Efficacy baselines of seven insecticides against larvae of Pandemis heparana (Lepidoptera: Tortricidae)

A diet-incorporation larval bioassay was used to evaluate the response of the leafroller Pandemis heparana (Denis and Schiffermüller) to seven insecticides: tebufenozide (Mimic 23% a.i., Bayer), methoxyfenozide (Prodigy 23% a.i., Bayer), flufenoxuron (Cascade 50 DC 4.7% a.i., BASF), lufenuron (Match 5.32% a.i., Syngenta), indoxacarb (Steward WG 30% a.i., Du Pont), abamectine (Vertimec EC 1.9% a.i., Syngenta) and spinosad (Laser 44.2% a.i., Dow Agro Science). Both neonate and 12-day-old (third to fourth instar) larvae were used in the bioassay. The obtained efficacy baselines were compared with the response of the pest exposed to leaves treated with the same insecticides. The persistence of field-aged leaf residues of the seven insecticides was bioassayed on neonate larvae. Given the obtained LC₅₀ values in the diet-incorporation bioassays larval age was not always a significant factor affecting the response of P. heparana larvae. Differences in LC₅₀ values between neonate and 12-day-old larvae were not statistically significant for abamectine, tebufenozide and methoxyfenozide. Young larvae seemed to be more susceptible than older larvae to spinosad, indoxacarb and lufenuron, while flufenoxuron was more effective against 12-day-old than neonate larvae. When the larvae were exposed to the insecticides on treated leaves, all the tested compounds were less effective on older larvae than on neonates. When applied at the recommended field rates, all seven insecticides can be considered highly effective against both neonate and 12-day-old larvae of P. heparana because their security index (SI = recommended field rate/LC₉₀) always exceeded the threshold value of 1. The high persistence of insect growth inhibitors (IGIs) and moulting accelerating compounds (MACs) in the field compensates for their relatively low SI values. For this reason and given their activity against Cydia pomonella, IGIs and MACs are the most interesting insecticides for spring treatments for the combined control of both species, P. heparana and C. pomonella.