Synergism of Insecticides in Tests with Resistant Soybean Looper Larvae (Lepidoptera: Noctuidae) in the Laboratory and Field

Abstract

The F1 generation of several field-collected strains of soybean looper larvae, Pseudoplusia includens (Walker), were tested for synergism of permethrin by piperonyl butoxide (PB) and S, S, S tributylphosphorotrithioate (DEF) in 1992. The synergism of permethrin and methomyl by PB was tested in 1993. Permethrin was completely synergized by 10 µg PB per larva in strains from Louisiana; these strains were 2.7-14.1-fold resistant. Permethrin toxicity was increased 8.6-fold by 10 µg PB per larva in a strain from Texas, which was 14.8-fold resistant. Toxicity was increased only 4.6- and 7.5-fold by 10 and 20 g µPB per larva, respectively, in a Puerto Rico strain; this strain had the highest reported level of permethrin resistance (426.6-fold). We observed no significant synergism of permethrin in any of the strains tested with DEF. Methomyl was synergized to a lesser degree than permethrin by PB in a strain from Louisiana. Field trials were conducted in 1992 and 1993 in soybean in Louisiana with 0.11 kg AI/ha permethrin and several rates of PB. Soybean looper control with permethrin + 1.12 kg Al/ha PB was comparable to that achieved with the current recommended standard insecticide, thiodicarb, at 0.50 kg(AI)/ha, and significantly greater than that achieved with permethrin alone.