2000 INTRODUCTION Sexual behavior, including response to sexual signals, should be plastic in animals that have distinct reproductive cycles or undergo periods of reproductive inactivity, such as reproductive diapause in insects (Teal et al., 2000). Response to sexual signals should occur at reproductive maturity under suitable environmental conditions for mate finding, mating and production of offspring. Hormones act as mediators between the environment and animal nervous systems to ensure that mate-finding behavior is evoked when animals are physiologically able to produce offspring (Anton et al., 2007). Response to sexual chemical signals or sex pheromones depends on the receiver’s hormonal state in several taxa. In two fish species, hormonal status influences male sensitivity to female-produced sex pheromones. Adult male Puntius schwanenfeldi increase peripheral nervous system sensitivity to pheromone with increased androgen titer (Cardwell et al., 1995) whereas endocrine effects on the central nervous system mediate pheromone responsiveness in the goldfish Carassius auratus (Sorensen et al., 1987). Male hamsters respond to sex pheromone only if a threshold level of testosterone occurs in the brain (Wood and Swann, 2000). Young locusts treated with the gonadotropic hormone juvenile hormone (JH) respond to aggregation pheromone like sexually mature locusts whereas mature locusts deprived of JH have a reduced response to aggregation pheromone similar to that of young locusts (Ignell et al., 2001). In moths (Lepidoptera), mate finding occurs via male response to female-produced, long-distance, sex pheromones. Pheromone response can increase predation risk and be energetically costly (Cardé and Haynes, 2004) and should be confined to a time when moths are sexually mature and the probability of locating a receptive female is high (Anton et al., 2007). Plasticity in pheromone response occurs in long-lived moth species in response to photoperiod and temperature (Dumont and McNeil, 1992), moth age (Turgeon et al., 1983; Dumont and McNeil, 1992; Gemeno and Haynes, 2000), mating status (Gadenne et al., 2001) and hormonal state (Gadenne et al., 1993). JH is the major gonadotropic hormone in many Lepidoptera and regulates egg maturation in females and sex accessory gland development and mating behavior in both sexes (Denlinger, 2002; Ramaswamy et al., 1997). In migratory and long-lived Lepidoptera in which mating is delayed, JH controls sex accessory gland development and induction of pheromone response in adult males (Peter et al., 1981; Cusson et al., 1993; Duportets et al., 1996; Duportets et al., 1998; Anton et al., 2007). JH acid production in adult male Pseudoletia unipuncta correlates with responsiveness to female sex pheromone (Cusson et al., 1994). JH-deprived male Agrotis ipsilon show no response to female sex pheromone and response is restored with the implantation of the gland that produces JH, the corpora allata (CA) (Gadenne et al., 1993). Newly eclosed male A. ipsilon treated with a juvenile hormone analogue (JHA) are responsive to female sex pheromone whereas untreated males do not respond until three days post-eclosion (Gadenne et al., 1993). In moth species studied to date, pheromone response plasticity associated with delayed reproduction is mediated in the central nervous system. The antennae of sexually immature male A. ipsilon are as responsive to female sex pheromone as antennae from reproductively active or JHA-treated males (Gadenne et al., 1993). Intracellular recordings show that JH affects the sensitivity of the The Journal of Experimental Biology 212, 2000-2006 Published by The Company of Biologists 2009 doi:10.1242/jeb.030858 Peripheral and behavioral plasticity of pheromone response and its hormonal control in a long-lived moth Joelle Lemmen* and Maya Evenden Department of Biological Sciences, CW405 Biological Sciences Building, University of Alberta, Edmonton, AB, Canada T6G 2E9 *Author for correspondence (e-mail: jlemmen@ualberta.ca) Accepted 14 April 2009 SUMMARY Reproductive success in many animals depends on the efficient production of and response to sexual signals. In insects, plasticity in sexual communication is predicted in species that experience periods of reproductive inactivity when environmental conditions are unsuitable for reproduction. Here, we study a long-lived moth Caloptilia fraxinella (Ely) (Lepidoptera: Gracillariidae) that is reproductively inactive from eclosion in summer until the following spring. Male sex pheromone responsiveness is plastic and corresponds with female receptivity. Pheromone response plasticity has not been studied in a moth with an extended period of reproductive inactivity. In this study, we ask whether male antennal response and flight behavior are plastic during different stages of reproductive inactivity and whether these responses are regulated by juvenile hormone. Antennal response to the pheromone blend is significantly reduced in reproductively inactive males tested in the summer and autumn as compared with reproductively active males tested in the spring. Reproductively inactive autumn but not summer males show lower antennal responses to individual pheromone components compared with spring males. Treatment with methoprene enhances antennal response of autumn but not summer males to high doses of the pheromone blend. Behavioral response is induced by methoprene treatment in males treated in the autumn but not in the summer. Plasticity of pheromone response in C. fraxinella is regulated, at least in part, by the peripheral nervous system. Antennal and behavioral response to pheromone differed in reproductively active and inactive males and increased with methoprene treatment of inactive males. Key words: juvenile hormone, pheromone response plasticity, Caloptilia, Lepidoptera, sex pheromone. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY