Bt Resistance Monitoring in European Corn Borers and Western Corn Rootworms

Scientific Opinion on an application from Pioneer Hi‐Bred International and Dow AgroSciences LLC (EFSA‐GMO‐NL‐2005‐23) for placing on the market of genetically modified maize 59122 for food and feed uses, import, processing and cultivation under Regulation (EC) No 1829/2003

“…However, a side-by-side comparison between the surface and incorporation treatment led to similar levels of variability in susceptibility, indicating that there are no major differences between both techniques (Saeglitz et al, 2006;Siegfried et al, 2007). Further, Siegfried and Spencer (2012) pinpointed the fact that strict quality control of bioassays using surface treatment through visual inspection is essential to minimise potential inconsistencies in terms of nonuniform treatment and inconsistent exposure of larvae (see also Gaspers et al, 2011). Given the costs associated with Bt-protein preparation, its instability and limitations in the amount that can be produced, Siegfried et al (2007) considered that the advantages of the surface treatment outweigh the possible increased uniformity of exposure that may be associated with incorporating the Bt-protein in rearing diets.…”

“…This baseline susceptibility should represent the natural variability in response to the Bt-proteins among WCR populations across their geographic distribution range, preferably prior to the first introductions of maize 59122 (Siegfried et al, 2005). To obtain comparable data and to detect actual shifts in susceptibility at an early stage, a consistent methodology in terms of sampling, laboratory bioassays and toxin standardisation is required (Andow, 2008;Tabashnik et al, 2008aTabashnik et al, , 2009Siegfried and Spencer, 2012;Devos et al, 2013). …”

“…Alternatively, susceptibility testing is performed with larvae (F 1 offspring) obtained from field-collected individuals using a diagnostic or discriminating dose of the Bt-protein incorporated into an artificial diet (Siegfried et al, 2007;Andow, 2008;Tabashnik et al, 2008aTabashnik et al, , 2009Siegfried and Spencer, 2012 50 or LC 99 for susceptible larvae is commonly used (Andow, 2008). The discriminating dose bioassay is a cost-effective method that allows the testing of many individuals at an appropriate dose, and will detect low frequencies of both polygenic and multiple resistance .…”

Understanding successful resistance management

“…Annual assessments of Bt susceptibility involving diagnostic bioassays based on the upper end of the 95% confidence interval of the LC 99 derived from baseline susceptibility studies have been conducted since the initial registration of Cry1Ab expressing events in 1996. 15,16,49 In the approximately 15 y that Cry1Ab susceptibility has been assessed and among almost 200 different populations that have been assayed, only one population (Kandiyohi County, MN 2001) was identified that did not exhibit mortality in excess of 99% in the diagnostic bioassay. 15,16 USEPA registrations of Bt corn events mandate that if resistance is suspected, a series of additional tests must be initiated to determine whether the resistance is heritable, to quantify the magnitude of resistance, and to measure the ability of the resistant strain to survive on Bt expressing corn plants.…”

“…11 In spite of what has been perceived as intense selective pressures and a likelihood for resistance evolution, O. nubilalis populations apparently remain susceptible to all the currently deployed Cry toxins that have been registered as PIPs by the USEPA. [15][16][17] While it is not possible to determine whether this lack of resistance is a consequence of regulatory IRM mandates, the predictions from initial theoretical models about Bt susceptibility and the HDR concept, 11,[18][19][20][21][22][23] appear to have been realized. Moreover, in other insects when there has been an apparent increase in resistance allele frequencies among field populations of lepidopteran pests targeted by Bt corn 24 or where there has been documented field control failures, 25,26 certain aspects of the pest's biology or attributes of the technology do not comply with the assumptions of the high dose/refuges concept.…”