Efficacy of neonicotinoid seed treatments in Bt maize

Transgenic (GMO) maize, Zea mays L. (Poaceae), that produces insecticidal toxins by way of genes derived from Bacillus thuringiensis (Bt), occupies nearly 75% of maize acreage in the United States. The primary below-ground target of Bt maize is the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), which is most threatening to continuous maize, especially in Heartland region states such as Indiana. Continuous maize grown in the Eastern Uplands region, such as in western Virginia, can also harbor WCR populations, as well as a range of secondary soil-dwelling pests. For this reason, nearly all maize seeds (79-100%) have neonicotinoid seed treatments (NSTs), which are marketed to protect seedlings against WCR and other secondary early-season pests. The practice of combining these treatments is extremely common, but datasets disentangling the relative benefits of combined use are limited. Concerns regarding the environmental fate of neonicotinoids raise questions on the costs and benefits of widespread neonicotinoid applications. Field experiments were conducted in both regions to evaluate these treatments for their efficacy against soil-dwelling pests in continuous maize. Plant stand, root node injury, and yield were measured. Results from compact method soil samples showed that pest pressures were low for all sites. Generally, stand, yield, and root damage rating results showed inconsistent benefits of NSTs. Given that NSTs were not always necessary under our experimental conditions, our results imply that research to develop more region-specific soil pest management options for growers may be necessary.