Cycas micronesica was listed as Endangered under the International Union for Conservation of Nature (IUCN) in 2006 (Marler et al., 2010) and Threatened under the United States Endangered Species Act (ESA) in 2015 (United States Fish & Wildlife Service, 2015). The species is Guam’s only native gymnosperm and the only native Cycas species within the United States. The abrupt change in conservation status was caused by the invasions of several specialist insect pests to the islands of Guam and Rota, with the primary threat developing from the 2003 invasion of A. yasumatsui (Hemiptera: Diaspididae) (Marler, 2012; Marler and Muniappan, 2006). This armored scale species feeds on the surfaces of all vegetative and reproductive organs, and the lack of biological control in Guam’s insular settings caused mortality of more than 90% of the Cycas plants in urban and natural habitats (Marler and Lawrence, 2012).
The local community of biologists has responded to the need to conserve threatened native plant species without complete knowledge, so nascent conservation projects have adopted an adaptive management approach. This is a common phenomenon, and much has been written about funders, policymakers, and practitioners needing to respect the vital importance of evidence-informed conservation decisions, especially when empirical information is scarce or scattered (Marler and Lindström, 2017; Pullin and Knight, 2009). The paucity of relevant local research on threatened terrestrial plant species has limited conservation successes in the Mariana Islands (Marler, 2017; Marler and Lindström, 2014). This phenomenon is not limited to the Mariana Islands with regard to cycad conservation issues, as publications on horticulture and physiology of cycads are deficient worldwide (Cascasan and Marler, 2016).
The potential for the use of large stem cuttings for rescue projects designed to transplant C. micronesica tree populations from military construction sites was recently evaluated (Marler and Cruz, 2017) because excavation of intact roots for traditional transplant operations was cost-prohibitive. This unprecedented attempt to initiate adventitious roots on large cycad stems resulted in 41% success in adventitious root formation under management by experienced cycad horticulturists and 100% mortality under management by silviculturists possessing no cycad experience. The outcomes illuminated 1) the necessity of applied cycad horticultural experience for practitioners of cycad conservation and restoration and 2) that large stem cuttings may be further considered for tree restoration projects only if more research identifies constraints to improved success rates.
Asexual propagation of healthy Cycas plants is routinely accomplished by inducing root formation on small adventitious stem cuttings (Norstog and Nicholls, 1997). The relatively low success rate for large C. micronesica stem cuttings may have resulted from the unhealthy status of the stems due to chronic A. yasumatsui damage to the donor plants (Marler and Cruz, 2017). To more fully understand how Cycas plants respond to chronic A. yasumatsui infestations, Cycas revoluta Thunb. plants were experimentally infested and then tissues harvested to determine carbohydrate status (Marler and Cascasan, 2018). The results confirmed that 87% of the nonstructural carbohydrates were depleted during the 60 weeks of infestation that were required to reach C. revoluta plant mortality.
Several locations throughout Guam were selected in 2007 for attempts to protect in situ trees from the A. yasumatsui threat by using insecticides. The endeavor began with stem injection protocols, but the damage to the persistent, live cortex tissue of Cycas stems was too extensive to implement stem injection on a large scale or sustained basis (Fisher et al., 2009). Root drenches of imidacloprid and dinotefuran systemic insecticides were subsequently evaluated for efficacy. The dinotefuran was not effective under Guam’s climate and soils, but the efficacy of imidacloprid was adequate to suppress A. yasumatsui pressure. This form of plant protection within the dedicated sites was continued with sustained drenches every 3–4 months.
A manipulative experiment was clearly needed to confirm the role of carbohydrates in the compromised adventitious root induction of unhealthy C. micronesica stem cuttings. Therefore, two of the Guam sites containing healthy trees that had been protected with imidacloprid were selected to conduct a propagation experiment that included direct carbohydrate quantification of each cutting. The objectives were to confirm the direct connection between nonstructural carbohydrate depletion in stems by A. yasumatsui herbivory and failures in asexual propagation attempts with C. micronesica trees.
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