Geographic Distribution and Effects of Transgenic Corn Hybrids and Chemical Insecticides on Northern and Western Corn Rootworms (Coleoptera: Chrysomelidae) in North Dakota
Calles Torrez, Veronica
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Northern (Diabrotica barberi Smith & Lawrence) and western (D. virgifera virgifera LeConte) corn rootworms are major corn (Zea mays L.) pests in North America. Integrated pest management (IPM) strategies used for corn rootworm control include crop rotation, chemical insecticides, and corn hybrids expressing Bacillus thuringiensis (Bt) toxins. Field and laboratory experiments were conducted between July 2013 and October 2017 to address the following objectives: 1) determine the geographic distribution and densities of D. barberi and D. v. virgifera in North Dakota (ND) and compare the effectiveness of unbaited green ScentryTM Multigard and yellow Pherocon® AM/NB sticky traps for monitoring both species; 2) access the impacts of corn hybrids expressing the Cry3Bb1, Cry34/35Ab1, and Cry3Bb1 + Cry34/35Ab1 proteins, tefluthrin soil insecticide, and clothianidin insecticidal seed treatment on beetle emergence, larval feeding injury, and corn grain yield; and 3) evaluate if either species has developed resistance to these Bt-proteins. Both species were commonly distributed in east-central and southeastern ND, and capture rates ranged from 0 to >10 beetles per trap per week. Green ScentryTM sticky traps captured greater mean numbers of D. barberi beetles in 68% of fields containing this species, while the yellow Pherocon AM/NB trap captured higher mean numbers of D. v. virgifera in 57% of fields containing this species, but their relative trap efficiency was inconclusive. Overall beetle emergence was significantly lower in Bt-protected than in non-Bt corn. Bt hybrids produced higher grain yield and sustained consistently lower levels of larval root-feeding injury than non-Bt in fields with higher rootworm densities. Tefluthrin and clothianidin did not significantly impact beetle emergence or larval root injury. Results also documented the first confirmed field-evolved incomplete resistance in D. barberi populations to Cry3Bb1 and Cry34/35Ab1 proteins. The Ransom population of D. v. virgifera was confirmed as resistant to the Cry3Bb1 toxin. Theses findings indicate reduced efficacy of corn rootworm control using tefluthrin, clothianidin, and Bt corn. Continuously scouting for rootworms using sticky traps or other methods prior to making rootworm management decisions, and the use of an IPM approach to their control is needed to mitigate the development of Bt-resistance rootworms in ND.
Doctor of Philosophy