Utah Pests News Fall 2007

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Western Corn Rootworm Can Lodge Profits

 

You may not think of corn as a huge agricultural crop in Utah – and you are right. We only harvest about 42,000 acres per year for silage and 12,000 acres per year for grain. Utah’s average harvest per acre for silage and grain is about 22 tons and 163 bushels, respectively. Even though we aren’t in the Corn Belt, our corn can still have insect problems. In the past, aphids, spider mites, corn earworm, seedcorn maggot, dusky sap beetles, and cutworms have all caused stand and yield problems. However, this summer I heard several reports of western corn rootworm causing lodged corn.

Western corn rootworm is native to North America, but never built up to damaging levels until continuous-corn production became popular about 50 years ago. As the name suggests, the larvae are the damaging stage and feed on corn roots. Initially, root tips will be discolored from larval feeding, but the roots can develop brown lesions or be pruned completely off over time. As you could imagine, damaged roots cannot absorb and move water throughout the rest of the plant. A weak root system also makes corn susceptible to "goose-necking" or lodging. Lodged plants will fall over during wind or rain storms and make harvest difficult or nearly impossible.

In many cases, crop rotation will minimize corn rootworm problems. Sometimes western corn rootworm can still be a consistent problem with corn rotation if new fields are planted in close proximity to harvested fields. Using a granular insecticide at the time of planting can significantly reduce the survivorship of feeding larvae. In-furrow or banded applications are effective and work best if followed by cultivation and irrigation. Foliar or aerial products applied later in the season are typically not effective because the canopy prevents the product from reaching the soil-dwelling larvae.

Later in the summer, adults like to feed and mate on the silks. Under normal conditions, adult western corn rootworms rarely build up to damaging levels. Adults can be used to predict larval pressure in continuous-corn the following year, so fields should be scouted weekly. To learn more about scouting, economic thresholds and treatment options for western corn rootworm, click here.

-Erin Hodgson, Extension Entomologist