Beetles, Aphids, Rootworms Taking Hold Despite Soggy Fields – DTN

    Japanese beetles in corn.

    Wet weather has allowed crop diseases to thrive so far this summer and overshadow the work of their pest co-conspirators, insects.

    However, a few hardy bug species have risen to the challenge and are causing problems for growers, entomologists told DTN. Japanese beetles are chomping away in some Missouri cornfields. Soybean aphids are embracing the drier climate of the northern Midwest and western corn rootworm beetles have been emerging, despite the soggy soils of many cornfields.


    In a year of heavy rain and variable yield potential, southwestern Missouri farmer Steve Cubbage found himself battling both insects and diseases. The only good news is the problems coincided so he could combine spray applications.

    When he loaded fungicides into his sprayer this July to control the corn diseases, he also added insecticide to the mix. Japanese beetles were trimming the silks, he said during a DTN webinar “Solving Production & Economic Challenges for the 2015 Season” held last week. The webinar was sponsored by BASF.

    The shiny beetles have been a nasty additional blow to some Missouri farmers, who are already facing record rainfalls, delayed planting and crop disease, University of Missouri Extension entomologist Wayne Bailey told DTN.

    Between the seven traps set up across the state, entomologists are pulling in about 80,000 to 100,000 of the beetles every day, he said. “Right now, we have a healthy population of these beetles and we’ve had some silk damage on some of the corn.”

    Although the beetles often prefer to feast on soybean leaves, the state’s delayed, immature and — in some cases — unplanted bean crop has forced them to settle for the corn silks emerging across the state.

    Bailey reminded growers to wait until the established economic threshold is reached: three or more beetles on a corn plant before pollination is 50% complete, with green silks eaten down to less than half an inch from the ear. Japanese beetles also tend to cluster in fields or feed along edges, so it’s easy to overestimate damage if sampling only occurs in congregated areas.

    In soybeans, treatment is recommended if the beetles have defoliated 20% of the leaves in reproductive stages, and 30% in pre-bloom stages.

    See this helpful threshold guide for the Japanese beetle, from University of Wisconsin entomologist Eileen Cullen.

    So far, Missouri is experiencing the worst populations of the pest this year, Bailey noted. However, the Japanese beetle is a slow but persistent migratory pest, and states to the north and west of Missouri are likely to see it in coming years, he warned.


    While much of the Midwest has been overwhelmed by rainfall, parts of the northern Soybean Belt are seeing drier weather — and soybean aphids are paying attention.

    Between June 29 and July 11, scouts in North Dakota and Minnesota found the tiny yellow insects in 71% of the soybean fields they examined, North Dakota State University Extension entomologist Janet Knodel noted in a university Crop & Pest Report. She recommended growers in those regions continue scouting bean fields from R1 through R5 growth stages.

    Soybean aphids are champion procreators and populations can increase exponentially in a matter of days. However, entomologists are urging growers not to spray too early.

    Most of the fields surveyed were well below the economic threshold for treatment, which is 80% of plants infested with 250 aphids per plant and growing, Knodel said.

    Spraying before this threshold is met comes with a host of disadvantages, University of Minnesota Extension entomologist Robert Koch explained in a university crop news release.

    Low populations of the pest do not cause yield loss, so overzealous sprays will likely waste money, he said. Most insecticide treatments for aphids (namely pyrethroids and organophosphates) are also equally deadly for the beneficial insects that dine on the aphids. Without these friendly bugs around later in the season, re-infestations are much more likely and more applications may be required. Continual overtreatment of aphids can also increase the chances of resistant populations, Koch added.

    While it might be tempting to save a trip across the field by tank mixing herbicide and insecticide, the different timing and equipment needs of each treatment is likely to lead to both performing poorly, he warned.

    For more details, see Koch’s article here and Knodel’s assessment of the aphids this year here.


    It might be tempting to write off the threat of western corn rootworm this summer, after drenching summer rains coincided with their vulnerable egg hatch period in early June.

    Don’t underestimate this pest, warned Monsanto technology development manager Sean Evans. “The eggs are really resilient,” he pointed out. “They can withstand some pretty extreme swings and can remain under water for a long period of time.”

    However, University of Illinois entomologist Mike Gray told DTN he does expect populations to be dampened by the wet summer start in his state. “I suspect the population will be significantly reduced due to record-breaking precipitation totals in many areas of Illinois in June,” he said. “Also, the widespread use of Bt hybrids combined with the use of planting-time soil insecticides should have an impact on densities.”

    Nonetheless, adult rootworm beetles have been emerging across the Corn Belt and beyond, from Pennsylvania to Nebraska, according to university pest alerts. University of Nebraska entomologist Bob Wright warned growers to keep an eye out for silk clipping in a Crop Watch article.

    Corn could be more susceptible to damage this year, as well, Evans noted. “Adverse conditions for the rootworm means corn is not growing as it should, either,” he pointed out. “Smaller root systems will not be as efficient, and the slower it grows and the smaller the plant mass, the smaller the number of insects that can have a greater impact.”

    Growers who traditionally battle this pest should still keep an eye on silks for now, and plan to dig roots around the end of July to get a good sense of populations and damage levels, he concluded.

    For more information on how to evaluate root damage, see this University of Nebraska guide, and for details on evaluating and treating adult beetle damage, see Bob Wright’s Crop Watch article here.

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