TOXICITY OF PYRETHROID INSECTICIDES TO CARROT WEEVILS - ENHANCEMENT BY SYNERGISTS AND OILS

In laboratory bioassays over 24 h, pyrethroid insecticides, with the e xception 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 in creased. LC(50) values for pyrethroids increased by approximate to 2- to 3-fold between 24 and 48 h as weevils recovered from treatment. The synergists piperonyl butoxide and DEF enhanced the toxicity of permet hrin by 3- to 5-fold in both 24- and 48-h posttreatment observations, but recovery rates were similar to those for permethrin alone. Additio n of herbicidal (mineral) oil increased the toxicity of permethrin and deltamethrin, whereas the addition of olive oil was either antagonist ic or ineffective. The increased toxicity of the herbicidal oil-pyreth roid mixture appeared associated with more rapid penetration of insect icide. In field trials. cypermethrin was as effective for control of t he carrot weevil as the currently recommended insecticide phosmet and is, therefore, a suitable alternative where control of both carrot rus t fly, Psila rosae (F.), and carrot weevil is necessary. Application o f cypermethrin in herbicidal oil was not more effective than cypermeth rin alone against the carrot weevil at the concentrations tested.