Variable effect of larval stage on the efficacy of insecticides against Listronotus maculicollis (Coleoptera: Curculionidae) populations with different levels of pyrethroid resistance

The annual bluegrass weevil, Listronotus maculicollis Kirby, is a major insect pest of golf course turf in eastern North America. The weevil has been primarily managed with insecticides with applications targeting adults or the first through third instar larvae. It is not understood how effective larvicides are when significant numbers of fourth or even fifths instars are present. In greenhouse and field experiments we exposed larval populations of the weevil to the insecticides cyantraniliprole, spinosad, indoxacarb, and trichlorfon when the larval instar average was 2.5, 3.2, and 4.0, respectively (henceforth called L2.5, L3.2, and L4.0 timing, respectively). Greenhouse and field experiments showed similar trends in insecticide effectiveness. At the L2.5 timing, cyantraniliprole was the most effective and trichlorfon the least effective insecticide. At the L3.2 timing, all insecticides provide similar control levels. At the L4.0 timing, spinosad was more effective than the rest. Consequently, spinosad efficacy did not differ between application timings, cyantraniliprole and indoxacarb were more effective at the L2.5 and L3.2 than at the L4.0 timing, and trichlorfon was more effective at the L3.2 than the L2.5 and L4.0 timings. Our observations show that these insecticides remain effective until the larval instar average is approximately 3.2, giving applicators more time to accurately determine the need for insecticide applications which can lead to reduced insecticide use and thereby alleviate pressure for resistance development.