Carbohydrate Depletion during Lethal Infestation ofAulacaspis yasumatsuionCycas revoluta

Abstract

Premise of research. The armored scale Aulacaspis yasumatsui Takagi has invaded numerous geographic regions to devastate cultivated populations of Cycas L. and the native population of Cycas micronesica K.D. Hill on the islands of Guam and Rota. Questions remain as to how this small hemipteran insect can so rapidly kill a large cycad tree. One possibility is depletion of nonstructural carbohydrates (NSCs) until the storage pool can no longer sustain plant viability.Methodology. We infested Cycas revoluta Thunberg plants with A. yasumatsui and harvested whole plants periodically until plant mortality at week 60. Harvested plants were separated into leaves, stems, and roots, and tissue was frozen and then lyophilized. The dry mass of each organ was measured, and then fructose, glucose, maltose, starch, and sucrose were quantified. Concentration and total pool size within each organ were calculated for each NSC.Pivotal results. Concentration and total pool of all five NSCs were greatest in stems. Concentrations exhibited linear or quadratic declines with time of infestation for all NSCs and organs, with the exception of glucose in stems and maltose in leaves. The total pool of all five NSCs declined linearly or quadratically in all three organs, with the exception of maltose in leaves. Initial plant dry mass of 152 g was 23% NSC, and ending plant dry mass of 59 g was 26% NSC. Carbohydrate stoichiometry shifted in favor of sugars over starch, and sugar stoichiometry shifted in favor of hexoses over disaccharides as time of infestation progressed.Conclusions. These results are consistent with carbohydrate depletion as the mechanism that explains rapid Cycas tree mortality during chronic A. yasumatsui infestation. This is the first demonstration of stress-induced expenditures of carbohydrates by cycads.