Corn Rootworm Scouting and Management for Conventional Corn Hybrids
Ken Wise, NYS IPM Program-Cornell Cooperative Extension
Many growers are exploring possible premiums for growing crops that meet non-GMO production standards. There are a few pests that will remerge as a problem in the absence of hybrids containing the Bt (GMO) trait, such as corn rootworm. Many growers have not scouted or managed for this insect pest without Bt hybrids for a long time. The keys to managing corn rootworm is prevention and scouting. We should start at the beginning and know the life cycle and how to correctly identify corn rootworm.
Corn Rootworm Life Cycle: Corn rootworm adult beetles fed on corn pollen and some on the silks. Once they have feed enough, so they can breed, and eggs can develop, eggs are laid in the soil in a corn field in anticipation for the next season. They over winter as eggs and will start hatching about the same time fireflies are present. You will find wormlike larvae –with a white body and brown head, about the diameter of a pencil lead–on corn roots.
Corn Rootworm Larva (Photo by Scott Bauer, USDA Agricultural Research Service, Bugwood.org)
The larvae will feed on the roots until a few weeks before pollination when they will emerge as an adult beetle. These larvae can reduce the plant’s ability to get water and nutrients from the soil, thus significantly reducing yield of silage and grain.
Extreme corn rootworm damage
Heavy feeding can even cause lodging (sometimes called goose-necking). Goose-necking is the plant’s attempt to straighten up again after it has begun to lean or lodge.
To avoid damage in continuous corn fields it is important to determine if the population of the pest in the soil will cause economic yield losses. The way this task is conducted is to count the number of corn rootworm beetles on the plants in the field. The key is to make sure you know how to identify them. There are two species of corn rootworm, northern and western. You will have to keep your counts for each in the field separate because the western corn rootworm does twice the damage than northern.
Western corn rootworm (WCRW) adults are black and yellow beetles that are approximately 1/4-inch long. The female is yellowish with 3 black stripes on its back, while the male is solid black with a pale-yellow area at the tip of its abdomen.
Northern corn rootworm is slightly smaller than the western, and it is bright green in color (see photo).
The northern corn rootworm (NCRW) used to be the predominant species in New York State, but since the arrival of the western in the 1980’s and 90’s, the western has become the dominant species. When scouting, 1 western corn rootworm equals 2 northern corn rootworm adults.
Scouting for Corn Rootworm: You will need to scout all corn fields that will be kept in corn next year. A first-year corn field will not need to be scouted since there will be no rootworms in the field, unless the field was in pumpkins the previous year. Corn rootworm beetles feed on pumpkin pollen and will lay eggs in the field. You will need to start scouting a week before pollination. Pollination occurs for three weeks and monitoring takes about 20 minutes per field. You will need to monitor each field once a week until you reach a threshold or until pollination is over. Taking beetle counts is important but make sure you stop to check a portion of the female western CRW’s for the actual presence of eggs. Squeeze the abdomens of the CRWs and look for the small yellow – white eggs. It takes CRW about three weeks from the time the adult beetles emerge from the soil and mate until the time the females are gravid. In this time period you may find high CRW numbers in a field but since the females are not yet capable of laying eggs they are not causing an economic problem. This is the reasoning behind sampling the same field 2-3 times before making the management decision. Being pollen feeders and highly mobile, CRW’s may relocate to another pollinating fields during the 3-week period. Comparing the two types of fields, the second field is at greater risk from subsequent CRW damage since females (and their eggs) will have matured and are ready for deposit. Remember, to start scouting 30 to 50 feet into the field. The edge of fields many times does not represent what the population is for corn rootworm across the rest of the field.
HHHere’s how you scout:
- Are female beetles present? Mature and capable of egg laying? Conduct the squeeze test (see above) to determine if they are ready to lay eggs.
- Approach a corn plant carefully because the beetles will fly off if they disturbed too much.
- Grasp the silk with one hand.
- Count the beetles on the entire plant.
- Start counting at the top working down.
- Gently pull leaves away from the stalk so you count any beetles that may be hiding in the whorls.
- For each corn plant monitored, record the total number of beetles observed. See the sequential sampling chart below. Since western corn rootworms are potentially more damaging than their northern cousins, count each western (yellow striped) beetle observed as “one” and each northern (green type) as “1/2”.
- Check several plants at random (not next to each other!) in several parts of the field.
- Continue sampling at seven-day intervals until the ear silks are brown, approximately 3 weeks after tassels are first visible, pollination is complete or an above threshold number of beetles are found.
Using the Sequential Sampling Card for Corn Rootworm
- Keep a running total (RT) of the number of corn rootworm beetles you have counted on each plant. Each northern corn rootworm has half the value of each western corn rootworm. The western corn rootworm does twice the damage to corn than does the northern. So if you count 3 westerns and 4 northern (2 western equivalent) on a plant you would have a total of 5 beetles.
- If the number of corn rootworm beetles observed is smaller than the “N” (“Not at threshold”) number stop and scout 7 days later.
- If the number of corn rootworm beetles observed is larger than the “T” (“At threshold” or “Treat”) number then you need to manage rootworms next year.
- If the number of corn rootworm beetles observed fall between “N” and “T”, continue sampling additional plants until you finally go over or under.
- In a very low or very high rootworm population a sampling decision can be made in sampling as few as 3 to 8 plants. For moderate populations more samples may be necessary to insure accuracy.
Remember all management options taken are for the next season if an economic threshold is reached.
- Rotation is the best way to control this insect pest. When a field is rotated you take away their food source for the next year. Rotating a field every 1 to 3 years will reduce the risk of corn rootworm reaching threshold.
- In second and third year corn with moderate corn rootworm populations an insecticide seed treatment can be used at the corn rootworm rate. This is Poncho 1250 or Cruiser 1.25. Note that if you use these seed treatments consider the effect on pollinating insects like bees. Use bee safe seed lubricates in the planters to reduce dust that can drift off target and kill these important aspects of our agroecosystems.
- The last management option is the use of a soil-applied insecticide at planting. To select an insecticide registered for corn rootworm, please consult the Cornell Guide for Integrated Field Crop Management. Note that soil applied granular insecticides are more effective than soil applied liquid insecticides.
For more specific information on corn rootworm management with conventional hybrids please see Dr. Elson Shield’s (Cornell University Field Crops Entomology Professor) “What’s Cropping Up” article: Managing Corn Rootworm in Non-GMO Corn
The following is a short video on corn rootworm scouting and tactics for managing corn rootworm. Video on IPM Scouting for Corn Rootworm
The following link is the corn rootworm scouting form: corn rootworm scouting form