Neuroendocrine Control of Pheromone Biosynthesis in Moths

Ada Rafaeli    ARO, Volcani Center, Institute for Technology and Storage of Agricultural Products, Department of Stored Products, Bet Dagan 50250, Israel

I Introduction

The process of reproduction represents the defining feature of all life-forms. Thus, the events concerned with reproduction may be considered as the ultimate objective of all other life processes. Species diversity, relying on the exchange of genetic information, therefore depends on the successful meeting between two individuals of the same species. Many insect species use species-specific sex pheromones for mate finding. Among the moth species, generally nocturnal and active during the night (scotophase), sex pheromones are prevalent. These are volatile chemical substances, synthesized and emitted by one partner (usually the female) and perceived by the opposite sex. Emission of pheromone by female moths occurs during calling behavior in which the female moth exposes its pheromone gland by extruding its ovipositor tip. Perception of these chemical substances triggers stereotypic orientation responses in the members of the opposite sex. The evolutionary and ecological success of the insect therefore depends on its ability to initiate and terminate pheromone biosynthesis, and the successful outcome of these behavioral events depends on their synchronization. In many moth species this synchronization is achieved by neuroendocrine mechanisms that, in turn, are influenced by various environmental and physiological events (temperature, photoperiod, host plants, mating(s), hormones, neurohormones, and neuromodulators).

This review considers the available evidence concerning the regulation of sex pheromone production at the level of the organism, tissue, and cell and attempts at providing a unifying hypothesis for the apparent variations among different moth species.

II Reproductive Behavior in Moths

Females of many moth species are attractive to males during specific periods of the photoperiod. In these moths, therefore, pheromone production and emission are controlled by an endogenous circadian rhythm that is entrained by photoperiodic cues. Additionally, some moths (e.g., Helicoverpa zea) have been shown to delay production of pheromone until a suitable host plant is found for egg laying (Raina, 1988). These facts suggest that a regulatory mechanism must play an important role in the synchronization of mating behavior.

Raina and Klun (1984) were the first to discover that sex pheromone production is regulated by a neurohormone in H. zea. The neurohormonal activity was found in homogenates of brain complexes [consisting of brain–subesophageal ganglia–corpora cardiaca–corpora allata (Br-SOG-CC-CA) complexes] during both the photophase and the scotophase. This activity was detected in the hemolymph only during the scotophase, thereby indicating a hormonal function. The neurohormone was termed pheromone biosynthesis activating neuropeptide (PBAN). The activity was present in brain complexes of both male and female H. zea as well as in females of other moth species (Ostrinia nubilalis and Lymantria dispar) and in the cockroach (Blatella germanica). Many workers subsequently detected PBAN-like activity in neural tissues of several other moth species as well as in other insect orders (Table I).

The regulatory mechanisms of pheromone production (physiological response) do not depend on calling behavior (behavioral response), although both responses may be synchronized and entrained to the photoperiod. In some species (Sesamia nonagrioides, Plodia interpunctella, Ephestia cautella, and Helicoverpa spp.), pheromone production and calling behavior are synchronous and peak pheromone production occurs during peak periods of calling behavior (Bablis and Mazomenos 1992a; Coffelt et al., 1978; Raina, 1988). However, in other moth species pheromone titers either remain high throughout the 24 h (Trichoplusia ni and Pectinophora gossypiella) (Shorey and Gaston,...