Thresholds for Helicoverpa zea (Boddie) in cotton Gossypium hirsutum L. have been understudied since the widespread adoption of Bt cotton in the United States. Our study was possible due to the widespread presence of H. zea populations with Cry-toxin resistance. We initiated progressive spray timing experiments using three Bt cotton brands (Deltapine, Stoneville, and Phytogen) widely planted across the U.S. Cotton Belt expressing pyramided toxins in the Cry1A, Cry2, and Vip3Aa19 families. We timed foliar insecticide treatments based on week of bloom to manipulate H. zea populations in tandem with crop development during 2017 and 2018. We hypothesized that non-Bt cotton, cotton expressing Cry toxins alone, and cotton expressing Cry and Vip3Aa19 toxins would respond differently to H. zea feeding. We calculated economic injury levels to support the development of economic thresholds from significant responses. Pressure from H. zea was high during both years. Squares and bolls damaged by H. zea had the strongest negative yield associations, followed by larval number on squares. There were fewer yield associations with larval number on bolls and with number of H. zea eggs on the plant. Larval population levels were very low on varieties expressing Vip3Aa19. Yield response varied across experiments and varieties, suggesting that it is difficult to pinpoint precise economic injury levels. Nonetheless, our results generally suggest that current economic thresholds for H. zea in cotton are too high. Economic injury levels from comparisons between non-Bt varieties and those expressing only Cry toxins could inform future thresholds once H. zea evolves resistance to Vip3Aa19.

中文翻译：

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棉花中抗 Bt 玉米（鳞翅目：夜蛾科）的经济伤害水平

自从美国广泛采用 Bt 棉花以来，棉花 Gossypium hirsutum L. 中的 Helicoverpa zea (Boddie) 的阈值一直没有得到充分研究。由于具有 Cry 毒素抗性的 H. zea 种群的广泛存在，我们的研究是可能的。我们使用在美国棉花带广泛种植的三个 Bt 棉花品牌（Deltapine、Stoneville 和 Phytogen）启动了渐进式喷雾定时实验，在 Cry1A、Cry2 和 Vip3Aa19 家族中表达金字塔毒素。在 2017 年和 2018 年期间，我们根据开花的周数对叶面杀虫剂处理进行定时，以在 2017 年和 2018 年期间随着作物的生长控制 H. zea 种群。我们假设非 Bt 棉花、仅表达 Cry 毒素的棉花以及表达 Cry 和 Vip3Aa19 毒素的棉花会做出不同的反应给 H. zea 喂食。我们计算了经济损害水平，以支持从重大响应中制定经济阈值。来自 H. zea 的压力在这两年都很高。被 H. zea 破坏的方块和棉铃具有最强的负产量关联，其次是方块上的幼虫数量。与棉铃上的幼虫数量和植物上的玉米穗蛾卵数的产量相关性较少。表达 Vip3Aa19 的品种的幼虫种群水平非常低。不同实验和品种的产量反应各不相同，这表明很难确定精确的经济损失水平。尽管如此，我们的结果普遍表明，目前棉花中玉米的经济门槛太高。一旦 H.