Time To Start Scouting Corn For Rootworm In Iowa

Time To Start Scouting Corn For Rootworm In Iowa

Cooler spring temperatures slowed down corn rootworm egg hatch. But recently warmer weather has stimulated rootworm development.

Every field should be scouted for corn rootworm damage regardless of the seed selection that you plant (i.e., corn rootworm populations are the highest priority for inspection. You need to scout fields, dig up some plants and look at the roots. Assess the amount of corn rootworm root injury and adjust your management strategies if the average injury is above 0.5 on a 0-3 rating scale.

BIG YEAR FOR CORN PEST? Saturated soils during egg hatch diminish overall corn rootworm pressure, and the widespread planting of rootworm resistant corn (Bt hybrids) by farmers should decrease rootworm populations in most fields. "But every field should be scouted for corn rootworm damage regardless of whether or not you planted Bt corn hybrids," says ISU entomologist Erin Hodgson. "Dig up some corn plants and check the roots for signs of pruning."

A few areas of Iowa were approaching 50% corn rootworm egg hatch now on June 14 (see Figure 1). Many other areas were projected to reach the 700 degree days mark within 7 to 14 days, depending on future temperatures. Cooler spring temperatures in 2013 have slowed down development, especially compared to rootworm hatch last year. Iowa State University Extension entomologist Erin Hodgson and pest management specialist Adam Sisson offer the following information and observations.

Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average hatching date around June 6. Development is driven by soil temperature, which is measured by degree days. Research suggests about 50% of egg hatch occurs between 684 to 767 accumulated degree days (base 52°F, soil). Shortly after each egg hatch, young larvae will begin feeding on root hairs and inside roots.

As young rootworms (larvae) develop (see Photo 1) they will begin feeding on tips of corn roots. A severe infestation can destroy nodes 4 through 6, which interferes with water and nutrient uptake by the corn plant and makes the plants unstable (Photo 2).

Severe corn rootworm larvae feeding can cause plants to lodge. Photo by Gary Munkvold, Iowa State University.

Roger Elmore, ISU Extension corn agronomist, summarized how this spring's weather delayed corn planting in 2013. But with the egg hatch starting in late May and early June, the rootworm larvae should have sufficient root tissue to feed on because corn was germinated. Saturated soils during egg hatch will diminish overall corn rootworm pressure, and the high adoption of Bt corn (widespread planting of rootworm resistant corn hybrids) by farmers should decrease rootworm populations in most fields.

However, every field should be scouted for damage regardless of the seed selection that you plant (i.e., corn rootworm populations are the highest priority for inspection), says Hodgson. You need to scout fields, dig up some corn plants and look at roots. Assess the amount of corn rootworm root injury and adjust your management strategies if the average injury is above 0.5 on a 0-3 rating scale. Aaron Gassmann, ISU corn research entomologist, has a webpage where you can go for additional corn rootworm management information, including an interactive node-injury scale demonstration and evaluations of various rootworm control insecticide products.

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