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The cycad aulacaspis scale (CAS), Aulacaspis yasumatsui Tagaki Lleg, arrived on Guam in 2003. Its spread throughout urban ex- sit (Cycas revoluta Thunberg) and in-situ (C. micronesica K. Hill) populations was rapid. The predator Rhyzobius lophanthae was introduced and released in February 2005 as a biological control mea sure. All Cycas plants in the commercial and residential landscape died in 2004"2005 unless they were chemically protected. Death rate of in-situC. micronesica varied with timing of CAS entry and Rhyzobius beetle migration or release into each habitat. Seedlings and juvenile plants less than 1 m in height were more susceptible and declined to 1% of the initial population by January 2008. Several pre-existing minor threats became major threats as plant health was compromised by CAS, and invasion by the cycad blue butterfly (Chilades pandava) in 2005 added to that list of threats. Phenology, cone morphology, seed set, and seed germination were among the many characteristics of in-situ populations that were altered by the invasion. Experiences from Guam may benefit managers of cycad populations facing similar threats.