Date of this Version
Da Silva, K. F. 2015. Assessment of variation in susceptibility of the fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) to Bacillus thuringiensis toxins
The fall armyworm, Spodoptera frugiperda, is a polyphagous insect pest affecting multiple crops. Fall armyworm is managed with insecticides and corn hybrids expressing insecticidal proteins derived from Bacillus thuringiensis. The early detection of insect resistance is important for making appropriate management decisions informs IPM and IRM recommendations.
The objective of the first study was to establish baseline susceptibility of fall armyworm populations to the Cry1F Bt insecticidal protein, emphasizing collections from locations where fall armyworm overwinters in the U.S. Fall armyworm neonates were exposed to artificial diet treated with increasing Cry1F concentrations, and mortality and growth inhibition were evaluated after 7 days. Differences in Cry1F susceptibility between the most susceptible and the most tolerant field populations were 2- and 6- fold for 2012 and 2013, respectively. These results are consistent with other baseline studies of Bt toxicity in other species although reduced susceptibility in some populations may suggest resistance development.
The second study was designed to identify possible sources of variability in laboratory bioassays. Efforts to standardize the laboratory methods used in bioassays of microbial products have been part of an overall attempt to minimize variation. The objective of this second project was to determine if pre-treatment conditions contribute to variation in a Spodoptera frugiperda laboratory population response when exposed to Vip3Aa19 insecticidal protein. Neonates were exposed to the LC70 of Vip3Aa19 under five pretreatment conditions: 1) larval storage time prior to exposure, 2) prior feeding on artificial diet, 3) larval storage at reduced temperature, 4) larval storage at reduced humidity, and 5). lab colony introgression with field collected individuals. Extremes of photoperiod settings used during the course of the bioassay itself were also tested. Significant effects of pre-treatment conditions were observed when neonates were fed prior to bioassay, when stored overnight at 14oC and when exposed to extreme photoperiod conditions. There was no significant difference observed in the other pretreatment conditions.
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