Biological control of powdery mildew (PM) plant pathogens may offer a solution to chemical control problems such as resistance, worker safety, and effects on nontarget organisms. Unfortunately, mildew consumption by arthropods is seldom studied and poorly understood. The coccinellid tribe Halyziini is composed entirely of obligate consumers of various PM fungi and is represented in the Western Hemisphere by members of the genus Psyllobora Chevrolat. In western North America Psyllobora vigintimaculata Say feeds on PM in natural and managed systems. Here, we report the insect's biology and phenology through the use of a laboratory colony, rectangular degree-day calculation and field sampling in an urban landscape. The natural presence of this beetle in Davis, CA, has been documented over the course of 2 yr through weekly presence-absence and density sampling of native and exotic plants prone to PM. Sampling data indicates activity from late February through mid-December. The insect was observed feeding on the PM of >25 plant species in 13 different plant families. There is a positive relationship between PM severity and the relative density of insects, suggesting an aggregative numerical response. The potential of P. vigintimaculata for biological control and the possibility of using this insect as an indicator of PM in horticultural systems are both discussed.