Larval injury can produce several visual symptoms. Plants may be stunted, chlorotic, wilted, or show signs of nutrient deficiencies because of root feeding. The most obvious sign of root injury during the season is lodging and goosenecking that can occur following heavy winds, and yield losses may occur at the end of the season. During an ideal growing season or in high-quality soils, economic levels of feeding injury may not be visible aboveground.
Scouting for larvae
You can scout for the presence of corn rootworm larvae during or after peak hatching in your area (see the Larval Survival and Movement section on the Biology and Ecology page). You may start scouting for corn rootworm larvae once hatching begins in late May, but it may be very difficult to find larvae as they will be small, first instars. You will need a shovel or spade and a black garbage sack (or a dark-colored cloth) or a bucket with water. Corn rootworm larvae are small, but most people can see them with the naked eye. Consider using a hand lens, and refer to the identification tips to confirm the presence of corn rootworm larvae.
- Select a random plant, and dig out the plant and surrounding soil. Make sure to go wide enough (at least 6 inches [15 cm] from plant) and deep enough (at least 6 inches [15 cm]) that you capture most of the root system. Make sure to collect several plants per field (~10 from different areas), especially if multiple hybrids are present.
- Look for larvae – 2 methods:
- Hand-sorting: Place the plant on the dark bag or cloth and gently break apart the soil. Larvae should be apparent on the contrasting background. Look for larvae on the bag or cloth, in the soil, and near the roots.
- Float test: Place the plant and soil in a 5-gallon bucket with water. Break apart the soil with your hands or by gently tapping the plant on the bottom of the bucket. Larvae should float to the top of the water. Adding salt to the water will make larvae “float” more easily.
- Make sure to give ample time for larvae to become apparent with each method. Record the number of larvae on each plant, and find the average.
Rescue treatments for corn rootworm larvae are not usually recommended since their efficacy is highly variable. Instead, use the float test and larval sampling techniques to determine whether larvae are present and their size. Then, plan to sample for adults later in the season.
Scouting for root injury
To get an accurate assessment of root injury in a field, sampling needs to occur while the injury is fresh. We recommend that people scout for root injury once the first adults are observed in the field. Root-damage ratings are the standard method of measuring the effectiveness of larval management techniques. To assess root injury, you will need a shovel or spade and a power washer or access to several buckets or a tank of water.
- Choose several random plants to extract from the field. Use a shovel to dig out the plants, making sure to go wide and deep enough (at least a 6-inch [15 cm] cube) to avoid cutting roots.
- If there is a lot of soil on the root system, you can gently tap the plants on the ground to dislodge the soil. Make sure not to break any roots in the process.
- Wash the roots – 2 methods:
- Power washer method: Soak roots in water, gently tapping the root on the bottom of the bucket to dislodge some soil. Then, power wash roots to remove all remaining soil. Keep the root away from the nozzle and use appropriate pressure to avoid damaging the roots.
- Soaking method: Simply leave the roots in the water for several hours (or overnight) to allow time for the water to wash soil away from roots. Then, swirl them around in the water and use your hands to dislodge any remaining soil. Roots do not generally get as clean using this method.
- Use the 0-3 Node-Injury Score (NIS) rating system developed by Iowa State University to assess root injury.
According to the USDA, a NIS rating above 1 is unacceptable for single-trait corn hybrids, and a NIS rating above 0.5 is unacceptable for pyramided corn hybrids. If these thresholds are reached, an alternative management strategy should be implemented the following year. Additionally, research demonstrates that a NIS rating of at least 0.25 indicates economic injury is occurring, and each whole number rating translates to approximately 15% yield loss (ex: NIS of 1 = 15% yield loss, 2 = 30%, etc).
Scouting for adults
Scouting for adults is recommended to begin around the time corn silking begins (R1 growth stage). Similar to larvae, there are several ways to sample for adults, and the sampling plan may depend on the purpose of scouting activities (current or future management). Generally speaking, there are two methods of sampling for corn rootworm adults to identify whether to expect high larval densities the following year: sticky traps and in-field plant counts.
Sampling using sticky traps in corn:
- Identify a field of interest. Perhaps it is a field that has suspected resistance, or maybe a management tactic did not perform as well as you thought.
- Use a flag, stake, marking tape, or a combination of things to mark at the field edge where your transect is located. This will help you locate your traps in the future.
- Place the first trap at least 165 feet from any field edge. Attach it directly to the corn plant at ear height.
- Continue placing traps along the same row and at least 165 feet apart. It is recommended to use 6-8 traps per transect to get a representative sample.
- Place at least one additional transect in the field (total of 2 or more transects). If a field has multiple hybrids, consider each hybrid a different field.
- Leave the sticky traps in the field for a week. Record the number of corn rootworms (NCR + WCR) per trap, and record how many days the trap has been in the field.
- Replace the traps with a new set. Continue to monitor traps for at least 6 weeks (the emergence period for adults is ~8 weeks), or until the threshold is reached.
If more than 2 beetles per trap per day (beetles/trap/day) are captured, it is recommended that a different management tactic is implemented. If rotation to a different crop (ex: soybean) is an option, that is preferred. If corn is to be planted the following year, consider a different management tactic for larvae.
Sampling using in-field plant counts in corn:
- Select 20 random areas in a field (remember, if multiple hybrids are in a field, consider each hybrid separately) – this can be done using GPS or by walking in a defined walking pattern (U-shaped or W-shaped are popular).
- At each area, select 2 plants within 10 feet of each other to inspect for adult corn rootworm. As you approach the plant, take note of any rootworm beetles that drop to the ground or fly away.
- Hold the silks tightly in your hand to prevent beetles from leaving while you inspect the rest of the plant. Slowly open your hand and count the number of beetles on the silks. Examine thoroughly, because beetles can be tangled in the silks or hidden deep in the ear tip.
- If multiple ears are present, inspect the silks on all ears.
- Repeat for the second plant, and move on to the next area.
- After sampling is complete (40 total plants), determine the average number of beetles per plant.
If the goal of sampling for corn rootworm adults is to determine if the amount of silk damage will interfere with pollination, a separate sampling plan exists:
- Randomly select 5 plants in an area to inspect for silk clipping. This process will be repeated in 5 areas of the field (25 total plants).
- If silk clipping is noted, measure the length of the remaining silk on each plant. Then, take an average for the entire field.
- Take note of the number of beetles present on each plant and determine the field average.
- Estimate how much pollination has taken place (percent) by carefully pulling back the husk leaves and gently shaking the ear. If the ovule has been fertilized, the silk will drop from the ear.
Note whether pollen shed is complete by inspecting tassels.