Field Notes talks corn pests and plant disease

Angie Peltier, UMN Extension crops educator, Fei Yang, UMN Extension corn entomologist and Anthony Hanson, UMN Extension IPM educator and Brett Arenz, UMN Plant Disease Clinic director

The following information was provided during the July 22, 2026 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session.

It’s time to talk about corn rootworms!

Minnesota has two rootworm species that threaten corn yield potential, western and northern corn rootworms.

Western corn rootworm

Western corn rootworm adults are small, yellow beetles with long antennae and black heads. Thoraxes of both male and female rootworms are yellow with black in different patterns across their wing covers. Female rootworm beetles have black striping across their wing covers running from their thorax to their rear end while males have continuous black coloring across their wing covers (Figure 1).

Figure 1. Western corn rootworm beetles on a corn ear leaf; a female beetle is facing up while a male is facing down. Picture: Angie Peltier.

Western corn rootworm (WCR) beetles are the first rootworm beetles to emerge as adults each summer, emerging approximately 1 week prior to northern corn rootworm beetles, with male beetles of each species emerging approximately 1 week earlier than females. WCR doesn’t tend to travel long distances from the field from which they emerge in the summer and so tend to be more abundant in fields with continuous corn production. However, variant WCR populations are able to lay their eggs in neighboring soybean fields, rendering them essentially ‘rotation resistant’.

Northern corn rootworm

Northern corn rootworm (NCR) beetles are pear-green in color (Figure 2). Unlike western corn rootworm beetles, northerns have a more mobile lifestyle and so can travel farther from the field from which they emerge and so all corn fields can be targets for egg laying.

Figure 2. Northern corn rootworm beetles on a pumpkin blossom. Photo: Angie Peltier

 

Some NCR populations have evolved what is called extended diapause, in which the eggs have a delay in hatching so they can survive more than one winter before hatching. This means that even if one rotates to a non-host crop such as soybeans, NCR eggs that were laid in a corn field may not hatch the following year in the rotation crop but in the following growing season. A strict corn/soybean rotation is a recipe for disaster in areas with high extended diapause NCR populations; rotating away from corn for two growing seasons is needed in these fields for rotation to significantly impact corn rootworm pressure.

Rootworm life cycle

Corn rootworms overwinter in soil as eggs laid in ~September. Larvae hatch in May and June and seek corn roots on which to feed. Feeding injury to roots not only disrupts water conduction from the roots to the rest of the plant but can create entry points for plant pathogens. Severe feeding injury can significantly reduce corn yield potential and can lead to plant lodging and subsequent standability issues and harvest headaches.

Corn larvae progress through multiple molts, growing larger as they feed until they pupate below ground and hatch as adult beetles.

Monitoring corn rootworms in 2026

The majority of management for corn rootworms takes place before and at planting through hybrid selection and in-furrow insecticide applications. However, scouting for rootworm beetles is still critical for effective corn rootworm management in future corn years. Once corn silks have dried, begin to scout for corn rootworm beetles weekly, visiting up to 12 locations per field and examining two plants per location. Risk to a 2027 corn crop is heightened if there is an average of 0.75 to 1 beetle per plant in continuous corn or an average of 4.5 beetles per plant in rotated corn.

An alternative way to determine the corn rootworm population in a field is by placing yellow sticky traps in corn fields. Placing 6 to 8 sticky traps in each field spaced approximately 50 paces apart, traps should be changed weekly for a total of 4 weeks beginning approximately 2 weeks after beetles begin to emerge. The threshold of beetles that should cause concern and additional management consideration in 2027 is an average of 4 to 6 beetles per trap per day.

Managing corn rootworms

There are three main means of managing corn rootworms, crop rotation, Bt hybrids and insecticides. Because most corn rootworms lay eggs in corn fields and larvae can survive only on corn roots, rotating to a non-host crop such as soybean, alfalfa, sugarbeet, potato, dry bean, small grains or sunflower can reduce the risk of a corn crop encountering a damaging corn rootworms larval population.

Bt proteins

Bt corn hybrids have been genetically modified to express a protein from a soilborne bacterium called Bacillus thuringiensis (Bt for short). Because of their crystalline nature, these proteins are called Cry proteins and when ingested by rootworm larvae Cry proteins create small holes in rootworm larval guts leading to their death. There are four Bt proteins with two modes of actions that are specific to corn rootworms, three similar ones in the Cry3 family proteins (Cry3Bb1, mCry3A, eCry3.1Ab) and Cry 34/35Ab1.

