Laboratory tests were conducted to evaluate the ability of three formamidines to synergize permethrin in tobacco budworm (TBW), Heliothis virescens (F), using a topical application of 1:1 wt/wt ratio of permethrin + formamidine against third instar larvae of susceptible and resistant strains. There was no synergism found with the permethrin + formamidine combinations against the susceptible strain, a slight synergism was found in a field strain, and an increased synergism was shown in the resistant strain, in which chlordimeform (CDF) and SN 49844 (an analogue of amitraz) showed a higher degree of synergism than amitraz.

A comparison study to test the synergistic effects of combinations against a cross-bred strain of a laboratory resistant and a laboratory susceptible strain showed slight synergism with the SN 49844 but no synergism with amitraz.