Although pyrethroids are increasingly being used to control a number of agricultural insect pests, especially the cotton leafworm, Spodoptera littoralis (Boisd.), pyrethroid resistance is a major obstacle limiting effective control. With the aim of maintaining the effectiveness of pyrethroids in managing pests, a study was undertaken to evaluated the effectiveness of oxidase and esterase inhibitors for synergizing pyrethroids in S. littoralis larvae. Compared with the insecticide-susceptible strain (L-SS) of S. littoralis, the resistance ratio (RR) in the field population (F-RS) was 271.43–fold to cypermethrin. The use of profenofos as an esterase inhibitor significantly increased larval susceptibility to cypermethrin in the F-RS strain, with a synergy ratio (SR) of up to 192.57–fold. Significant inhibition of esterase by profenofos in the F-RS strain was found in vivo. Also, piperonyl butoxide (PB) as an oxidase inhibitor had slight effect of cypermethrin toxicity, so its addition is not a solution for pyrethroid resistance. Thus, modifying the toxicity of cypermethrin by mixing it with organophosphorus compounds (OPs) increased its toxicity and decreased the population of S. littoralis, which is a successful strategy for managing pyrethroid resistance.

Chemical plant protection is still an indispensable method in effective potato protection against Colorado potato beetle – CPB – (Leptinotarsa decemlineata Say) in Poland. This species is able to develop strong resistance against all active substances used in chemical and biological insecticides. The phenomenon of resistance is variable in time and in space. Therefore the objective of the study was to determine the present susceptibility level of Polish populations of CPB to main groups of insecticides recommended in Poland for CPB control.