Balancing Bt Toxin Avoidance and Nutrient Intake byHelicoverpa zea(Lepidoptera: Noctuidae) Larvae

Abstract

To evaluate how the Cry1Ac Bacillus thuringiensis (Bt) toxin and the ratio of dietary protein to carbohydrate (P:C) independently and jointly affect Helicoverpa zea (Boddie) larval feeding performance and behavior, we conducted no-choice and binary choice experiments using chemically defined diets. We tested two related strains of this polyphagous pest: a strain selected for resistance to Cry1Ac in the laboratory (GA-R), and its field-derived parent strain (GA). In no-choice tests, feeding performance in GA and GA-R was superior on a non-Cry1Ac 80P:20C diet compared to a non-Cry1Ac 35P:65C diet or an 80P:20C diet containing Cry1Ac. Based on consumption and position measurements in binary choice tests, larvae preferred the non-Cry1Ac 80P:20C diet over the other two diets mentioned above. However, the association between preference and performance was weaker when comparing the Cry1Ac 80P:20C diet versus a Cry1Ac 35P:65C diet, and when comparing the Cry1Ac 80P:20C diet versus the non-Cry1Ac 35P:65C diet. In all choice situations, consumption preference occurred mainly by the percentage of larvae that fed almost entirely from one or the other diet, rather than from variation in the extent of diet mixing by individuals. Resistance to Cry1Ac affected the balance between toxin avoidance and nutrient intake: larvae from the more resistant GA-R strain consumed significantly more of the Cry1Ac 80P:20C diet when paired with non-Cry1Ac 35P:65C diet, while GA larvae consumed more of the latter, though not significantly. The results show that dietary P:C ratio, Cry1Ac, and resistance to Cry1Ac affected feeding behavior and performance of H. zea larvae.

Effects of seasonal changes in cotton plants on the evolution of resistance to pyramided cotton producing the Bt toxins Cry1Ac and Cry1F in Helicoverpa zea

“…26 The ratio of dietary protein to carbohydrate (P:C) affected both the toxicity of Cry1Ac and the fitness costs of resistance to Cry1Ac in H. zea, indicating that seasonal changes in P:C ratios could also affect suitability of Bt and non-Bt cotton. [36][37]63 In accord with a previous study, 5 we did not detect significant fitness costs affecting survival on young non-Bt cotton (feeding was initiated 79 DAP in Brévault et al 5 and 83 DAP for young cotton here). However, seasonal changes in cotton induced large fitness costs that reduced survival on old cotton in this study (feeding experiment initiated 122 DAP).…”

“…The two subsets of each strain were crossed every two or three generations to reduce effects of inbreeding depression . The two subsets of GA‐R were periodically selected for resistance to Cry1Ac, by exposing at least 2500 GA‐R neonates per subset (total selected 5000) to MVPII (Dow Agrosciences, San Diego, CA, USA) overlaid on artificial diet . Larvae that were third or higher instars after 7 days of feeding on treated diet (20 or 30 µg cm −2 ) were scored as resistant to Cry1Ac.…”

“…36 The two subsets of GA-R were periodically selected for resistance to Cry1Ac, by exposing at least 2500 GA-R neonates per subset (total selected 5000) to MVPII (Dow Agrosciences, San Diego, CA, USA) overlaid on artificial diet. 37 Larvae that were third or higher instars after 7 days of feeding on treated diet (20 or 30 μg cm −2 ) were scored as resistant to Cry1Ac. MVPII is a liquid formulation of a hybrid toxin with an active portion identical to that of Cry1Ac.…”

Mutations in a novel cadherin gene associated with Bt resistance inHelicoverpa zea

“…For example, increased locomotory activity and ballooning behaviors are thought to be the result of larval toxin detection and avoidance (Benedict et al, 1992; Men et al, 2005; Prasifka et al, 2009; Goldstein et al, 2010; Ramalho et al, 2014). Furthermore, in binary choice tests, larvae from GA-R preferred to feed on a nutritionally optimal Cry1Ac diet relative to a non-Bt suboptimal diet, while larvae form GA consumed more of the suboptimal non-Bt diet (Orpet et al, 2015b). This suggests that detection and consumption of a nutritionally optimal diet supersedes toxin avoidance behaviors in GA-R.…”

“…Here, we used a genome scanning approach to compare previously described Cry1Ac-selected and unselected lines of H. zea (Brévault et al, 2013, 2015; Orpet et al, 2015a, 2015b; Welch et al, 2015; Carrière et al, 2018a). We identified six regions of the genome showing signatures of selection, one of which includes a novel gene from the cadherin family, which has been shown to comprise genes associated with Cry resistance in other lepidopteran species (Gahan et al, 2001; Fabrick et al, 2014; Zhang et al, 2017).…”