Forage alfalfa (Medicago sativa L. [Fabales: Fabacae]) is a major agronomic crop grown nationally and Montana ranks highly in acres harvested. The alfalfa weevil (Hypera postica Gyllenhal [Coleoptera: Curculionidae]) is the primary defoliating pest that requires insecticide applications to prevent yield loss, particularly pyrethroid active ingredients (a.i.) that are both efficacious and cost-effective. Reports from commercial alfalfa producers in Big Horn County, MT, suggested local populations of alfalfa weevil had developed resistance to the pyrethroid a.i. lambda-cyhalothrin (type II pyrethroid). Chemical control is an important component of integrated pest management (IPM) of alfalfa weevil and the loss of pyrethroid a.i. as an effective tool would result in additional production costs. Two locations in southern Big Horn County and nine locations in four other Montana counties where resistance has not been reported were sampled and assayed for resistance to lambda-cyhalothrin. Populations from three counties were susceptible, the concentration causing 50% mortality (LC50) ranged from 0.02 to 0.10 µg/cm2. In contrast, populations from Big Horn County did not reach 50% mortality at the highest concentration of lambda-cyhalothrin tested (3.30 µg/cm2), indicating high levels of resistance have developed in these populations. A field trial in Big Horn County supported laboratory results of resistance; lambda-cyhalothrin at the highest label rate did not reduce alfalfa weevil populations. Additional bioassays suggest cross-resistance to zeta-cypermethrin (type II pyrethroid), but only partial cross-resistance to permethrin (type I pyrethroid).

中文翻译：

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苜蓿象鼻虫（鞘翅目：Curculionidae）在蒙大拿州对高效氯氟氰菊酯的抗性首次报告

饲料紫花苜蓿（Medicago sativa L. [Fabales: Fabacae]）是全国种植的主要农作物，蒙大拿州的收割面积排名很高。紫花苜蓿象鼻虫（Hypera postica Gyllenhal [Coleoptera: Curculionidae]）是主要的落叶害虫，需要使用杀虫剂来防止产量损失，尤其是既有效又具有成本效益的拟除虫菊酯活性成分 (ai)。来自蒙大拿州大角县商业紫花苜蓿生产商的报告表明，当地的紫花苜蓿象鼻虫种群对拟除虫菊酯 ai λ-三氯氟氰菊酯（II 型拟除虫菊酯）产生了抗药性。化学防治是苜蓿象鼻虫综合虫害管理 (IPM) 的重要组成部分，而作为一种有效工具的拟除虫菊酯 AI 的丧失将导致额外的生产成本。对大霍恩县南部的两个地点和蒙大拿州其他四个县的九个地点进行了采样，并分析了对 λ-三氟氯氰菊酯的抗性。三个县的人群易感，导致 50% 死亡率的浓度（LC50）范围为 0.02 至 0.10 µg/cm2。相比之下，大霍恩县的人群在最高浓度的高效氯氟氰菊酯测试（3.30 µg/cm2）下死亡率没有达到 50%，这表明这些人群已经产生了高水平的抗药性。大角县的一项现场试验支持了实验室的抗性结果；最高标记率的 λ-三氯氟氰菊酯并没有减少苜蓿象鼻虫的数量。其他生物测定表明对 zeta-cypermethrin（II 型拟除虫菊酯）具有交叉耐药性，