Compatibility of Rhinocyllus conicus1 and 2,4-D (LVA) 2 for Musk Thistle Control

Abstract

Herbicidal effect on Rhinocyllus conicus Froet., a thistle head weevil, was studied by examining the mortality, emergence rates and weights of weevils developing from plants treated with 2,4-D (LVA). Infested heads, obtained by caging ovipositing R. conicus on primary heads of musk thistle ( Carduus nutans L.) (resembles C. thoermeri Weinmann), were treated with 2,4-D at 1.68 kg/ha 0–3 wk after oviposition. Mortalities of larvae developing from untreated plants and those plants sprayed 1–3 wk were significantly lower than mortality from plants sprayed within 48 h of oviposition. The latter failed to support larval development beyond the 2nd instar. Developmental times and weights of weevils that emerged from blooms sprayed at 1, 2, and 3 wk were not significantly different from controls. Plants sprayed up to 2 wk after oviposition (late-bud to early-bloom) did not produce viable seeds, but treatments at 3 wk after oviposition (full-bloom) allowed 10% germination of seeds not damaged by R. conicus in primary heads, and plants survived to produce additional heads. Treatment of musk thistles with 2,4-D at late-bud to early-bloom stage of the primary heads prevented formation of viable seeds without adversely affecting R. conicus development.