Outbreaks of Rachiplusia nu (Guenée) in southeastern and southern Brazil are associated with its field resistance to Cry1Ac toxin

DANIEL RICARDO SOSA-GÓMEZ; ALEXANDRE SPECHT; ANNA CHRISTINE WILCKEN FÉLIX PESSOA1; MARÍA GABRIELA MURÚA

Congreso; International Congress on Invertebrate Pathology and Microbial Control & 53rd Annual Meeting of the Society for Invertebrate Pathology; 2021

Transgenic soybean varieties expressing Cry1Ac toxin have been extensively cultivated in Brazil since the 2013/2014 growing season, increasing the probability of selecting resistant insects. Outbreaks of Rachiplusia nu (Guenee) have been detected in 2020/2021 in southeastern and southern Brazil, together with moderate soybean defoliation. Sampling performed in Bt soybean fields in the 2020/2021, using the beat cloth method and light traps, showed an unusual prevalence of R. nu, in some cases ranging from 6 to 8 larvae per linear meter. Very high prevalence of this species was also observed in the light traps, with most of the owlet moths belonging to it. Total number of R. nu moths collected in two light traps in 14 sampling nights, between January and February 2021 in Warta, PR State was 8,461 (93% of the collected moths). The maximum number of captures in one night was 3,629 specimens of R. nu moths (February 16, 2021) in two light traps. Bioassays performed in January 2012 with field populations of R. nu collected in Bento Goncalves, RS and Ponta Grossa, PR revealed median lethal concentration values ranging from 0.70 to 1.20 μg.mL?1 Cry1Ac. In contrast, field populations of R. nu collected in Bt soybean fields during the 2020/2021 growing season were able to grow and reach the adult stage rearing on artificialdiets containing 3, 50, and 100 μg.mL?1 Cry1Ac as well as on leaves of soybean varieties expressing Cry1Ac.