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Top Pheromone variables
Certain environmental factors control sex pheromone communication of insects in two different ways (Bartell and Shorey 1969). Firstly, each factor may operate directly, influencing the intensity at which the pheromone activity will be expressed. Examples of these direct effects are given in the following four sections. Secondly, the daily fluctuations of certain environmental factors, particularly light intensity, may control the timing of pheromone activity by phase-setting of circadiam rhythms. This cyclic recurrence of pheromone behavior will be discussed later (sect. 4.3.1).
Light intensity, varying between > 100,000 lux at midday and <0.0l lux on a moon—less night, is one of the most pronounced and most predictable of the environmental factors that fluctuate on a daily basis. It is little wonder, then, that light intensity is a major factor controlling whether insect sex pheromone activity will occur and, sometimes, the intensity at which it will occur. Thus, females of the Queensland fruit fly, Dacus tryoni (Froggatt), which normally approach males and mate at dusk, were maximally responsive to the male-produced sex pheromone when held in a ‘dim’ light intensity of 8 lux (Fletcher and Giannakakis 1973). Female responsiveness decreased at both higher and lower light intensities. in another insect that typically mates at dusk, the light-brown apple moth, Epiphyas postvittana (Walker), light intensities greater than 3.5 lux suppressed the responsiveness of the top pheromones at
Pheromone density
Population density may have a profound effect on insect behavior. This factor probably should be considered in interpreting the results of many field and labora- tory studies in which pheromone production, release, or responsiveness are deter- mined. For instance, male boll weevils, Anthonomus grandis (Boheman), released more pheromone, as indicated by an increased attractancy to other boll weevils, when isolated individually than when confined together in groups (Hardee et al. 1969). On the other hand, female yellow mealworm beetles, Tenebrio moiitor (L.), released more sex pheromone in the presence of other females than when isolated individually (Happ and Wheeler 1969).
Populations of the Virginia-pine sawfly, Neodiprion pratti (Dyar), typically have outbreaks lasting two to three years, followed by a rapid decline in population size. Bobb (1964, 1972) found that sawfly females released relatively high levels of sex pheromone during outbreak years and little pheromone during low population years. He proposed that the lack of female attractancy was the factor leading to a lack of mating and, therefore, to the population decline. Although no evidence is available, perhaps the crowding during the outbreak years operated as the control factor, resulting in female offspring that had a reduced tendency to produce or release sex pheromone. Check out top pheromones at and
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The influence of diet on subsequent sex pheromone behavior has received little attention. In the few species studied, prior exposure to a sub-optimal food source has led to a decrease in sex pheromone release (California red scale females Aoni- diella aurantii (Maskell) (Rice and Moreno 1969), boll weevil males (Hardee et al 1970), Australian sheep blowfly females Lucilia cuprina (Wied.) (Bartell et al. 1969)). The influence of nutrition on pheromone responsiveness has been investi- gated only in the Australian sheep blowfly. Males provided a protein food were more responsive when exposed to female sex pheromone than were non protein-fed males (Bartell et al. 1969). None of the above experiments have evaluated whether the appropriate food had a selective controlling action on pheromone behavior, or whether the observed effects occurred in conjunction with many other behavioral and physiological effects as a result of an increased vigor due to the proper nutrtion.