Farm Horizons, February 2013

Corn rootworm update

By Dave Schwartz, Certified crop advisor, Gold Country Seed

Of all the insects that feed on corn, corn rootworms seem to be the most resilient and able to tolerate pretty much everything we have thrown at them. They have adapted to soil insecticides, crop rotation, foliar insecticides, and now are becoming resistant (in portions of Minnesota) to the Bt trait used to protect corn roots.

So, I thought it may be helpful to review what we know about corn rootworm, so growers have the tools they need to make wise decisions.

Corn rootworms have been around for a long time, at least 50 years. Their life cycle in Minnesota starts with egg hatch, beginning about June 1 and peaking around June 10. Larvae feed on corn roots until early July. If larvae are not able to find corn roots within 48 to 72 hours, they starve. Rootworms feed for about three weeks and then pupate. Five to 10 days later, beetles emerge from the soil and begin feeding on corn pollen and corn silks. Two weeks after feeding, female beetles begin laying eggs in the soil, and continue egg-laying until frost occurs.

Female beetles are mobile, and have been known to fly at least 25 miles. In very dry soil conditions (fall 2012), beetles tend to lay their eggs deep in the soil. Wet weather during June can take a toll on rootworm larvae, and cold open winters increase mortality of eggs. This is especially true if fields have little snow cover in February and March, so weather plays a crucial role in rootworm populations.

Fall tillage, on the other hand, doesn’t appear to be a big factor, no matter what type of tillage system is used.

Corn yield loss is the result of larvae feeding on roots, causing plants to take up less water and nutrients, especially in droughty growing seasons. Root feeding leads to stalk lodging and root rots that can also take a toll on corn yields. Corn growers have several management options available for controlling corn rootworms.

• The most commonly chosen option is to plant hybrids with one or more corn rootworm Bt genes. Planting hybrids with two genes for rootworm control provides assurance against rootworm damage in continuous corn operations.

• The second option is to use soil rootworm insecticides at planting. In general, they are quite effective. In droughty weather, growers normally see better results with dry granules than with liquid insecticides.

• A third option to consider is to select hybrids with good roots. This is just common sense, but should be considered if facing potentially-high rootworm populations.

• The fourth management tactic is to scout for beetles in August. If beetles are being found in high numbers in mid-August, it’s a good bet the field will have rootworm pressure the following year.

On a side note, we are finding that corn fields which tassel late, and soybean fields with volunteer corn attract adult beetles from nearby fields. This can lead to rootworm damage the following year when corn is grown.

• The final corn rootworm management option to consider is crop rotation.

An effective corn rootworm management plan may incorporate all of the options listed above.

Finally, two types of corn rootworm species are found in Minnesota. The greenish colored northern rootworm has been the most common specie, because it is quite winter hardy. The second specie, but not as tolerant to cold temperatures, is the western corn rootworm. It is more common in states to the south. It is the same size as the northern specie, but has stripes on its back. The western specie has shown tolerance to the rootworm Bt gene. The good news is that most of the rootworm resistance that is being found is in counties south of Wright County.

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