Population phenology studies in the native range of a potential biological control agent are indispensable to understand and manage the establishment of populations. At the same time, is important to know the responses of this agent to climatic variation if it were to be released in its target range. The life cycle and phenology of the armored scale Rhizaspidiotus donacis Leonardi, 1920 (Hemiptera: Diaspididae) under field conditions in part of its native range, the southeast of Spain, was studied from 2008 to 2010 as part of an evaluation of this scale for biological control of the giant reed, Arundo donax L. (Poaceae) in the south—western United States and Mexico, as this rhizomatous grass becomes invasive and extremely harmful in riparian areas. The results observed were that R. donacis has one generation per year. The crawlers are the infective and dispersive life stage and they occurred during spring, from March to June and the reproductive period identified by the adult males emergence, was found from May to July. Temperature was one of the climatic variables considered with most influence on the variation of stages abundance.