Among the wasps, the Masarinae are a peculiar subfamily. It is the only group of wasps that provisions brood cells with pollen and nectar. The studied species Trimeria howardi Bertoni, 1911, was until recently abundant in Ribeirão Preto, São Paulo, southeastern Brazil. This paper deals with plant species visited by T. howardi, in relation to the species annual cycle of activity, based on periods of nest foundation, flight activity, and development stages of immatures present in the cells. During the study period (five years) T. howardi visited four species of plants in two botanical families: Bidens pilosa L.(Asteraceae); Xanthium spinosum L. (Asteraceae); Stylosanthes guianensis (Aulb). Sw. (Leguminosae), and Zornia diphylla (L.) Pers. (Leguminosae). Based on the number of visited families, T. howardi can be considered a narrow polylectic species. In general, the species annual activity cycle consists of three phases: Active phase: January to July; Inactive phase: August - September; Active phase: October to December. The annual cycle is bivoltine or multivoltine and the diapause is facultative, because it occurs in a single generation during the year.