Another challenge for managing WCR however, is that some populations in Minnesota have evolved resistance to two of the Bt Cry proteins that had been bred into hybrids: Cry3Bb1 and Cry34/35. Unfortunately, once WCR populations have evolved resistance to one of the Cry3 family of proteins, the entire mode of action (all Cry3 proteins) are rendered ineffective due to a phenomenon known as cross resistance. Similar to tank mixing herbicide active ingredients from more than one site-of-action family, pyramiding Bt proteins, or deploying more than one Bt protein from more than one family in the same hybrid, is required for longer-term trait durability.

There are fewer cases of Bt resistance in NCRs, one in Meeker County Minnesota and another in North Dakota.

RNAi is a newer mode of action in genetically modified hybrids (Bayer hybrids). These hybrids express a messenger RNA molecule that turns the rootworm beetle’s own immune system against it. A new Bt trait, the CRW4 trait (as it is called) has proven quite effective in company-run trials. The two new Cry proteins included in CRW4 are named Mpp75Aa1.1 and Vpb4Da2. Dr. Yang’s lab is expected to have access hybrids with the CRW4 trait to impartially test in the 2027 growing season and the CRW4 trait is expected to be available for MN corn producers in the 2028 growing season.

Chemical insecticides

Soil applied insecticides are labeled for corn rootworms, but those from the pyrethroid class of insecticides have proven ineffective in some areas due to insecticide resistant populations. At multiple UMN research locations in which Dr. Yang’s lab has tested in-furrow insecticides however, few (including those containing nurizma and plinazoline) have proved effective. However, those insecticides with organophosphate active ingredients (ex. Aztec and Index) have proved quite effective at the UMN field sites where there is moderate corn rootworm pressure.

Some seed treatment insecticides from the neonicotinoid class are effective against low populations of corn rootworms. But only the high concentrations of seed treatment insecticides (ex. Poncho 1250) have proved effective against low rootworm populations. However, the effectiveness of seed treatment insecticides declines in moderate and high populations of corn rootworms.

University of Minnesota Plant Disease Clinic

The University of Minnesota Plant Disease Clinic accepts diseased plant samples for diagnostic purposes. All plants of economic importance in Minnesota other than turf grass species are accepted at the plant clinic and for a fee will be examined to provide the submitter with a diagnosis and associated management recommendations. During the 2025 growing season, the UMN Plant Disease Clinic (PDC) accepted samples from more than 180 different plant species. Without an accurate diagnosis of a disease problem, effective management now and into the future is much less likely. The PDC accepts plant samples both through shipment and by drop off throughout the work week. Regardless of how the samples get to the lab and because foliar disease symptoms can result from root or stem infections, it is important to provide as much plant material as possible, with whole plants (including roots) preferred.

Samples should arrive to the lab in good condition to allow diagnosis. Dr. Arenz suggests that roots be wrapped in plastic and foliage in newspaper. Samples should not be watered before shipping as this can simply accelerate plant tissue degradation and make diagnosis more difficult. In addition, sending samples overnight and early in the week is important so that they do not sit baking in a hot delivery truck for too many days or over a weekend, as this can complicate diagnosis. There is paperwork that needs to accompany samples that asks submitters to provide answers to diagnosis-pertinent questions such as when symptoms were first observed, the weather conditions experienced by the plant and the pattern of symptoms in the field and on plants.

There is a fee for diagnosis, ranging between $50-100 and largely depending upon the amount of staff time and materials required for diagnosis. Clinic staff would be glad to visit with someone (via email: pdc@umn.edu) or phone: 612-625-1275) before sample submission to best determine which plant parts to send.

The PDC is on the lookout for red crown rot, a new disease of soybeans in the state found for the first time last August. Above-ground symptoms of red crown rot are similar to the yellowing of the tissue between leaf veins followed by the death of this tissue observed in plants suffering from either brown stem rot or sudden death syndrome. Similar to the pathogens that cause sudden death syndrome and brown stem rot, the fungus that causes red crown rot produces a phytotoxic compound that travels in water being pulled from the soil and into the leaves by the plant. However, unlike plants that die prematurely due to sudden death syndrome that often experience leaflet loss, leaves of plants that are killed by red crown rot remain attached to plants. The fungus that causes red crown rot causes red discoloration of the outside of the crown of the plant (just at/above the soil line) and produces small, bright red fungal fruiting structures called perithecia can be observed at the crown of the plant. Perithecia can be easily observed using a hand lens or your smart phone’s camera app for magnification.

Audience questions

The presenters answered various audience questions asked either during webinar registration or live during the webinar, including: are insecticide applications to rootworm beetles (“beetle bombing”) an effective rootworm management strategy?; do corn rootworm beetles effect corn pollination?; how does one effectively manage NCR populations that have the extended diapause trait?; in general is Verticillium become more prevalent in Minnesota soils?; have you been getting many white mold samples this year as it has been very dry?; how should one package corn leaf samples so that they arrive to the PDC intact?; has anyone seen soybean aphids yet this year?

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.