| Summary: | The scale predator <i>Rhyzobius lophanthae</i> Blaisdell was introduced to Guam and Rota to control invasive <i>Aulacaspis yasumatsui</i> Takagi armored scale infestations on the native <i>Cycas micronesica</i> K.D. Hill populations. The predator demonstrated a clear preference for <i>A. yasumatsui</i> infesting adult plants, resulting in 100% seedling mortality due to the lack of a biocontrol of the scale on seedlings. A Y-tube olfactometer was employed to determine if scale-infested seedling leaves were less attractive to <i>R. lophanthae</i> than scale-infested mature tree leaves. Five paired combinations of seedling versus mature tree leaves were used. The <i>R. lophanthae</i> adults navigated toward scale-infested and un-infested leaves of adults and seedlings when paired with an empty chamber. However, a clear preference for adult leaves occurred when paired with seedling leaves. The results were unambiguous in charcoal-filtered air, intermediate in unfiltered air from an open laboratory, and most ambiguous when conducted with unfiltered in-situ air. The number of predators that did not make a choice was greatest for in-situ air and least for charcoal-filtered air. These results indicated that the substrate used in olfactometry influenced the results, and interpretations of charcoal-filtered air assays should be made with caution. Volatile chemical cues are involved in <i>R. lophanthae</i> preferring <i>A. yasumatsui</i> located on <i>C. micronesica</i> adults when infested adult and seedling leaves are present.
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