Managing and Identifying Corn Rootworm

The corn rootworm has been a long-time pest of corn, with the first major damage reported in Colorado in 1909. In recent years, the use of Bt corn, or corn genetically engineered with the bacterium Bacillus thuringiensis, which is toxic to corn rootworm, has lowered the corn rootworm pest pressure. However, a recent study by Christian Krupke suggests that corn rootworm has been slowly developing a resistance to the Bt toxins. Today, we will take a look at the three different major species of corn rootworm and examine their differences. We will also discuss how crop insurance comes into play, if at all, when dealing with the insect.

While there are three different major species to note when it comes to corn rootworm, they all have a few similarities. All species have four stages of development, starting as an egg, moving to larva, then pupa, and then the adult. Once the larva hatches in each species, it will immediately begin hunting for corn roots to feed on. These larvae nibble on the roots and burrow deep into them, eating as they go. The damages can often cause root rot or lodging in corn, in addition to decreased nutrient uptake from the pruned and damaged roots.

Adult rootworms often feed on the leaves of the plant and the pollen during silking, which can further stunt the plant’s growth. In addition, the lowered pollen count can result in poorly filled ears. The adult rootworms tend to have a similar proportion amongst the species, with color palettes and patterns along the back being the easiest distinguishing factor, making each species easily identifiable. Unlike the adults, the larvae look extremely similar, making early determinations more difficult. While the actual lifecycle does vary a bit amongst the species, for most corn rootworms, freshly pupated females begin laying eggs rapidly, generally after about two weeks.

The western corn rootworm, or WCR, is the most common of the two corn rootworm species that overwinter as eggs in the soil. The eggs go through a suspended development known as diapause, after which they remain dormant in the soil until temperatures reach 52 degrees farenheight, and have been able to absorb some water. For WCR, if there is no corn present in the field when the larvae hatch, they will die of starvation. Overly dry conditions and flooded conditions can also lead to some larvae dying out.

The female WCR needs pollen for proper egg development, which is why they tend to be attracted to corn silk and pollen. Once a field has finished pollination, the WCR will move from the field to either a new, later planted corn field or less desirable hosts to continue the development of the eggs. While capable of flying a fair distance ( much longer distances are possible during heavy wind or storms), the WCR tends to prefer the field it emerged from for laying the next brood.

Similar to other corn rootworms, the WCR devours roots in its larval stage and munches on leaves, silk, and pollen of the corn plant in its adult stage. The damages to the roots cause root rot, lodging, and nutritional deficiency, depending on the number of WCR present. Large numbers of WCR larvae can severely damage root structure, which can lead to harvesting issues later. The damage caused by the larvae can also lead to other plant diseases, which use the damaged areas as entry locations. Adult feeding can lead to even less nutrient intake by damaging the leaves, as well as reduced ear fill, as pollination is interrupted.  Effectively, this pest causes problems and damages throughout most of a corn plant’s lifecycle. Everything from nutrient uptake to ear production to harvesting can be affected, which can lead to large yield reductions if enough WCR are present.

Adult stage of Corn Rootworm. Photo by Tom Hlavaty from the USDA ARS Gallary

One of the first control steps, as with most pest and disease pressure, is scouting and monitoring. Multiple methods of scouting for corn rootworm exist, with several counting by hand methods, and a sticky trap method. You can also scout for rootworm presence by hunting for their larvae around late May, though you may not be able to determine the species of rootworm until they mature. This guide from the Corn Rootworm IPM provides several excellent scouting options to help determine your corn rootworm pressure.

Another early tactic is utilizing crop rotation to starve WCR young. If the larvae hatch and have no corn to eat, they are likely to starve. As the western corn rootworm adapts to corn-soybean rotations by laying eggs in soybean fields, it may reduce the effectiveness of the rotation. Additionally, planting early can help. If the pollination stage is done sooner, the rootworms have less time to reproduce, and when the roots are given more time to develop, the plant has a higher tolerance to root feeding damage.

There are also transgenic corn hybrids that are currently effective against the western corn rootworm. Bt corn is highly resistant to the rootworm, but repeated use can lower that resistance as the insect adapts. Varieties that produce strong, healthy root systems tend to withstand more rootworm feeding than other options.

Aside from Bt corn, insecticide is an option. Insectiside is only suggested when there are massive numbers of beetles, with the Colorado State Extension suggesting an average of 18,000 beetles per acre on a scouting day between August and September. Insecticide may mitigate the numbers, but it can fail, and it can also kill beneficial insects. Some WCR may also be resistant to the used pesticide. The Colorado State Extension has several tips for pesticide usage.

The damages inflicted by northern corn rootworm are extremely similar to its western cousin. The larvae feed on corn roots, devastating the root structure in large numbers, while the adults hit the plant from the other side, wrecking corn pollination. Just like with WCR, the damages can lead to plant diseases such as root rot, and can weaken the root structure enough that the plant lodges. As the damages can be so similar, it can be more difficult to determine the rootworm species without investigating the insects themselves.

While SCR can still do damage to corn, for many states, it’s not considered economically damaging to corn crops. To cucurbit plants, though, the damage can be far worse. Even so, they can inflict similar damage to corn as their northern and western cousins if their numbers are high enough.

The larvae of the SCR work similarly to NCR and WCR, once again destroying the roots of corn. However, the adults are attracted to far more than just corn pollen and silks. Adult SCR also enjoy the leaves, flowers, petals, pollen, and fruits of other plants. Cucurbits are a heavy target, which may be where they get their spotted cucumber beetle name. The pest can sometimes kill seedling cucurbit plants when in large enough numbers due to overfeeding, and even if the plant survives, it can damage the value of the fruit, which may be fed upon as well. Additionally, SCR is known to carry a form of bacteria that causes bacterial wilt.  Plants infected with this disease have their vascular tissue plugged, choking the plant, which will lead to it wilting and eventually dying. The variant from SCR is most known for affecting cucurbit plants, and the best currently known management strategy is to remove infected plants to prevent further spread.

So, how does crop insurance play into all of this? Crop insurance does not directly cover your crops from the damage inflicted by corn rootworms. However, it does help cover the financial loss experienced by reduced yields. If the damage done by corn rootworm is enough to hurt your yields, then crop insurance can help, depending on the plan selected.

Each operation is unique, and available crop insurance options vary across the country. Our expert agents have access to powerful software tools that can help you tailor a risk management strategy specific to your operation and location.

Photo from the USDA ARS gallary, shot by Ken Hammond.