Observations on the biology of <i>Prionus californicus</i> on hops in Idaho

Abstract

The damage potential, seasonal flight activity and larval development of <i>Prionus californicus</i> were investigated over five years in Idaho hop yards. Hop plants were not susceptible to oviposition and larval establishment until after their second year. Crowns of severely infested plants were reduced to masses of frass and rotted areas. Major roots were frequently tunneled and girdled. First foliar symptoms were loss of vigor and often one or more wilted and yellowing shoots. Some affected plants died within a few months while others became less vigorous over several years. Two kinds of damage occurred: the first was from larvae that developed within root systems in established yards; the second was root damage to new plantings from larvae left in the soil when the old yard was taken out and replanted to hops. Beetle flight occurred only during July with day-to-day variation at least partially related to minimum temperatures and to precipitation. Young larvae were found almost entirely in the vascular region of roots to a soil depth of about 200 mm. Older larvae were in both living and dead roots to a soil depth of about 500 mm. Based upon measurements of 243 larvae taken from 50 hop root systems, the life cycle of the species is usually 4 years.