Insect resistance management (IRM) is critical for corn rootworms because they are highly adaptable pests. It is important to follow an integrated pest management (IPM) plan to mitigate resistance development and ensure profitable corn production.
Scouting for corn rootworm is the best way to determine if management action is needed. Once scouting reveals that corn rootworm is present and causing injury to a cornfield, choosing the best management strategy depends on field history, resistance issues (insecticide, Bt, or crop rotation), financial constraints, and agronomic practices in the field.
Many corn hybrids available to farmers are now transgenic (genetically modified) Bt corn hybrids. Bt corn has been modified to express proteins derived from a naturally occurring soil bacteria, Bacillus thuringiensis. These proteins were chosen for their ability to kill specific corn insect pests.
There are many Bt traits available to farmers, and it is easy to be confused by all the different names and terminology (see definitions below). Bt hybrids have been developed that target coleopteran pests (beetles, specifically northern and western corn rootworm), lepidopteran pests (caterpillars), or both at the same time. Refer to the Handy Bt Trait Table for more information about Bt trait targets and trade names.
Single-trait hybrids – Single-trait hybrids, or hybrids that contain one Bt protein for a pest, are not that common anymore due to the likelihood of resistance development.
Pyramided hybrids – A Bt hybrid with two or more Bt proteins that target the same insect group (ex: two Bt proteins for caterpillars) is called a “pyramid”.
Stacked hybrids – A Bt hybrid that contains proteins that target two different insect groups (ex: one Bt protein for corn rootworm and one Bt protein for caterpillars) is called “stacked”. This also applies to a hybrid that is herbicide-tolerant and has at least one Bt protein.
Stacked pyramids – A Bt hybrid that contains two or more Bt proteins that target one insect group and at least one Bt protein that targets another insect group is called a “stacked pyramid” (ex: two Bt proteins for corn rootworm and one Bt protein for caterpillars).
More recently, Bayer CropScience has developed corn hybrids with a new transgenic mode of action for corn rootworm called RNAi, which stands for RNA interference. This new technology will always be paired with Bt but provides another barrier for resistance development. Corn hybrids with RNAi are not recommended for fields that have had issues with Bt-resistant corn rootworm populations, since the RNAi technology would be acting alone and rootworms would likely become resistant. IPM and IRM guidelines for Bt corn hybrids are applicable to new hybrids that contain RNAi.
Refuge seed does not have the Bt traits used for the target pest and requires the farmer to comply with location and area percentage requirements for resistance management. The idea behind a refuge is that any resistant individuals produced on Bt plants will mate with susceptible individuals produced on refuge plants, producing individuals that are heterozygous for resistance and susceptible to the Bt trait.
Some Bt hybrids will have refuge-in-the-bag (RIB), which means refuge seed is already mixed in with traited seed at the proper ratio. If a separate refuge is required, make sure to follow all instructions for planting a refuge, including location (adjacent or within the field) and area (percentage) requirements. Refuge area requirements are typically 5-20% for corn rootworms. Resistance evolves more quickly when the refuge area is decreased or resistance is more dominant.
Preventing Development of Bt Resistance
If a Bt hybrid is used and unexpected corn rootworm populations or severe root injury occurs in the field, you should contact the seed company and your local extension agent. Some warning signs that a field may have resistant corn rootworm:
- Severe root pruning. A node-injury score of >1 is unacceptable for hybrids with a single Bt trait, and >0.5 is unacceptable for pyramided hybrids.
- Symptoms of feeding. If a Bt cornfield has significant lodging, goosenecking, leaf-feeding, or silk clipping, there may be resistance developing.
- High adult populations. High densities of adults in Bt cornfields are a red flag. If using sticky cards to monitor adults, >2 beetles/trap/day suggests another tactic should be used the following year.
We recommend the following strategies to reduce the risk of rootworm populations developing resistance to Bt:
- Crop rotation. If possible, rotate fields annually from corn to a non-host crop. Fields should be rotated away from corn at least once every 4 years to break up the life cycle and slow resistance development.
- Comply with refuge requirements. Make sure to follow instructions on the seed tag.
- Weed management. Eliminate volunteer corn or grassy weeds that could serve as hosts.
- Scout and assess activity. Assess root injury by larvae when it is fresh, especially in fields with a history of rootworm injury, and scout for adults.
- Switch it up. If there is a lot of adult activity or larval feeding in a Bt cornfield, that is a sign to do something different the next year. If crop rotation is not an option, plant a non-rootworm-Bt corn hybrid the following year and use a soil insecticide. Soil-applied insecticides work best when rootworm pressure is low, but it can help reduce selection pressure for Bt resistance.
It is NOT recommended to combine soil insecticides and Bt hybrids for many reasons. It is typically not cost-effective to use both tactics at once, and it does not provide additional yield protection.
Insecticidal seed treatments do not generally provide protection from corn rootworm feeding. In general, seed treatments are only effective for a 2-3 week window after planting, which does not typically coincide with larval activity. Seed treatments may be effective with low larval densities.
Insecticides for corn rootworms are either granular or liquid forms that can be applied to the soil at planting or to the foliage post-emergence. Soil insecticides for larval control, applied in-furrow or as T-bands, generally provide root-zone protection that results in approximately 70% larval mortality. The efficacy of post-emergent applications for larvae is more variable and usually depends on rainfall or irrigation for activating the insecticide. These applications require incorporating the insecticide via tillage, which may not be possible in some fields.
The goal of insecticides for larval control is to protect the majority of the root system from larval feeding. Larvae may still feed on roots outside of the protected zone. Economic injury can still occur with high larval populations or if egg hatch is delayed.
Some considerations for insecticide applications:
- Make sure equipment is calibrated as this is the most frequent cause of performance issues.
- Use the full rate of insecticide as specified on the label. Using a lower rate can result in poor performance and lead to resistance development. Using a higher rate is not profitable (and illegal!).
- Make sure insecticide is evenly applied to the soil, drift is reduced as much as possible, and the furrow is fully closed.
- Prevent volatilization by lightly incorporating the insecticide into the soil.
- Soil moisture is important. If it is too dry, the insecticide may not be distributed throughout the soil, and if it is too wet, insecticides can leach or runoff.
- Highly alkaline soils deteriorate insecticides faster than neutral or slightly acidic soils.
Sometimes, foliar insecticides are used in an attempt to suppress adult corn rootworms in a field for two reasons: to minimize silk clipping or to decrease oviposition and subsequent larval numbers in the spring. Using foliar insecticide applications to target adult corn rootworm during the summer is NOT recommended because of the long emergence period of adults. It would likely take several applications to reach an acceptable level of adult control, which is rarely cost-effective. Plus, applications for reducing spring larval populations would need to target gravid females (females ready to lay eggs) to be effective. In general, the costs associated with scouting adults and applying foliar insecticides are much higher than using a soil insecticide to target larvae. Additionally, foliar insecticides have a negative impact on natural enemies in the field, which can cause flare-ups of other pests.