Toxicity of pyrethroid insecticides to carrot weevils: enhancement by synergists and oils

In laboratory bioassays over 24 h, pyrethroid insecticides, with the exception of tefluthrin, were more toxic than azinphosmethyl or phosmet to the carrot weevil, Listronotus oregonensis (Leconte). The toxicity of permethrin decreased as posttreatment holding temperatures were increased. LC50 values for pyrethroids increased by approx 2- to 3-fold between 24 and 48 h as weevils recovered from treatment. The synergists piperonyl butoxide and DEF enhanced the toxicity of permethrin by 3- to 5-fold in both 24- and 48-h posttreatment observations, but recovery rates were similar to those for permethrin alone. Addition of herbicidal (mineral) oil increased the toxicity of permethrin and deltamethrin, whereas the addition of olive oil was either antagonistic or ineffective. The increased toxicity of the herbicidal oil-pyrethroid mixture appeared associated with more rapid penetration of insecticide. In field trials, cypermethrin was as effective for control of the carrot weevil as the currently recommended insecticide phosmet and is, therefore, a suitable alternative where control of both carrot rust fly, Psila rosae (F.), and carrot weevil is necessary. Application of cypermethrin in herbicidal oil was not more effective than cypermethrin alone against the carrot weevil at the concentrations tested