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CONTROLLING BEETLES: Golden Harvest Seeds agronomist Spencer McIntosh warns that additional means of rootworm control may be needed to help protect against development of resistance to BT by the beetles whose larvae are corn rootworms.

Agronomist warns of rootworm resistance to BT varieties

Soil insecticides or beetle sprays may need to be added, even on BT varieties.

In western Kansas and Nebraska and the panhandles of Texas and Oklahoma, the corn rootworm is a big problem.

Over the years, as populations have steadily grown, plant varieties utilizing BT technology have been the best answers to combat the rootworm pest. It has been uniquely effective. That’s a source of concern for Spencer McIntosh, agronomist at Golden Harvest Seeds.

“Planting varieties with the BT trait is the most cost-effective tool we have and there has been a great deal of reliance on BT technology since it was introduced 15 years ago,” McIntosh says. “My concern is that there has been too much reliance on one technology and we are putting too much selective pressure on the corn rootworm.”

Corn rootworms are not really much of a problem in southeast and south-central Kansas, he says.

“Rotation is very important both for crops and for pesticides,” McIntosh says. “In recent years, we have seen some drop in the control we are getting with the BT trait. We need to take a look at going back to adding another insecticide in areas where there is heavy pressure.”

Soil insecticides over the top of BT varieties are available and were the tools most often used before the introduction of the BT trait.

Counter and Force are both still on the market and either is a good add to BT varieties to help assure that rootworm control is complete, he says.

“One thing that farmers do have to be aware of is that organophosphates can have an impact on how herbicides are metabolized,” McIntosh says. It is manageable, but producers need to be aware of it.

He said there are insecticide boxes on many planters that can apply Counter and Forxce and can be set up for injection into the fertilizer lines.

It is important for growers to watch for the beetles that lay the eggs that become rootworms, he says. If they are not getting complete beetle control, there is an increased risk for problems to develop.

“We are seeing increased tolerance for bifenthrin in both the beetles and the larvae,” he says. “In some cases, even resistance is being noted. We need to try to relate back to our experience with weeds. High selective pressure results in resistant species.”

He stressed that the beetles do not mutate as a result of exposure to BT. Rather, in every species, there are some individuals with natural immunity to the chemical. When there is high selective pressure, the susceptible beetles are killed, leaving the resistant ones to multiply.

One insecticide that farmers may want to consider is Steward, which has a different mode of action and can help manage beetles later in the season.

McIntosh warns that farmers make mistakes in deciding on the need for insecticides.

One mistake is to assume that a second or third year of corn on corn fields will have only light pressure. Unfortunately, the beetles are mobile. They like to feed on silks. They can then migrate into first-year corn and lay eggs. Those eggs will hatch in second-year corn and you can have heavy rootworm pressure, McIntosh says.

Some corn varieties may have early maturity dates and some corn is cut early for silage. In those instances, the beetles will fly out of those fields and head for fields where there is still food for them.

“I can’t stress enough that you can’t rely on just one mode of action,” he says. “The old saying ‘an ounce of prevention is worth a pound of cure’ really applies here. We need to invest now to prevent resistance from developing.”

He says the season of rootworm feeding has also lengthened in recent years.

“Before BT, we used to spray for beetles in late July,” McIntosh says. “Now, we are seeing rootworms all the time. I got a call in 2010 from a grower who had larvae feeding in August.”

He says the longer season is the result of active control measures being applied early. Soil insecticides will start to lose effectiveness after about six weeks. BT is very effective until about the 4 to 10 leaf stage then begins to wane.

“That tells me that we need to be more aware of beetle populations later in the season,” he says.

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