Impact of Spodoptera frugiperda neonate pretreatment conditions on Vip3Aa19 insecticidal protein activity and laboratory bioassay variation.
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
LC50 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.


CONCLUSIONS
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.
Department of Entomology, University of Nebraska-Lincoln, Lincoln, NE, USA.
Karen F
KF
Da Silva
Department of Entomology, University of Nebraska-Lincoln, Lincoln, NE, USA.
Terence A
TA
Spencer
Department of Entomology, University of Nebraska-Lincoln, Lincoln, NE, USA.
Carolina
C
Camargo Gil
Department of Entomology, University of Nebraska-Lincoln, Lincoln, NE, USA.|Entomology and Nematology Department, University of Florida, Gainesville, FL, USA.
Blair D
BD
Siegfried
Syngenta Crop Protection, LLC, Research Triangle Park, NC, 27709, USA.
Frederick S
FS
Walters
D000818:Animals; D000831:Animals, Newborn; D001681:Biological Assay / Q000379:methods* / Q000592:standards; D005260:Female; D006813:Humidity; D007306:Insecticides / Q000378:metabolism / Q000494:pharmacology*; D007753:Laboratories; D007814:Larva / Q000187:drug effects; D008297:Male; D017440:Photoperiod; D011506:Proteins / Q000235:genetics / Q000494:pharmacology*; D012015:Reference Standards; D018411:Spodoptera / Q000187:drug effects* / Q000502:physiology; D013997:Time Factors
D007306:Insecticides; D011506:Proteins
Vip3A; bioassay; fall armyworm; variation