The Relationship of Weight of Lepidopterous Larvae to Effectiveness of Topically Applied Insecticides1

Abstract

Four species of lepidopterous larvae were treated by topical application with several insecticides. LD-50 values were established on a μg. insecticide per gram of larval weight basis for a number of different weight groups for each species. The southern armyworm, Prodenia eridania (Cram.), gave a nearly constant LD-50 value for the different weight groups using DDT or Phosdrin® (1-methoxycarbonyl-1-propen-2-yl dimethyl phosphate, 60% technical) as the insecticide, as did the tobacco hornworm, Protoparce sexta (Johan.), with endrin. The corn earworm, Heliothis zea (Boddie), gave no increase in the LD-50 for Phosdrin but gave more than 1,000-fold increase with DDT from larvae weighing 0.1 gm. to those weighing 0.5 gm. Of the eight insecticides tested on the tobacco budworm, Heliothis virescens (F.), only Phosdrin remained constant for the different size groups. Guthion® (O,O-dimethyl S-(4-oxo-3II-l,2,3-benzotriazine-3-methyl) phosphorodithioate), Dipterex® (0,0-dimethyl 2,2,2-tr ichloro-1-hy droxyethy lphosphonate), and malathion gave increases of 2 to 10 times, while endrin, DDT, toxaphene and TDE give increases of from 20 to 100 times.