Surveying for corn rootworm

December 5, 2024 | Shane Beck

Corn rootworm continues to be an economically significant pest across Iowa.

In Iowa, the northern and western corn rootworms are of greatest concern.

Corn rootworms overwinter as eggs in the soil and hatch in late May to mid-June. The larvae feed on the roots of corn plants until they pupate and change into beetles.

Feeding damage on the roots can lead to a decrease in water and nutrient uptake and decreased standability.

Regardless of the observed species, two corn rootworm beetles per trap, per day, represent the economic injury level (EIL).

This means the EIL across four traps in seven days equals 56 beetles. This is the minimum number of beetles required to sustain a yield loss that is equal to the cost of a management tactic used to control the beetle population in that field.

Surveying

To help understand best management practices for corn rootworm, the Iowa Soybean Association (ISA) conducted a multi-year survey, sampling 120-140 fields each year, with an emphasis on fields that have been continuous corn or in counties with significant continuous corn acres.

A year-over-year comparison of the counties sampled, and the average pressure observed in each county is shown below.

The trapping duration lasted four weeks and started around the VT/R1 growth stage. This method consisted of placing four sticky traps in one row of a cornfield at 200 feet intervals. The traps were checked and replaced every week with the number of northern corn rootworms and western corn rootworms recorded separately.

Field management data was also collected to determine relationships between management practices and corn rootworm pressure observed in the fields.

There was a year-over-year reduction in the likelihood that corn rootworm pressure observed in the field following soybeans exceeded the EIL.

Across all years, there was a consistent reduction in the likelihood of a field exceeding the EIL when the corn crop followed soybeans.

In the case of fields following corn, the EIL was exceeded in 41% of cases, while for fields following soybeans, it was exceeded in 20% of cases.

Of the fields that followed soybeans and exceeded the EIL, northern corn rootworm was the predominant species in 57 fields. Western corn rootworm was the predominant species in 26 fields. This is likely because of the extended diapause that northern corn rootworms have developed. Extended diapause is when beetles lay their eggs in the cornfield, but then the larvae hatch two years later, making the rotation management strategy less effective.

From 2017-2024, ISA sampled 993 fields across the state in 89 counties. Farmer participants indicated that 413 of those fields used a Bacillus thuringiensis (Bt) or RNA interference (RNAi) trait technology to help control corn rootworm pressure. Most fields surveyed used two traits to control corn rootworm, BT proteins Cry34/35Ab1 and Cry3Bb1 were the most reported Bt proteins.

When looking at the effectiveness of control with trait technology, stacking traits is a key to maintaining efficacy when paired with crop rotation.

Starting in 2022, the percentage of fields using three traits increased by adding new RNAi trait technology using the dvSnf7 trait.

From 2022 to 2024, 31% of the fields that used two traits were above the EIL, and 23% of the fields that used three traits were above the EIL.

Managing corn rootworm

Strategies for managing corn rootworm vary by farmer and relative pressure, but there are three main strategies to manage this pest. It is recommended to rotate strategies to manage resistance pressure.

• Crop rotation
• Use of Bt and RNAi traits
• Preplant and/or foliar applied insecticides

Overall, there is no one-size-fits-all approach to managing corn rootworm. Effectively managing the pest from year to year requires a multi-layered approach. It’s important to rotate to a non-host species crop when the pressure gets significantly above two beetles per trap, per day.

Using in-furrow and foliar applied insecticide can also be effective at managing larvae and beetles already present in a field. We advise against utilizing both an in-furrow insecticide and a trait. This can lead to increased resistance and decrease the individual effectiveness of in-furrow insecticides and traits individually.

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