Secondary insects can be primary pests

WEST LAFAYETTE, Ind. – When deciding whether to treat for corn rootworm, don’t forget to factor so-called secondary insects into the mix.

Wireworm, white grub and seed corn maggot are known as secondary insects only because the total economic damage caused by them is low compared to the damage caused by rootworm.

But that term is misleading. In some cases, any one of those insects can become a field’s primary pest.

And in many cases, the combination of them leads to an economic infestation, defined as a level where the expected economic loss is equal to or greater than the cost of treatment.

Recognizing the problem.

The incidence of secondary insects in the Corn Belt has increased over the past few years, said Purdue University entomologist Larry Bledsoe.

“Although we’re not sure if insect populations are increasing, or if the perception of secondary insect problems is greater,” he explains.

“More people seem to be aware of them and are looking for them.”

Kevin Steffey, Extension entomologist at the University of Illinois, agrees.

“There have been an increased number of reports of wireworm and white grub problems in Illinois fields,” he notes.

A telltale sign of secondary insect damage is a failure in plant emergence expressed in skips along the rows.

Later in the season, wilted or stunted plants might be a sign of soil insect damage, although these symptoms can also be caused by such foliar insects as webworms and stalk borer.

Scouting techniques.

Wireworm is the only secondary soil insect in corn that can be detected relatively easily prior to planting.

To do so, set out solar bait stations two to three weeks before planting: Dig 4 to 6-inch holes in the field, fill with one cup each of corn and wheat seed, and cover with soil and black plastic to warm up the area.

Up to two bait stations per acre should be set. Wireworms will be attracted to the germinating seed. In two weeks, dig up the stations.

An economic infestation is one or more wireworms per station. Numbers below that level would not justify a soil insecticide on their own, but should be added to the damage potential of other soil insects in the field.

Predicting infestations of other soil-borne insects is very difficult, said Steffey.

“Ultimately, a grower needs to dig up damaged plants and look in the ground to see what insects are causing the problem.”

Steffey recommends that growers dig up three feet of row at several locations in a field within a week of planting.

Determine the total number of seeds still viable, and the proportion that have been damaged by seedcorn beetles or wireworms.

After emergence, if skips are seen in a field, digging up some plants and scratching about in the top inch of soil will show the presence of grubs and other larvae.

The presence of wireworms and white grubs should be documented so that a field history can be established and used to determine the need for a soil insecticide the following year.

Treatment strategy.

To decide whether a soil insecticide treatment is necessary in your corn fields, first look at your potential for corn rootworm.

In many areas of the Corn Belt, rootworm alone justifies the use of a soil insecticide. Jim Oleson, Iowa State University entomologist, estimates that half of the state’s continuous corn fields have economic infestations of corn rootworm.

Second, look at your potential for cutworm. In the southern Corn Belt, cutworms can be an annual occurrence capable of reducing stands by several percent overnight.

In the northern Corn Belt, cutworms are more localized, “a luck-of-the-draw problem,” said Bledsoe. “They appear wherever the air masses happen to move them.”

Rescue treatments are available for cutworm, but university studies also show that a T-banded application of a broad-spectrum soil insecticide such as Force 3G controls them.

Three approaches.

If your only concern is cutworm, university entomologists recommend a scout-and-treat approach.

For those growers who also have rootworm and/or secondary soil insect concerns, the potential for cutworm should be factored in when determining the need for a soil insecticide.

Third, for those growers lucky enough not to have rootworm or cutworm problems, look at your field history. Fields that are coming out of sod or CRP are especially vulnerable.

Fields that occasionally show skips, or have had soil insects in the past are prime candidates. That’s especially true for wireworms and white grubs.

Some types of white grubs have a life cycle of three years. Wireworms have a life cycle of up to six years, all but one of which is spent in the soil in the larval stage.

So watch the field history for several years – a field that was in sod as long as four years ago might have a problem with wireworm. And look at fields that just don’t yield as well as they should.

“We know secondary insects are out there,” said Bledsoe, “because we sometimes see a yield response with a soil insecticide in the absence of rootworm pressure. So we know something else was there.”

University of Missouri entomologists have summarized eight years worth of research data, comparing soil insecticides for control of wireworms, corn rootworms and cutworms.

All insecticide-treated plots were compared to an untreated plot that contained, on average, almost 50 percent damaged plants.

Among some commonly used soil insecticides, the percentage of healthy plants were as follows: Force, 81 percent; Aztec, 80 percent; Counter 15G, 80 percent; Regent, 76 percent; Counter 20CR, 73 percent; and Lorsban, 73 percent.

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