Abstract
The corn- and rice-strains of Spodoptera frugiperda exhibit several genetic and behavioral differences and appear to be undergoing ecological speciation in sympatry. Previous studies reported conflicting results when investigating male attraction to pheromone lures in different regions, but this could have been due to inter-strain and/or geographic differences. Therefore, we investigated whether corn- and rice-strain males differed in their response to different synthetic pheromone blends in different regions in North America, the Caribbean and South America. All trapped males were strain-typed by two strain-specific mitochondrial DNA markers. In the first experiment, we found a nearly similar response of corn- and rice-strain males to two different 4-component blends, resembling the corn- and rice-strain female blend we previously described from females in Florida. This response showed some geographic variation in fields in Canada, North Carolina, Florida, Puerto Rico, and South America (Peru, Argentina). In dose-response experiments with the critical secondary sex pheromone component (Z)-7-dodecenyl acetate (Z7-12:OAc), we found some strain-specific differences in male attraction. While the response to Z7-12:OAc varied geographically in the corn-strain, rice-strain males showed almost no variation. We also found that the minor compound (Z)-11-hexadecenyl acetate (Z11-16:OAc) did not increase attraction of both strains in Florida and of corn-strain males in Peru. In a fourth experiment, where we added the stereo-isomer of the critical sex pheromone component, (E)-7-dodecenyl acetate, to the major pheromone component (Z)-9-tetradecenyl acetate (Z9-14:OAc), we found that this compound was attractive to males in North Carolina, but not to males in Peru. Overall, our results suggest that both strains show rather geographic than strain-specific differences in their response to pheromone lures, and that regional sexual communication differences might cause geographic differentiation between populations.
Highlights
Geographic variation in the sexual communication signals of animals is a widespread phenomenon, being reported in frogs [1,2,3], birds [4,5,6], fish [7] and insects [8,9,10]
Geographic variation in sexual communication systems has been reported in several lepidopteran species [29,30,31,32,33,34] and is of interest because changes in the sex pheromone signal and/or response to sex pheromones could result in reproductive isolation and subsequently may lead to speciation [35,36,37,38]
The field tests showed that S. frugiperda males of both strains exhibited some geographic variation in their attraction to two different synthetic 4-component-blends (Blend 1 and Blend 2) in North America, the Caribbean and South America (Figure 1)
Summary
Geographic variation in the sexual communication signals of animals is a widespread phenomenon, being reported in frogs [1,2,3], birds [4,5,6], fish [7] and insects [8,9,10]. Smith) (Lepidoptera: Noctuidae), consists of two genetically and behaviorally distinct strains, the corn- and rice-strain, occurring sympatrically throughout North- and South America [42] Both strains appear to be undergoing ecological speciation in sympatry and reveal several possible prezygotic isolation barriers [43]. These include differential host plant choice [42,44,45,46,47], strain-specific mating times in the scotophase [48,49], as well as differences in the female sex pheromone composition [50,51,52]. Besides being an excellent model to study the evolution of reproductive isolation [43], S. frugiperda is a serious pest species that feeds on a large variety of agricultural crops [56], and can cause annual damages of up to ,300 million dollars in the United States [57]
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