Sound production in three sympatric Ips (Coleoptera:Scolytidae) species co-inhabiting Sitka spruce (Picea sitchensis) Public Deposited


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  • Three sympatric species of Ips, colonizing Sitka spruce, were investigated with regard to their infestation habits, stridulating apparatus and acoustic signals. The above phenomena are evaluated qualitatively and/or quantitatively and the acoustic signal differences are proposed as a possible factor for maintaining species isolation, although function is not shown. Field and laboratory observations show that I. concinnus and I. tridens tridens have similar microhabitat preferences, emergence periods, brood development and pairing behavior. Although field observations were not obtained for I. plastographus, previous studies indicate that their ecological and attack behavior is probably similar to the two other species. Statistical comparisons of measured parameters of the male and female I. concinnus gular-prosternum stridulatory organ revealed several intra-specific differences, which, in several parameters, were correlated with beetle size. Utilization of the electron microscope and compound light microscope did not reveal any discernible stridulatory organs on male and female I. t. tridens beetles. Electron micrographs and measurements of length, width and number of ridges are presented for the vertex pronotam stridulatory organs on female I. plastographus. Stridulatory structures were not found on male I. plasto raphus. The stridulatory structure differences encountered in the three species are considered a major influence in acoustic signal disparities. Stridulations produced by I. concinnus beetles in the behavioral contexts of attraction, female stress and male aggression are illustrated and statistically compared, using the parameters of pulse number per chirp, chirp duration and pulse rate. I. concinnus attraction stridulations, tape recorded as five consecutive females join a male, show that the chirps of the first female differ statistically from chirps of following females in number of pulses per chirp. Additionally, pulse rate and duration of chirps of only the third female differed significantly from chirps of the first female. These stridulation differences appear to be correlated with attack behavior and Increasing male resistance to entering females as number of females increases in the nuptial chamber. Although the brief clicks emitted by male and female I. t. tridens males in an attraction context are similar to male aggression and female stress clicks, they are distinctly different in structure, duration and frequency when compared to I. concinnus and I. plastographus attraction chirps. Oscillographs of stridulations produced by female I. plastographus in the natural behavioral contexts of attraction, rivalry and stress are presented. Statistical comparisons show that attraction chirps are quicker in pulse rate and that rivalry chirps possess a greater number of interrupted chirps. Attraction chirps of I. concinnus and I. plastographus are statistically compared and appear distinct on the basis of chirp duration and pulses per chirp. Oscillograms depicting internal pulse structure of chirps reflect stridulatory apparatus differences between I. concinnus and I. plastographus. Pulse structure as an important facet of intra-specific signal recognition is discussed. The distinctiveness of each species' attraction stridulations and the apparent importance of acoustic signals in pairing behavior suggests that stridulation contributes to reproductive isolation in these three species of Ips.
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