Corn, soybean insect updates track new concerns

Ray Mueller
University of Wisconsin Extension Service entomologist Bryan Jensen explains how to count corn rootworm beetles during a video shoot.

FOND DU LAC – Changes in federal standards on the use of crop products, a finding in Nova Scotia, and the appearance of a new insect in states bordering Wisconsin were among the highlights in the insect review by Extension Service entomologist Bryan Jensen at the 2019 Pest Management Update.

On the federal regulation front, Jensen reported that the Environmental Protection Agency (EPA) is considering an increase in the width of vegetative filter strips to 25 feet, possibly 15 feet, from the current 10 feet, as the required setback from bodies of water at any site where the very toxic synthetic pyrethroids are applied.

Insecticide Re-Registration

In an action already taken by the EPA, the sulfoxaflor active ingredient insecticide has been re-registered for controlling insects with piercing or sucking mouth parts in fields of soybean, corn, small grains, and alfalfa, Jensen stated. Although sulfoxaflor is classified as a neonic, it does not carry a bee advisory warning, does not require a pollinator mitigation strategy, and is not very toxic to the natural predators of the soybean aphid, he pointed out.

On a related point, Jensen continued, the EPA is conducting a new pollinator risk assessment for the neonic class products with thiamethoxam and clothianidin as the active ingredient. It plans to issue a proposed interim decision, followed by a period for public comment, by the end of 2019. He noted that corn and soybeans account for the greatest use of the neonic class of products.

Regarding the use of neonicotinoid seed treatments on soybean seed to control seed maggots, Jensen cited a major study and a report by 23 authors which indicated that the benefits from such a practice are minimal at best. He noted that insect pests are seldom an early season problem in soybeans and that water contamination and negative effects on bees and aquatic and terrestrial invertebrates are known risks from the use of neonics.

Insect Resistance to Bt Proteins

Although it happened in Nova Scotia, where only about 30,000 acres of corn are grown, resistance by European corn borer to the CRY1F protein has been documented there, Jensen reported. Because he considers “a canary in a corn mine” type warning about the bacillus thuringiensis (Bt) trait which was introduced in 2001 with the trade name Herculex, Jensen urges corn growers and crop consultants to “be aware, be vigilant, and say something if you see something” regarding resistance by the insect which was once the main pest in corn.

These western corn rootworm adult beetles emerge late June through
August and lay eggs almost exclusively in corn fields from early August through
early September.

A similar concern is the confirmed resistance by the northern corn rootworm to both the Cry3Bb1 (Yieldgard) and Cry35/34Ab1 (Herculex) Bt proteins in North Dakota, Jensen continued. That finding in late 2018 did not detect any cross resistance to those proteins by the western corn rootworm, he added.

Within Wisconsin, the northern rootworm was the most prominent in 2017 and 2018 but western rootworm population increased in 2019, Jensen pointed out. He noted that resistance by the western rootworm to all four of the Bt proteins has been documented at a few locations.

From a historical background of trying to control the rootworm with crop rotations, foliar applied insecticides, soil-applied insecticides, and the growing of Bt corn hybrids, Jensen emphasized that the first step in reacting to the potential of the rootworm resistance is not to immediately change management practices.

Verifying Rootworm Resistance

Start by verifying the cause of corn lodging and calculate the extent of rootworm damage on the node injury scale, Jensen advised. If that rating tops 1.0 with corn having a single corn rootworm trait or 0.5 with stacked traits, then it's time to contact one's seed dealer, he stated.

The companies, which are the registrants of those Bt traits, will kept that information confidential until there is a “trigger point” accumulation of reports, Jensen explained. They then need to inform the EPA, which could then provide a management plan to the affected growers.

Rootworm Control Protocols

In lieu of that, Jensen suggests regular scouting for rootworm beetles in continuous corn, particularly on tracking the population trend on second and third scoutings to notice if the economic threshold of .75 beetles per corn plant is reached. As a followup, monitor the root feeding in continuous corn to identify resistance before it becomes widespread, he adds.

Jensen doesn't have an answer on how long the interim should be before growing a Bt hybrid again in a field where there was a failure with that same protein. To reduce the odds of facing that question, he sees crop rotation as the best overall management tool, followed by having an awareness of the Bt proteins, rotating them as well on continuous corn, choosing a pyramid of protein toxins rather than a single one, and being aware that differing trade names might have the same trait package.

With conventional corn hybrids, soil-applied insecticides (liquid or granule) can be a remedy for high rootworm populations but probably not for ultra high populations, Jensen observed. With Bt hybrids, he would not use a soil-applied insecticide with normal rootworm populations but possibly with very high populations.

Fortunately, the latest surveys show that the corn rootworm beetle populations were fairly low throughout Wisconsin, Jensen concluded. With good monitoring and existing practices, he hopes that will continue to be true.

Larval feeding and plant injury caused by the soybean fall midge is usually restricted to the base of the plant.

Soybean Gall Midge

Although there has not been a confirmed finding in Wisconsin, Jensen is nonetheless concerned about the soybean gall midge, which has established itself in dozens of counties in Minnesota, Iowa, Nebraska, South Dakota, and Missouri. He said there was some brief confusion with the identification of a white mold gall midge during August of 2019 in northwest Wisconsin but noted that it is a different insect despite a very similar appearance of the larvae of the two midges.

Jensen explained that the soybean gall midge, which produces two generations per year, causes most of its damage at the edge of fields as it girdles the base of soybean plants. He cited damage estimates of yield losses approaching 100 percent at the edge of fields to up to 20 percent as far as 300 feet from the edge.

In 2016, experts from Iowa State University Extension and Outreach reported a new soybean pest in western Iowa - the soybean gall midge. The pests' larvae are clear and eventually turn bright orange as they mature.

Despite its name, the insect does not predispose a soybean plant to the white mold disease, Jensen observed. He announced that looking for the soybean gall midge will added to the organized field surveys in Wisconsin in 2020.