Pyrethroid resistance of Helicoverpa armigera in Spain: current status and agroecological perspective

The cotton bollworm Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) is a major pest on a wide range of crops. Insecticide treatments have been widely used and are currently indispensable for its control in almost all crops, which has resulted in some insecticide resistance. However, information about insecticide resistance of H. armigera in Spain is very limited. The pyrethroid resistance status of H. armigera was investigated in Spain during a 5 years period (1995–1999). Toxicological bioassays were completed in the laboratory on F1 offspring of 35 field-derived strains collected from a range of crops and other host plants or from light traps. Seven pyrethroids, cypermethrin, bifenthrin, cyfluthrin, lambda-cyhalothrin, deltamethrin, permethrin and fenvalerate were tested. Dose–mortality regressions, LD50s and their fiducial limits were estimated by probit analysis. Resistance factors (RF) were estimated at the LD50 level as RF=LD50 field strain/LD50 susceptible strain. A substantial inter-strain variation in pyrethroid resistance was evident. Although susceptibility (RF=1), low (RF=2–10) or moderate (RF=11–30) insecticide resistance were prevalent for most insecticide–strain combinations (107 of 111, 96%), high resistance (RF=31–100) to cypermethrin and deltamethrin and very high resistance (RF>100) to lambda-cyhalothrin and deltamethrin were recorded in four cases, some of which were associated with field control failures. Such resistance levels to pyrethroids in European populations of H. armigera had not been previously reported by using experimental bioassay procedures. However, on the whole, pyrethroid resistance in H. armigera in Spain was not as high or widespread compared with situations found in other areas of the world. The current situation in Spain is discussed in relation to potential factors accounting for the development of insecticide resistance: IPM implementation and seasonal insecticide resistance patterns as well as migration and cropping structure leading to the existence of refugia.