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Not all crop pests died this winter

By Staff | Apr 15, 2014

JAPANESE BEETLES attack the silks on an ear of corn.

With the colder-than-usual temperatures thanks to nine polar vortexes that covered Iowa during the inter, the question has been, did that cold weather kill off those insects that tend to find a safe place to over-winter?

Cory Lundgren, an agronomy sales specialist with NEW Cooperative in Fort Dodge, said producers will still need to be aware of insects for the upcoming growing season.

“Winter mortality for many insects can be 80 percent in a normal winter and less if it is a mild winter,” said Lundgren. “It also greatly depends on how the insect over-winters.”

According to Lundgren, typically adults or nymph insects will have greater mortality than an insect over-wintering in an egg stage.

“Overall, there will be insect pests survive this winter, and we can’t just turn our back and ignore the problem in our fields,” he said. “Insect pests like aphids and Japanese beetles migrate great distances before becoming a problem in our fields, so just because weather in our part of the country is harsh, it doesn’t mean that all of the insects are gone and won’t move in.”

SEVERAL GENERATIONS OF aphids are visible on this soybean leaf. There are also lady beetles feeding on them.

The pests that are most likely to invade Iowa fields this year, Lundgren said, are bean leaf beetles, corn rootworm beetles and larva, aphids and Japanese beetles.

Although the chances are high that all of those insects will be present, Lundgren said no one can predict how severe the infestations will be.

“There is a high likelihood that insect pest pressure will be less than what it has been the past several years,” he said, “however, insects adapt and change and, just like the rest of farming, it all depends on the weather.”

What’s first?

Lundgren said bean leaf beetles will be the first concern to growers who choose to not treat their soybeans with an insecticide seed treatment.

CORN APHIDS feed of a leaf. They draw so much sap, it leaves the leaf veins looking skeletal.

“Over-wintering adult BLB typically emerge early to mid-May and will feed on the first emerging soybeans in a given area,” he said. “They typically feed on the stems or hypocotyl of young bean plants as well as the cotyledons.

“The damage looks like small lesions in the tissue and with severe feeding they can clip the stem or leaves off completely, thus killing the plant.”

About 50 percent of corn rootworm larva, Lundgren said, will hatch after an accumulated 400 degree days have occurred.

“So if we have a normal year,” Lundgren said, “we typically see pressure in mid to late May.

“If we have unseasonably warmer temperatures in March or April, such as we did in 2012, we could see the hatch earlier.”

“Overall, there will be insect pests survive this winter, and we can’t just turn our back and ignore the problem.” —Cory Lundgren Agronomist, NEW?Cooperative

Fields at greatest risk for CRW infestation are continuous corn. Other factors, Lundgren said, that may increase the pressure would be field that were not sprayed with an insecticide in 2013.

“If adult rootworm beetles were living and feeding in a field in 2013,” he said, “then there is a greater chance they laid eggs there as well.”

The CRW beetles can also cause issues later in the growing season, during pollination.

“The adult beetle will feed on the silk of corn plants,” Lundgren said, “and can, in some cases, feed severely enough to prevent pollination.

“This pressure will depend on the summer’s weather and how many CRW larvae survive to adult stage.”


Although producers haven’t seen much pressure from soybean aphids the past couple of years, they are still an insect to watch.

Lundgren said the greatest risk of aphids are soybeans planted near timber-lined fields, rivers and streams, and fields surrounded by corn; and fields planted with untreated seeds.

“Seed treatments will not completely control aphids,” Lundgren said, “but they tend to keep populations lower longer.

“Aphids are the pest that seems to change their habits the most often and can be unpredictable.

“In the past it seemed like aphids favored one variety of soybean over another in one year and have completely opposite tastes the next year. As a rule of thumb, aphids will do something different every year.”

Lundgren said corn aphids are unrelated to the soybean aphid, but they still behave in a similar fashion.

“They feed on leaf tissue and tassels robbing the plant of fluids and nutrients and, in turn, secrete honeydew.”

That sticky sugar film, Lundgren said, becomes a host for sooty mold and reduces the plant’s ability to photosynthesize.

The corn aphid populations rise typically when corn plants are under stress, especially during a drought in mid to late July.

Japanese beetles

Japanese beetles have been increasing in numbers in producers’ fields, causing increasing economic damage.

“They are a bronze and metallic green beetle,” Lundgren said, “about one-half to three quarters of an inch in size and have little white tufts of hair on the sides of their abdomen.

“They feed on leaf tissue and corn silks and leave the veins causing a skeleton-like affect. In most cases, the economic damage comes from silk clipping during pollination reducing the kernel count on ears.”

When to treat

According to Lundgren, every insect has its own threshold as to when damage is great enough to warrant an insecticide application.

These thresholds, he said, have been determined by universities, including Iowa State University, and typically, these economic thresholds are published and occasionally modified, depending on the value of the crop and the cost of application.

What is not known, he said, is the economic threshold of multiple pests.

“Most universities will agree that even though an individual pest count may be below threshold,” said Lundgren, “adding multiple pests together will cause economic damage.

“Universities will not however, make the recommendations or determine thresholds of this nature.”

That decision, will be up to the producer, he said.

“They need to weigh their options and work with their agronomists to determine the best course of action,” he said.

When it comes to spraying there are a few things to consider, he said, including timing and the possibility of further crop damage.

Timing, Lundgren said, is different for every pest but, depending on the insect, the product being sprayed, coverage is important.

“Most insecticides only kill what they come in contact with or are ingested by the insect,” said Lundgren. “Like a contact herbicide, getting enough pressure and volume to provide adequate coverage is important. Many insecticides do not move once they have been applied.”

Lundgren warned that growers need to be aware that some insecticides can cause crop injury if the crop had already been treated with certain herbicides.

“Make sure to consult your agronomist or chemical provider before applying any pesticide to your crop,” said Lundgren.

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