Observed plant recovery of the endangered Cycas micronesica populations on the island of Guam: plant resistance or biocontrol

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The island of Guam’s only gymnosperm and historically one of the island’s most abundant trees, Cycas micronesica, has been devastated by high mortality due primarily to the armoured scale Aulacaspis yasumatsui. This cycad-specific scale pest invaded Guam in 2003 and, in only a few years, this pest caused over 90% mortality to maturing trees and 100% mortality to seedlings. In 2015, C. micronesica was listed under the United States Endangered Species Act. Continuous surveillance of tree mortality throughout the island showed extreme decline in health until recent surveys have demonstrated that there has been improved tree health with little evidence of leaf herbivory on some remaining trees. Suggested explanations for this observed reduction in scale herbivory include some form of resistance in the surviving trees or a biocontrol agent (or several agents) that previously existed or has been introduced unintentionally and is controlling the scale to a greater degree. A combination of resistance and biocontrol are possibly both involved. We discuss in depth these possibilities and then propose experimental approaches that will help resolve this question.