ARS | Publication request: Conventional Screening Overlooks Resistance Sources: Rootworm Damage of Diverse Inbred Lines and Their B73 Hybrids is Unrelated

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 12, 2009
Publication Date: N/A

Interpretive Summary: The western corn rootworm is a major insect pest in continuous corn production in the US. The rootworm larvae feed on corn roots and cause economic losses due to plant lodging and decreased nutrient uptake, both resulting in yield loss. The screening of germplasm collections for resistance to western corn rootworm is ongoing and materials with promising levels of resistance are being identified. However, it¿s been observed that hybrids (resulting from a cross of two inbred parents) tend to be more resistant than inbreds. We evaluated 25 corn inbreds and their hybrids in a rootworm trial and found that, indeed, hybrids are more resistant than inbreds. Furthermore, we could not predict the level of resistance of a particular hybrid by looking at the corresponding inbred parents. Therefore, it is likely that valuable inbred sources of resistance have passed unnoticed in previous germplasm screens since they were not evaluated as hybrids. The results of this study have important implications regarding how germplasm collections should be screened. Specifically, inbred germplasm accessions should be crossed with a tester line to form hybrids prior to screening. This information is important to breeders and entomologists involved in screening germplasm for resistance to western corn rootworm, and other disease and insect pests of corn and other crops. These results will aid breeders and entomologists in making more informed decisions in screening germplasm for sources of resistance, and could ultimately result in the discovery of resistance that can be incorporated into hybrids for producers.

Technical Abstract: The western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte) is a major pest of maize in the USA and Europe, and is likely to increase in importance as the trend towards continuous corn favors larger WCR populations. Although current transgenic approaches are effective, native resistance sources are desirable. The polygenic nature of native resistance is likely more difficult for the insect to overcome and transgenic sources of resistance are not registered for use in Europe. Screening germplasm (landraces, populations, inbreds) for resistance to WCR is a labor and time intensive process, and is usually conducted on the germplasm per se and not on topcrossed materials. However, it has been observed that topcrossed materials tend to have reduced WCR damage. To formally test this observation, we evaluated 25 diverse inbred lines and their B73 hybrids for WCR damage and other associated root traits in seven environments. Overall, hybrids had less damage than inbreds and the correlation between inbreds and hybrids was not significant. These findings have important implications regarding screening germplasm for WCR resistance, namely that inbred materials and perhaps populations should be topcrossed prior to screening for WCR damage. Although the extra step of topcrossing would require significantly more time and effort, it would ensure that valuable sources of resistance to WCR are not missed during the screening process.