Northern, Diabrotica barberi Smith & Lawrence, and western, D. virgifera virgifera LeConte, corn rootworms (Coleoptera: Chrysomelidae) are major economic pests of corn, Zea mays L., in North America. Corn hybrids expressing Bacillus thuringiensis Berliner (Bt) toxins are commonly used by growers to manage these pests. Several cases of field-evolved resistance to insecticidal proteins expressed by Bt corn hybrids have been documented in many corn-producing areas of North America, but only for D. v. virgifera. In 2016, beetles of both species were collected from five eastern North Dakota corn fields and reared in a growth chamber. In 2017, larvae reared from those populations were subjected to single-plant bioassays to screen for potential resistance to Cry3Bb1, Cry34/35Ab1, and pyramided Cry3Bb1 + Cry34/35Ab1 Bt toxins. Our results provide the first documented report of field-evolved resistance in D. barberi to corn hybrids expressing Cry3Bb1 (Arthur problem population) and Cry34/35Ab1 (Arthur and Page problem populations, and the Ransom and Sargent populations) proteins in North America. Resistance to Cry3Bb1 was also observed in the Ransom population of D. v. virgifera. Increased larval survival on the pyramided Cry3Bb1 + Cry34/35Ab1 hybrid was observed in both species. No cross-resistance was evident between Cry3Bb1 and Cry34/35Ab1 in any of the D. barberi populations tested. Our experiments identified field-evolved resistance to Bt toxins in some North Dakota populations of D. barberi and D. v. virgifera. Thus, more effective control tools and improved resistance management strategies are needed to prolong the durability of this technology for managing these important pests.

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A 3-yr investigation was conducted in commercial corn, Zea mays (L.), fields in eastern South Dakota to determine how reduced application rates of planting-time soil insecticides would influence temporal emergence patterns and survival of northern and western corn rootworms, Diabrotica barberi Smith and Lawrence, and D. virgifera virgifera LeConte, respectively. Beetle emergence was monitored at 2-d intervals throughout the entire adult emergence period of three growing seasons from corn plots treated with planting-time applications of labeled (1X) and reduced (0.5 and 0.75X) application rates of terbufos, tefluthrin, and chlorethoxyfos. No consistent insecticide- or rate-related impacts on mean total emergence per trap were recorded for any of the compounds investigated. However, terbufos applications resulted in a 52% reduction in the number of beetles captured per trap, 53% reduction in maximum rate of adult emergence, and a 59% reduction in overall rate of emergence over time for male D. virgifera during 1994. Terbufos also significantly extended the time required for emergence to peak and linear emergence of female D. virgifera to end in 1994. Tefluthrin applications delayed onset, end, and time of maximum emergence of female D. barberi by 9.9, 14.1, and 12 d, respectively, during 1993. Tefluthrin also reduced emergence rates over time for male (38%) and female (46%) D. barberi during 1994. Overall, application rate was inconsequential regarding total emergence, seasonal emergence pattern, or level of plant protection provided for all insecticides we tested in this 3-yr investigation. Our findings demonstrate that, if properly applied, the reduced application rates used in this study provide adequate root protection and will not significantly impact the biology of these pest species.

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