Corn rootworm is consistently the most prevalent insect pest in field corn across the corn belt and other corn production areas of the US. It earned the nickname, ‘the billion-dollar bug’ as a result of data collected by the USDA citing that the reduction of profits totaled $800 million in yield loss, and another $200 million spent on treatment expense.
Corn rootworm tends to consistently take yields and profits from the corn producer each year. Over the years, infestations have become more complex, and the responding prevention and control of corn rootworm has become more sophisticated.
There are three strains of corn rootworm. The Southern Corn Rootworm, also known as the ‘spotted cucumber beetle’ is a lesser problem than the Northern and Western Corn Rootworm because the southern variety does not overwinter in most corn growing regions of the midwest and northeast, thus making its economic impact minimal compared to its counterparts.
The Northern and Western Corn Rootworm, however, are a different case. There are three ways these strains of rootworm can affect corn:
- Root damage is by far the most significant type of damage and should be the focus of any control regimen. Emerging larvae feed on the young root hairs of emerging roots after they hatch. As they develop, the outer root tissue of the plant becomes their source. Once they advance into later instars, they burrow into the roots and bring about their greatest damage when the secondary roots are established and brace roots are beginning to emerge. Root tips will appear brown, and evidence of tunneling and chewing is visible all the way back to the base of the plant. Severe infestations can consume almost all of the productive root tissue of the plant. Characteristic ‘goose-necking’ is clear evidence the damage is done.
- Leaf feeding while foraging around, breeding, and finding sites to lay their eggs can be significant at extremely high populations.
- Silk damage by clipping can become economically significant, but again, only at very high populations. Some claim that economic damage due to incomplete pollination can occur at as low a population as 5.0 beetles per silk; others claim as many as 10 or more per silk is needed. The only control available in this scenario is foliar application using a high-boy sprayer, or aerial spraying, and effectiveness is limited.
Corn Rootworm Risk Assessment
Corn rootworm is most prevalent in fields where corn is grown continuously from year to year. The root damage occurs the year after the adults have laid their eggs in the soil of the previous year’s corn crop. In the past, scouting for adults in first and second year corn have held priority for predicting the need for treatment the following year, and any years beyond that have been presumed to be infested. When (typically) 1.0 adults are found per silk during silk emergence and development; treatment or rotation the following year is warranted. Different situations, regions, and conditions may stray from this general average.
Corn rootworm larvae damage in first year corn is rare, but can occur within a few variants.
Corn rootworm variants – Corn rootworm is a highly adaptable insect pest, and have developed population variants to overcome simply crop rotation. To determine if such variants are setting up in your field, it’s good to check for larval root injury in first year corn as part of your Integrated Pest Management (IPM) practices. Further, you should know the difference between Northern (green without markings) and Western (striped) and monitor the prevalence of either or both while scouting.
Northern Corn Rootworm can be resistant to crop rotation via extended diapause. Eggs laid in corn by Northerns can lay dormant for two or even three years before hatching. If this variant population is abundant, the potential for root feeding in first year corn exists where the rotation might just be a year or two out of corn. It will rarely exist in rotations with alfalfa or other perennial crops. Extended diapause is somewhat geographically specific, so consult with your local Extension or certified crop advisor to learn whether they have been documented in your area.
Western Corn Rootworm can be resistant to crop rotation because they will lay eggs outside of cornfields, especially in soybeans. Those eggs will hatch the following year and can cause injury to first-year corn. The use of sticky traps in selected soybean fields as part of your IPM regimes is the best way to determine populations. Thresholds can be found for your area through your Land Grant University or Certified Crop Advisor.
Corn Rootworm Control
Resistance genetics have proven to provide a degree of control over corn rootworm. Certain genetic parentage of pyramided Bt hybrids have shown good control over corn rootworm larvae, but close attention to your localized efficacy of the varieties offered is important to entirely trust the results in your geographic region.
There are now a few options for seed treatments which have various effectiveness ratings against rootworm larvae, but again, consult your state’s extension recommendations before using them.
Soil insecticides applied at planting continue to be used widely for treating corn rootworm. Rotating families of insecticides is highly recommended to head off resistant populations before they predominate.
The best control against corn rootworm hasn’t changed: Rotation, rotation, rotation.