Pest Pressure Watch
April 6, 2010 
Pay attention to in-season developments. That’s the key piece of advice offered by University of Minnesota entomologist Ken Ostlie as he contemplates pest problems for the season ahead. “Each year is different. I don’t put too much stock in predicting insect problems because I’ve been humbled frequently,” he admits. But Ostlie and other insect and disease experts had a few observations to at least give dealers and growers a heads up.
Soybean Sickness
Soybeans faced some of the most startling pest outbreaks last year. Kiersten Wise, plant pathologist at Purdue University, reports that Indiana growers had problems with white mold in the crop, thanks to cool conditions in July and August. Illinois had similar problems, and Iowa actually had a white mold “epidemic” in 2009, per X.B. Yang, Iowa State University.
Iowa also saw a surprising incidence of sudden death syndrome (SDS), says Daren Mueller, Extension plant pathologist. Selecting varieties with high levels of resistance to SDS will be the key for managing the disease, as one potential management tool — fungicide seed treatment — has not been effective in reducing SDS on soybeans. Researchers had been hoping to see some control with this strategy, but so far no good results have been reported by other states, he says.
And white mold control with foliar fungicides continues to be tough, he admits. “By the time you see it in a field, it’s often too late to get the maximum benefit from a fungicide application,” he says. Unfortunately, the fungus survives well in the soil as well as on other hosts besides soybean, adds Carl Bradley, crop scientist at the University of Illinois.
The amount of foliar diseases seen in 2009 coupled with this winter’s snow cover are cause for concern, says Mueller. Inoculums in fields have likely been well protected, and fields planted to the same crop may be especially problematic in 2010, he says. Yang agrees: “It is a mistake to plant soybean after soybean in 2010. The risk can be high unless 2010 is a dry season.”
Mueller says there’s a good chance for a wet spring — which also doesn’t bode well for disease pressure. Growers may want to consider using seed treatments on soybeans to help manage seedling diseases associated with wet soils. Growers may be forced to plant less seed per acre to save money. Yang recommends avoiding planting soybean with narrow rows (15 inches or less) in fields that had white mold in the past. And, growers should use no-till if corn is the rotation crop, as tillage can bury white mold fungus, allowing it to survive in deep soil up to seven years.
Excessive moisture in Mississippi last year delayed soybean harvest for up to five weeks in some areas and caused pod and seed diseases to dominate the entire state, says Mississippi State’s Extension Plant Pathologist Tom Allen.
Cool conditions around and after corn silking — plus late rains — favored widespread ear rots in Indiana corn fields, says Purdue’s Wise. In Illinois, diplodia ear rot was especially prevalent, with some field’s having incidence as high as 70%, reports Bradley.
Iowa’s problems included some ear rots as well as eyespot, gray leaf spot, and Northern leaf blight. Fungicides are effective at reducing many of these diseases, says Mueller, “but with the cost of application and the price of corn, some corn growers were reluctant to pull the trigger on fungicides, even when they had dramatic eyespot and gray leaf spot pressure early in the season.”
In the South, Mississippi’s Allen attributes increased foliar diseases such as gray leaf spot and Northern corn leaf blight to producers deciding to plant continuous corn — and last year in particular — to the extended cool, wet weather throughout the state.
“The likelihood that these or any other diseases will be problematic in 2010 will depend mostly on the weather,” says Purdue’s Wise. “We advise growers to take good notes about the history of disease within a field — and if they are planting back into a field that has disease problems, the best thing is to choose a hybrid that has some level of resistance to the disease that was problematic.”
Excessive Aphids
The extremely late and heavy soybean aphid population in southern Indiana last season surprised Christian Krupke, entomologist with Purdue University. Late planting and replanting due to early season rains meant this area had relatively immature beans that attracted many winged migrants from adjacent states, he says. Fortunately, most areas did not reach threshold levels and did not require pyrethroid sprays.
Even better news: “Soybean aphid numbers are expected to be low early in the season, as we found very few overwintering on buckthorn plants in fall,” Krupke says.
Erin Hodgson, Iowa State University entomologist, says soybean aphid continues to be the state’s biggest insect problem, and last year’s cool summer temperatures certainly favored aphid development. In fact, she saw record numbers of fall migrant aphids moving to buckthorn. “We don’t know what this means for hatch numbers in the spring and subsequent movement to soybean,” she says. Spring planting may be delayed in 2010 — and this may be an advantage for the pest. “Start scouting before flowering and continue regularly until seed set,” she advises.
Aphid Data
At the University of Illinois, Michael Gray was surprised that soybean aphids came on very strong late in the growing season, especially across the southern part of the state. In fact, some growers actually described the aphid densities as “swarms.” Late summer/early fall flights to buckthorn were heavy, but an unexpected fungal disease infection helped to suppress aphid numbers. The result: Spring flights may be lower than many would have guessed, says Gray.
Hodgson has found that foliar insecticides are very effective in reducing populations if the coverage is sufficient. “Getting proper coverage is essential to contact aphids feeding on the undersides of leaves in the lower canopy,” she emphasizes, and making timely foliar treatments after aphids exceed the economic threshold (250 per plant) will protect yield.
Minnesota’s Ostlie warns that premature (below threshold) insecticide applications for soybean aphid may lead to resprays, as products only supply about a five- to 10-day window of control. “Be careful on your timing,” he says.
Minnesota ran into two-spotted spider mite problems as well, even in cool drought conditions. Ostlie says growers need to be careful about insecticide choice here, as pyrethroid products provide poor control — and may actually flare mite populations, leading to resprays in a couple of weeks or less, if the drought persists. He says insecticides containing chlorpyrifos and bifenthrin provide good control of both mites and soybean aphid.
Purdue’s Krupke saw few insect problems on Indiana corn last year, with late planting and excessive rain acting to reduce rootworm pressure in most of the state. He notes, though, that Western bean cutworm continued to rise in severity in northern counties, with some fields showing economic damage levels.
For this season, “rootworm numbers should again be low to moderate, although the historical high pressure areas will probably remain so, while Western bean cutworm will likely continue to expand its range eastward across Indiana and into Ohio,” Krupke says. Late, wet planting — common in Indiana — will exacerbate problems such as grubs and wireworms, he notes.
In Illinois last season, European corn borer populations reached historic low levels, reports Gray. Even densities of the Western corn rootworm were very low, and he suspects the very wet spring did not favor establishment of larvae.
To the north in Minnesota, slightly drier conditions during corn rootworm hatch and establishment enhanced pest survival and increased pressure. Across the state, Ostlie saw unexpected rootworm injury to triple-stack corn. “The performance issues with Bt (Bacillius thuringiensis)-RW corn were unprecedented,” he says. “There seems to be a fundamental shift in either the corn rootworm tolerance of the proteins or their levels in the plant were lower under the unusual weather conditions in 2009.”
Ostlie recommends growers pay attention to corn rootworm populations, especially in corn following corn fields where the same transgenic traits have been used for several years. And, producers should not neglect insect refuge management requirements in transgenic fields.
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