Two Times when Spraying Corn Rootworm Adults can Payoff
Corn rootworm (CRW) adults are showing up across many Midwest fields now and actively feeding. In some fields, it’s to the point that corn pollination is being adversely affected.
The question now – is it financially worthwhile to spray adult beetles? The potential return-on-investment this year indicates it could be.
The only way to know is to scout and evaluate the CRW numbers that are present, according to Veronica Calles-Torrez, Janet Knodel and Mark Boetel, North Dakota State University Extension entomologists.
Here are their recommendations on determining when a foliar-applied insecticide is warranted.
First, determine the CRW beetle population in your field: Randomly select five nonconsecutive plants in five to 10 different locations throughout the field. Count and record the number of rootworm adults while keeping track of the number of plants sampled, and then calculate the average number of beetles per plant.
If the field averages five or more beetles per plant during the first week of pollen shed, the entomologists say a foliar-applied insecticide is recommended to minimize CRW impact on pollination.
For example: If you checked 50 corn plants and found a total of 300 beetles, your average number of beetles per plant would be six, so an application would be warranted.
Another option is to check whether CRW are actively feeding on silks. If you find corn silks are clipped to within ½” of the ear tip on 25% to 50% of the total number of the plants you sampled, then a foliar insecticide application is recommended to prevent further silk damage.
For example: if you checked 50 plants across the field and 12 to 25 of them have silks clipped to ½” of the ear tip, you’re likely to see a payoff from treatment.
University of Nebraska Extension Entomologists, Robert Wright and Julie Peterson, offer one additional consideration. They note that using adult beetle control programs to reduce egg laying should begin treatments when the beetle threshold is exceeded and 10% of the female beetles are gravid (abdomen visibly distended with eggs).
"This is an important point since the first beetles to emerge are mostly male, and females require at least 10 to 14 days of feeding before they can lay eggs," Wright and Peterson say in an online article. "Treatments applied too early may be ineffective if large numbers of females emerge after the residual effectiveness of the treatment has dissipated."
Looking Ahead To 2023
Agronomists are encouraging farmers to switch up modes of action in their traited hybrids for maximum root protection next season. Corn rootworms will adapt to repetitive management tactics, especially in corn-on-corn environments, says Kevin Gale, AgriGold agronomist, in a recent blog.
“If farmers grow the same technology year after year, the rootworm population will adapt and do more injury to the root,” he says. “Rotating modes of action is more effective.”
Ribonucleic acid interference (RNAi) technology is adding a new component to farmers' battle with corn rootworm. RNAi works in CRW as it feeds on corn roots by turning off, or silencing, functions the pest needs for growth and sustainment.
The first corn hybrids containing RNAi technology to fight corn rootworm were launched commercially this season as SmartStax PRO. The hybrids offer three MOAs for corn rootworm – a double-stranded RNA targeting the CRW snf7 gene, in combination with Bt proteins Cry3Bb1 and Cry34Ab1/Cry35Ab1.
The SmartStax PRO lineup is expected to be launched more broadly across the seed industry next season.
Corteva Agriscience officials announce they have received U.S. Environmental Protection Agency approval for the company’s new Vorceed Enlist corn products with RNAi technology. The Vorceed Enlist hybrids will range in the 93- to 100-day relative maturity.
“The trait allows the broadening of our genetics which will result in higher-yielding hybrids,” says Tony Klemm, Corteva vice president, corn portfolio. “We'll quickly ramp up production, and this product will be our leading corn rootworm trait offer in the portfolio by mid-decade.”
He adds that the technology is expected to be available in products across all Corteva seed brands, including Pioneer, Brevant Seeds, AgVenture, Dairyland Seed, Hoegemeyer, NuTech and Seed Consultants.
The development of the RNAi technology continues “to move agriculture towards more environmentally friendly insecticides,” report researchers in a journal article published in Insects in January 2022 (see https://bit.ly/3u8Ogzc).
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