Extended Sentinel Monitoring of Helicoverpa zea Resistance to Cry and Vip3Aa Toxins in Bt Sweet Corn: Assessing Changes in Phenotypic and Allele Frequencies of Resistance

Abstract

Transgenic corn and cotton that produce Cry and Vip3Aa toxins derived from Bacillus thuringiensis (Bt) are widely planted in the United States to control lepidopteran pests. The sustainability of these Bt crops is threatened because the corn earworm/bollworm, Helicoverpa zea (Boddie), is evolving a resistance to these toxins. Using Bt sweet corn as a sentinel plant to monitor the evolution of resistance, collaborators established 146 trials in twenty-five states and five Canadian provinces during 2020–2022. The study evaluated overall changes in the phenotypic frequency of resistance (the ratio of larval densities in Bt ears relative to densities in non-Bt ears) in H. zea populations and the range of resistance allele frequencies for Cry1Ab and Vip3Aa. The results revealed a widespread resistance to Cry1Ab, Cry2Ab2, and Cry1A.105 Cry toxins, with higher numbers of larvae surviving in Bt ears than in non-Bt ears at many trial locations. Depending on assumptions about the inheritance of resistance, allele frequencies for Cry1Ab ranged from 0.465 (dominant resistance) to 0.995 (recessive resistance). Although Vip3Aa provided high control efficacy against H. zea, the results show a notable increase in ear damage and a number of surviving older larvae, particularly at southern locations. Assuming recessive resistance, the estimated resistance allele frequencies for Vip3Aa ranged from 0.115 in the Gulf states to 0.032 at more northern locations. These findings indicate that better resistance management practices are urgently needed to sustain efficacy the of corn and cotton that produce Vip3Aa.

Continental-scale migration patterns and origin of Helicoverpa zea (Lepidoptera: Noctuidae) based on a biogeochemical marker

“…Our results support the risk of selection pressure for resistant alleles and spreading resistance in local and migratory populations. Therefore, our results confirm the hypothesis that the adoption of insecticides and Bt crops in southern regions of the United States impacts the performance of these management tools in northern regions of the United States and in the southern regions of Canada ( Hutchison et al 2007 , Jacobson et al 2009 , Dively et al 2023 ).…”

“…Its migratory capacity enables the colonization of agricultural landscapes and the surge in population density in regions where it cannot survive the harsh winters ( Slosser et al 1975 , Eger et al 1983 , Lawton et al 2022 ). Finally, proposed long-distance migration is claimed as one of the major reasons for ineffective control of H. zea and increased practical resistance to pyrethroid insecticides, or toxins from Bacillus thuringiensis (Bt) Berliner expressed in transgenic crops ( Westbrook et al 1995 , Jacobson et al 2009 , Jones et al 2019 , Dively et al 2023 ). Therefore, H. zea serves as an exemplary model for studying migratory patterns of pest insects to advance our understanding of its ecology and improve Integrated Pest Management (IPM) and Insect Resistance Management (IRM) programs.…”

An Extended Investigation of Unexpected Helicoverpa zea (Boddie) Survival and Ear Injury on a Transgenic Maize Hybrid Expressing Cry1A/Cry2A/Vip3A Toxins

“…In maize fields, the mature larvae of H. zea usually move out from the ears and then drop into the soil for pupation. Thus, the larval occurrence data on maize ears in Trial-I to Trial-VI were adjusted using a similar method as described in [18]. Briefly, one 6th instar was added to the data if an ear with no accountable larvae was observed but had an ear injury area of >6 but ≤12 cm 2 , two 6th instars were added to the data if the ear injury area of an ear was greater than 12 cm 2 without accountable larvae, while one 6th instar was added for an ear with an ear injury area of >12 cm 2 and only one accountable larva.…”

“…No cross-resistance between Vip3A and Cry toxins has been reported to date in resistant insects [4,[13][14][15][16]. Moreover, Bt crop varieties containing vip3A genes have been effective in controlling H. zea, and to date, no practical resistance resulting in field control problems has been documented [4,5,17,18].…”

Development of a bioassay method to test activity of cry insecticidal proteins against Diatraea spp. (Lepidoptera: Crambidae) sugarcane stem borers

“…in sugarcane, transgenic plants producing Bt insecticidal proteins are the preferred alternative for controlling sugarcane stem borers [17]. The efficacy of these insecticidal proteins is commonly evaluated in bioassays under controlled conditions with the purified protein incorporated in an artificial diet and monitoring survival and development [18], or by direct observation of their efficacy in plants in the field [19]. Several studies have assessed the activity of diverse Bt proteins, such as Cry1Ab, Cry1Ac, Cry1F, Cry2Ab, and Vip3Aa20, on sugarcane stem borers, including Diatraea saccharalis (F.), Diatraea flavipennella Box and Elasmopalpus lignosellus (Zeller) (Lepidoptera: Pyralidae) [15,17,[20][21][22][23].…”