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BACKGROUND  Variation in response to insecticidal proteins is common upon repetition of insect bioassays. Understanding this variation is a prerequisite to detecting biologically important differences. We tracked neonate  Spodoptera frugiperda  (J.E. Smith) susceptibility to  Vip3Aa19  over 17 generations using standardized bioassay methods. Five larval pretreatment conditions and one bioassay condition were tested to determine whether susceptibility was affected. These included: storage time; prefeeding; storage at reduced temperature; storage at reduced humidity; colony introgression of field‐collected individuals. Extremes of photoperiod during the bioassay itself were also examined.    RESULTS   LC 50   values for two strains of  S. frugiperda  varied 6.6‐fold or 8.8‐fold over 17 generations. Storage time and humidity had no impact on  Vip3Aa19  susceptibility, whereas prefeeding significantly reduced subsequent mortality (by 27%). Storage at reduced temperature increased mortality for one colony (from 45.6 to 73.0%) but not for the other. Introgression of field‐collected individuals affected susceptibility at the first generation but not for subsequent generations. A 24 h bioassay photophase significantly reduced susceptibility (by 26%) for both colonies.    CONCLUSION  Certain pretreatment and bioassay conditions were identified that can affect  S. frugiperda   Vip3Aa19  susceptibility, but innate larval heterogeneity was also present. Our observations should help to increase the consistency of insecticidal protein bioassay results. © 2015 Syngenta Crop Protection, LLC.  Pest Management Science  published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Impact of Spodoptera frugiperda neonate pretreatment conditions on Vip3Aa19 insecticidal protein activity and laboratory bioassay variation