Aulacaspis yasumatsui Takagi (Hemiptera: Sternorrhyncha: Diaspididae) is a cycad-specific armored scale that invaded Guam in 2003 (Marler and Muniappan, 2006). This pest has combined with several other invasive insects to threaten Cycas micronesica K.D. Hill and Cycas revoluta Thunberg trees. These two Cycas species were widely planted throughout the urban landscape before the pest invasions, and the regionally endemic C. micronesica was a dominant forest species. Coccobius fulvus Compere & Annecke (Hymenoptera: Aphelinidae), an endoparasitoid, and the beetle Rhyzobius lophanthae Blaisdell (Coleoptera: Coccinellidae), a predator, were introduced as efforts to establish a biological control program (Moore et al., 2005). The endoparasitoid failed to establish despite multiple introductions, but the predator proliferated.
Cycas micronesica plant mortality after the scale invasion was epidemic, but slowed down after the widespread establishment of R. lophanthae (Marler and Lawrence, 2012). After seven years, the biological control by this predator has not proven to be completely effective, although the beetle is widespread and both adults and larvae feed on A. yasumatsui (Fig. 1). The decline in population of the endemic C. micronesica elicited an endangered status by the International Union for Conservation of Nature by 2006 (Marler et al., 2006), and continuing mortality predicts extirpation from local forests by 2019 (Marler and Lawrence, 2012). A few C. micronesica and C. revoluta trees linger in commercial and home landscapes, but they do not display their former aesthetic appeal and are thus less valuable in the horticulture trade.
In attempts to understand why R. lophanthae has not been completely effective in the urban and natural landscapes, we have noted that the beetle does not effectively protect C. micronesica seedlings from the armored scale (Marler and Terry, 2011). An understanding of the causal effects of the limited predation is required to inform ongoing horticultural management and biological control strategies. Toward that goal, our objective was to determine scale predation on seedlings positioned at the natural ground level and artificially elevated to the height of mature tree leaves. To improve interpretation, we monitored pest and predator incidence two years before and four years after the elevation experiment at the study site.
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