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Corn+Soybean Digest

Ask The Experts

How can I prevent corn rootworm problems?

(HARMS) Corn rootworm damage was severe in many fields in eastern Illinois this past year even with biotech corn, seed treatments and planting insecticide applications.

We need to take the opportunity to learn from the past year to better prepare for future problems:

  1. We had high adult populations in soybeans in the fall of 2003.

  2. Western corn rootworms laid lots of eggs in soybean fields in 2003.

  3. The 2004 corn crop was planted early.

  4. Rootworm hatch was at about the normal time.

  5. Seed coatings protect an area around the seed.

  6. Rootworms need to feed on roots of resistant corn before they die.

  7. In light infestations all of the programs worked, but in heavy infestations the planter applications worked the best.

  8. In summer 2004 we had even higher rootworm adult counts in many fields and over a bigger area than we saw in 2004.

In order to address this problem, we first have to monitor the soybeans in the summer for rootworm adults laying eggs.

If extremely high numbers are found, you may wish to consider an at-planting application and delay planting those fields until last. You should also monitor them closely from mid-May to mid-June for larvae feeding. If excessive larvae are found, consider a rescue treatment of a soluble insecticide and hope for rain.

Is corn rootworm diapause the only contributing factor to corn rootworm yield impacts in first-year corn?

(SAX) I have no doubt that corn rootworm diapause issues are creating root feeding problems in certain corn fields. However, I don't think this is the only factor or the major factor contributing to yield loss from corn rootworm in first-year corn.

As I have monitored soybeans for bean leaf beetle infestations from 2000 through 2004, I've noticed an increased level of corn rootworm beetles present in soybean fields.

I've asked entomologists about this and they say the numbers are low and they probably don't lay eggs in soybean fields.

I've begun checking rootworm beetles in soybean fields for gravidity. When I find gravid corn rootworm beetles in the middle of a soybean field, I find it difficult to conclude that they don't lay eggs in the soybean fields.

Over the past four summers, the number of corn rootworm beetles per sweep when checking for bean leaf beetles in soybean fields have increased from less than one per 10 sweeps to a consistent eight to 10 per 10 sweeps.

If three bean leaf beetles per sweep is equivalent to five bean leaf beetles per foot of row, then one corn rootworm beetle per sweep should be equivalent to about 1.6 corn rootworm beetles per foot of row, or about the same as one corn rootworm beetle per plant in a corn population of 28,000/acre. This level is an accepted treatment threshold for determining the need to use insecticide in corn-on-corn rotations.

Thus, it would stand to reason that this level of corn rootworm beetles in a soybean field would result in significant root feeding in the following corn crop. This more accurately explains the levels of rootworm feeding in first-year corn than attributing all of the feeding to diapause problems.

(HARMS) Extended diapause is minor compared to the western corn rootworm laying eggs in soybeans in the central Midwest. We have both problems, but extended diapause only occurs in a small percentage of the fields while the problem with westerns laying eggs in beans is occurring in a larger area each year.

It is now prevalent in about half of Illinois and Indiana and one third of Ohio. In addition, we're finding more white grub, wireworm and grape colaspis. The below-ground feeders are causing more stand reductions each year. I expect this trend to continue, thus making insecticide decisions more difficult.

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