratory two-choice assays. Further, using single sensillum recordings, we characterized a pheromone-sensitive odorant receptor neuron in a specific olfactory sensillum on male P. corruscus antennae and demonstrated its sensitivity to hydroxycamphor. Thus, this study has identified the first volatile pheromone and its corresponding sensory neuron for any firefly species, and provides a tool for monitoring P. corruscus populations for conservation and further inquiry into the chemical and cellular bases for sexual communication among fireflies.

Antennal sensilla diversity in diurnal and nocturnal fireflies (Coleoptera, Lampyridae)

“…The peg length is ~3-4 times the height of the dome base, the width of the peg is uniform throughout the length and the distal end of the peg is rounded. We found no pores along the length of B1 sensilla, however B1 sensilla identified by Lower et al [21] possessed pores along the sides of the peg. B1 sensilla were found in the diurnal (nonbioluminescent) species P. corruscus, Py.…”

“…Based on their numbers, B2 and B3 sensilla could be two other pheromone sensilla candidates, but in both cases, females have more B2 and B3 sensilla than males. Furthermore, sensilla similar in morphology to B3 sensilla were unresponsive to P. corruscus sex pheromones [21], confirming that B3 sensilla are not used for the detection of sex pheromones in this species. The nocturnal P. pyralis, a closely related congener of P. corruscus in this study, also has high numbers of B1 sensilla, and males have ~ 30% more B1 sensilla than their females (Table S4), suggesting that both Photinus species may use B1 sensilla as pheromone sensilla.…”

“…An interesting challenge was the identification of pheromone sensilla, especially in the absence of functional verification with electrophysiology. The first pheromone sensilla for fireflies were just recently identified in P. corruscus [21] and we used this opportunity to test our set of predictions for identifying potential sex pheromone sensilla in fireflies based on morphology alone. We hypothesized that the differences in mating signals between diurnal and nocturnal fireflies will be reflected in their antennal sensilla counts and possibly in their sensilla densities.…”

“…Among chemoreceptors most functional studies have focused on the detection of plant volatiles, sex pheromones, and/or aggregation pheromones [19]. For example, sensilla trichodea detected sex pheromones in the pine weevil Hylobius abietu (Curculionidae [20]), and sensilla basiconica were shown to detect sex pheromones in the firefly Photinus corruscus [21]. In addition, sensilla coeloconica have been identified as thermoreceptors in Siagona europaea (Carabidae [22]), and as hygroreceptors in the firefly Luciola cruciata [23].…”

“…The specific chemicals produced by female fireflies have only been identified for a hand full of species; Pyrocoelia oshimana [43], two unidentified Diaphanes species, Pyrocoelia praetexta , and Lamprigera tenebrosa [44]. However, functional female sex pheromones have only been identified for one species Photinus corruscus [21]. The specific antennal sensilla that respond to sex pheromones in P. corruscus , the diurnal winter firefly, have recently been identified as sensilla basiconica [21].…”