Field Guide for Integrated Pest Management in Hops

Biology and Life HistoryAdult root weevils begin feeding onleaves within 24 hours after emerging fromoverwintering sites beginning in late April.All adult weevils are females; males are notknown to occur. They cannot fly and areactive largely at night. Females must feed for25 to 30 days before they can begin layingeggs. Eggs are deposited on the soil surface,in soil crevices, and on leaves near the baseof plants. Egg-laying continues through lateSeptember and early October, with eachfemale laying an average of 300 eggs. Larvaeemerge from eggs in approximately 21 days,move through soil, and begin feeding onplant roots. Most root weevils overwinter aslate-instar larvae that pupate in the spring,but overwintering as adults can occur.Monitoring and ThresholdsPopulations of adult weevils can bemonitored with the use of grooved boardsand pitfall traps to determine when adultsare active in the spring. Scouting for leafnotching caused by adult feeding is alsouseful. Economic thresholds have not beenestablished for root weevils in hop.ManagementBiological control of root weevil inhop has been achieved using heterorhabditidand steinernematid nematodes. Nematodeapplications should be timed to coincidewith the presence of late-instar larvae, soiltemperatures above 50 °F, and adequate soilmoisture. Nematodes and foliar insecticidesare best applied in late summer or fallto reduce the abundance of large larvaefeeding on hop roots in the spring. Foliarinsecticides should be applied approximatelythree weeks after adult emergence butbefore egg-laying begins. They are moreeffective applied at night when adult weevilsare most active. See the Pacific NorthwestInsect Management Handbook at http://pnwpest.org/pnw/insects for a current list ofregistered insecticides.BELOW LEFT: Figure 80. Notched edge ofa leaf caused by adult weevil feeding. Thisfeeding injury is not known to cause economicloss. (Ken Gray Image Collection, OregonState University)BELOW: Figure 81. Root weevil larvae andassociated feeding injury on a root. (C. Baird)At-A-GlanceRoot Weevils◆◆Monitor forvine weevil adultsbeginning inApril.◆◆Treat foradults with foliarinsecticidesapproximatelythree weeksafter adults aredetected in hopyards.◆◆Treat for lateinstarlarvae inthe late summeror fall usingsoil-appliedinsecticides.◆◆Biologicalcontrol ofroot weevil inhop can beenachieved usingheterorhabditidandsteinernematidnematodes.17 45