Influence of delayed corn planting on corn rootworm (Diabrotica spp.) emergence phenology, Bt corn efficacy, and implications for scouting

Corn rootworms (Diabrotica spp.) have been economic pests of corn for over a century, resulting in numerous tactics aimed at controlling both the adult and larval stages. Scouting for beetles to determine population levels was a cornerstone of managing this pest for years before the introductions of transgenic corn producing Bt (Bacilus thuringiensis Berliner) Cry protein(s).  Due to the development of resistance to transgenic hybrids, there is a renewed need for scouting. While transgenic hybrids are meant to reduce corn rootworm population levels, they also delay emergence of adult beetles. Delayed corn planting has a similar affect, but little is known about how these two interact to affect peak adult emergence which has implications for scouting. This study determined whether transgenic corn rootworm-resistant hybrids (differing in Bt traits), when coupled with delayed corn planting, significantly reduce beetle emergence and alter peak beetle emergence, thereby requiring scouting windows to shift. The experiment contained a factorial treatment arrangement in a split-plot randomized complete block design main plots consisting of four planting dates and sub-plots consisting of three hybrids (differing in transgenic traits). Adult beetles were captured in modified cut-plant emergence cages and collected every two to three days throughout the season. Peak adult beetle emergence was calculated based on Julian date. Corn rootworm beetles emerged later with transgenic hybrids and had lower population levels when compared to a non-transgenic hybrid. Delayed planting dates also resulted in later emergence and fewer beetles. Implications for scouting windows will be discussed.