2012 Too Dry for Many Pests

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The past year was a tough one for crops in a lot of states. In the drought-stressed Midwest, it was just as tough on bugs—both pest and beneficial. The season started off well. A mild winter with a good snow cover improved overwintering survival of pests and beneficials alike. The early warm up could have set the stage for high insect populations, but the cold snap that appeared to threaten a lot of early planted corn knocked bugs back as well.

"Insects like nothing better than a nice, mild progression in the spring," said Ken Ostlie, Extension entomologist, University of Minnesota. "Dramatic temperature swings when the snow cover is gone can help turn the insect issue around a bit."

WHAT HAPPENED TO SOYBEAN APHIDS?

For some insects, the early spring was too much of a good thing. As temperatures warmed, insect life cycles shifted into high speed with faster maturation and reproduction. The problem was the absence of plant material at the right time. In Minnesota, which reported unusually high soybean yields in some areas, soybean aphids got snookered, suggested Ostlie.

"The early bud break on the overwintering host buckthorn had soybean aphid populations reproducing fairly early," he said. "The late freeze caught them and knocked the population back. Then we had rainfall during colonization that combined with delayed soybean planting in some areas. When the aphids were ready to make their move, the soybeans weren't there. When the soybeans were there, heavy rainfall washed the newly established aphid colonies off because the canopy was not developed to protect them."

The one-two-three punch effectively knocked aphids out for the season in many parts of Minnesota. While not as dramatic, other areas also saw the soybean aphid set back on its heels. "There weren't soybean aphids in our fields or reports of them," said Christian Krupke, Extension entomologist, Purdue University. "A lot of insects need free water, either dew or rain to keep them hydrated. There was neither in much of Indiana for most of the late spring and through summer. It was a rough summer for many things, not just crops."

Although his state's soybean growers didn't see aphids, they did see plenty of mites. However, the weather turned around, and the soybeans outgrew the mites, repelling them. While the mites might then have turned on corn, it was too late in much of Indiana—and elsewhere.

"Some mites stressed irrigated corn, but lots of dryland corn was already dead by the middle of July and not of interest to the mites," said Krupke. "Insects like stressed plants, but not dead plants."

NEW PROBLEM PESTS

Krupke reported that most other crop insect pests also failed to develop. Adult corn rootworms did more damage than many years, but didn't reach the problem level of 10 years ago, he added.

One pest that was a problem was the bean leaf beetle that invaded late in the season. The pod-feeding adult caused enough damage to justify treatment. However, by the time the damage was recognized, it was too late to take action—for this year.

"We've had high populations for two years in a row, so producers need to be scouting earlier, perhaps mid-August, for population levels and considering control," said Krupke.

Most of the insect pest populations in Missouri were also depressed. Spider mites and redheaded flea beetles were the exceptions, according to Wayne Bailey, Extension entomologist, University of Missouri. "Spider mites were severe in soybeans until it got too dry even for them," he said. "The flea beetle turned to corn in response to the drought. They would stand and feed on the liquid coming out of the silks, and then as it dried up, clip another half an inch of silk off to restart the liquid."

Wireworms were a problem early in the season before moving deep into the soil. Bailey expects them to return again next year as part of a five-year cycle. Although stinkbugs were a problem in corn (brown) and soybeans (green), they weren't as much of a problem as Japanese beetles. The pest is spreading from the East into the South and West and caused damage in soybeans and corn, although Bailey noted there was a lot of damage in some fields and not others.

"Early insect pressure included secondary pests like corn maggots and even seed corn beetles in some fields," he said. "Normally we get control from seed treatments, but this year it was so dry that the chemicals didn't move off the seed."

Like Krupke, Bailey noted problems with the bean leaf beetle. In Missouri, they were more severe on untreated fields. However, very high numbers early season feeding on vegetation and a later season second generation feeding on pods would have affected yields if the drought had not already taken its toll. The soybean podworm, also known as the corn earworm, can cause 100 percent yield losses in some parts of the state.

TREATING VS. BENEFICIALS

He also pointed to a growing problem with the spotted cucumber beetle or southern corn rootworm (CRW). Showing up in soybean fields, it can defoliate up to 50 percent of the plants and then feed on the flowers, taking out 100 percent of the flowers. Seed treatments don't stop it, and with fewer over the top insecticide treatments, the pest is flourishing. Ironically, the use of insecticides may be part of the problem, especially with the soybean podworm.

"The major natural control is a fungus that grows on green cloverworms," explained Bailey. "If you have a good green cloverworm population, the fungus moves to the soybean podworm. It can be a pest, but in most years it's simply a host for the fungus. In a dry year, you don't have as many beneficials, and growers who used an early season insecticide as a preventative took out the green cloverworms."

Bailey pointed to a growing interest in understanding pest-beneficial-plant interactions. "With soybean aphids we often don't have to spray because of a buildup of beneficials, including minute pirate bug, around 12 species of lady bugs and five or six others, like the big eye bug," he said. "If you hit soybeans with an insecticide early on, you can wipe out all the beneficials. Some years that won't matter, but if you get an infestation, it can be a problem. Because beneficials were down this past year, it will take time to build up the population in the spring. Pests will get a good start, making scouting early season even more important."

PLANNING FOR 2013

Ostlie noted the heavy snowfall that blanketed much of Minnesota in early December may bode poorly for the 2013 corn crop. Although beneficials will have a better rate of survival than with an open and cold winter, so will pests. In particular, he is concerned about overwintering corn rootworm populations. As Bailey noted with seed treatments, Ostlie pointed to poor activation of soil insecticides in the dry soils. This combined with overwintering to produce high populations into the summer. He expects a repeat in 2013 with the CRW surviving quite well.

"Where trait resistance is showing up, growers and consultants have commented that they've never seen so many corn rootworm beetles in the field as they did in 2012," said Ostlie. "With overwintering, there will be even more risk of silk clipping by western corn rootworm adults this coming summer. If they keep silks clipped to within half an inch of ear tips, you can end up with pollination issues."

Another element that Ostlie, Bailey and Krupke all noted was the influx of pests from the South with the early warm-up. Ostlie pointed out that it continued into the early summer with a variety of insects arriving on the prevailing jet stream out of the Southwest. "We saw some black cutworm infestation that we hadn't seen for many years," he said. "With the average temperature increasing, we are more likely to open up to migration earlier in the spring and be open for longer periods. It won't happen every year, but it will over the long term."


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