Bt toxins and insecticides against noctuid pests: susceptibility, parental effects, and growth potential

• Ano de defesa: 2020
• Autor(a) principal: Rabelo, Marcelo Mendes
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Viçosa
• Palavras-chave em Português: Algodão - Resistência a doenças e pragas; Aptidão; Tabelas de vida; Bacillus thuringiensis; Spodoptera spp.; Entomologia Agrícola
• Link de acesso: https://locus.ufv.br//handle/123456789/29228

Lepidopteran has around 120 thousand species described, and all of them are phytophagous. Several species, mostly Noctuidae damage row crops cultivated across the south and north Americas. Historically, lepidopteran pests have been managed with the use of synthetic insecticides, and plants expressing Bacillus thuringiensis toxins. This thesis has reported the effect of Bt toxins and insecticides against noctuid pests from Brazil and the United States. In Brazil, the susceptibility Spodoptera cosmioides (Walker), to three Cry toxins and its life- history traits feeding on dual-gene Bt cotton and an artificial diet containing sublethal concentrations of Cry1Ac was documented. In concentration-response bioassays, S. cosmioides larvae showed low susceptibility to Cry toxins. The decreasing rank of toxicity was Cry1F, Cry2Aa, and Cry1Ac. Foliage of dual-toxin Bt cotton varieties (Cry1Ac+Cry1F and Cry1Ab+Cry2Ae) caused higher larval mortality than Cry1Ac-expressing cotton. Spodoptera cosmioides showed reduced larval weight when growing on the Cry1Ac-treated diet, yet they reached adulthood and produced offspring. Interestingly, these offspring when grown on the control diet showed reduced weight gain associated with the toxin exposure of the previous generation, indicating a parental effect of the exposure to Cy1Ac. The reduced larval weight was recovered in later instars, and there was no significant change in the population fitness of the parental S. cosmioides or their offspring. In the United States, Helicoverpa zea (Boddie) life-history traits were determined for individuals of three field populations from a region where this pest likely overwinter. Triple-gene Bt cotton cultivars expressing Cry and Vip3Aa toxins killed 100% of the larvae in all populations tested. In contrast, dual-gene Bt cotton expressing Cry1Ac+Cry1F and Cry1Ac+Cry2Ab2 allowed population growth with the intrinsic rate of population growth (r m ) 38% lower than on non- Bt cotton. The insects feeding on Bt cotton plants expressing Cry1Ac+Cry2Ab2, Cry1Ac+Cry1F, and Cry1Ab+Cry2Ae exhibited reduced larval weight, survival rate, and increased development time. Additionally, fitness parameters varied significantly among the insect populations, even on non-Bt cotton plants, likely because of their different genetic background and/or previous Bt-toxin exposure. The susceptibility of eight noctuid speciesfrom the Florida Panhandle to bifenthrin (pyrethroid) and chlorantraniliprole (diamide) was also determined. Larvae from field and laboratory populations were exposed to commercial insecticide formulations using the leaf-dip method in concentration-mortality bioassays. The field populations of H. zea, S. frugiperda (Smith), S. eridania (Stoll), S. exigua (Hubner), and Chloridea virescens (Fabricius) had reduced susceptibility to bifenthrin compared with the laboratory populations, while there was no reduced susceptibility in Agrotis ipsilon (Hufnagel). The susceptibility to chlorantraniliprole was similar between the field and laboratory populations studied, except for S. exigua that exhibited 630-fold resistance to the diamide. The probit regression equations indicated that the larval mortality of S. exigua and S. frugiperda populations was less than 80% with bifenthrin at the concentration equivalent to the label rate. Likewise, the estimated mortality of S. exigua larvae with chlorantraniliprole at the label rate concentration was less than 80%. This study advances our understanding of potential responses of target and nontarget pest species exposed to Bt toxins and insecticides, which help to design pest management programs for lepidopteran species in the Americas. Keywords: Bioassay. Bacillus thuringiensis. Spodoptera spp.