Cycas micronesica density in cliff line areas of northern Guam infested with Asian cycad scale, Aulacaspis yasumatsui

Asian cycad scale (ACS), Aulacaspis yasumatsui, first detected in Tumon, Guam in December 2003, has killed nearly half of Guam’s 1.5 million indigenous cycads, Cycas micronesica, over the past five years. This project estimated the density of C. micronesica remaining in the inaccessible cliff lines of northwestern, northern, and northeastern Guam and assessed their current health status using 80 aerial photographs of the cliff line perimeter of northern Guam in April 2007. There were 6537 C. micronesica counted in the photographs, resulting in a density of 0.8 trees/ha. Of these, 2160 (33%) exhibited leaf yellowing, drooping leaves, or defoliation characteristic of A. yasumatsui infestation while healthy trees appeared green and vigorous. There were no statistically significant differences between the numbers of trees in the western, northern and eastern cliff line areas sampled, although there were fewer trees observed in the eastern sector. There were significant differences in the distribution of healthy trees between the western, northern and eastern cliff line areas, with a greater proportion of healthy trees found in the west and north than in the eastern cliff line area. Differences among sites may be related to differences in rainfall, with trees on the dryer leeward western and northern sites being scattered throughout the cliff line area while trees on the wetter windward eastern site appeared clustered on the lower rocky flats above the seacoast. Differences may also be due to differences in the density of the coccinellid predator, Rhyzobius lophanthae, in this remote area of Guam.