University of Nebraska-Lincoln
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University of Nebraska-Lincoln |
| Managing Corn Rootworm Larvae |
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Robert J. Wright,
Extension Entomologist, IPM, South Central Research & Extension Center, Clay Center, NE |
Updated March 25, 2000
For further information on corn rootworms, refer to EC 1563, Corn Rootworm Management and NebGuide G92-1108-A, Evaluating Corn Rootworm Soil Insecticide Performance.
 Mature Corn Rootworm Larva |
 Lodging of Plants |
 Severe Root Pruning |
Damage by corn rootworm larvae is most likely to occur in continuous corn fields. However, it has been estimated that more than 60 percent of Nebraska’s continuous corn fields do not have economic corn rootworm infestations in a given year. The only way to determine whether a field is likely to have an economic rootworm infestation is to scout for adults or larvae.
Corn should be scouted weekly from mid-July through early September or until the threshold is exceeded. If on any scouting date during the egg-laying period, counts indicate more than 18,000 beetles per acre (=0.75 beetles per plant, based on a plant population of 24,000 plants per acre),consider rotation or use of a soil insecticide as a precautionary measure the next spring. Beetle thresholds (expressed as number per plant) will vary with plant population (Refer to NebGuide G86-774-A, Western Corn Rootworm Soil Insecticide Treatment Decisions Based on Beetle Numbers).
First year corn is unlikely to benefit from soil insecticide applications for corn rootworm control in most situations. Crop rotation is at least 98 percent effective in protecting fields from economic levels of corn rootworms. Treatment of first year corn should be considered only if corn follows oat stubble, or soybean fields heavily infested with volunteer corn or weeds. In a very small percentage of fields following soybeans or other rotations, northern corn rootworm larvae may damage first year corn. This is because a certain percentage of the eggs may hatch two years after being laid --a phenomenon referred to as extended diapause. These fields cannot be identified with certainty, but tend to occur in localized areas. Growers in areas where extended diapause has been a problem must base their treatment decision on the presence of past rootworm problems in their fields and their neighbors’ fields. In this regard, extended rotations (i.e. more than two years) are encouraged if practical. The damage to first year corn from western corn rootworms reported from parts of Indiana and Illinois is due to beetles laying eggs in soybeans in a corn-soybean rotation system. This problem is not known to occur outside of these two states.
The effectiveness of soil insecticides can be reduced if the soil remains dry after application, if excessive rainfall occurs, if soils are highly alkaline, or if insecticides are applied at planting time on early planted corn. Insecticide performance is often more reliable if an insecticide is applied at cultivation time in late May or early June, especially if corn was planted before May 15. Cultivation treatment is particularly desirable if soil is alkaline (high pH accelerates decomposition of some insecticides) or if the field has developed a history of poor root protection when insecticides have been applied at planting.
Many insecticide performance failures can be traced to poor calibration of granular applicators. In many cases, amounts used are below those recommended on the label. Remember that label recommendations for pounds/acre are based on 40-inch row spacings. If corn is planted in rows narrower than 40 inches, there are more linear feet of row per acre, which requires more insecticide per acre to obtain the proper rate. By calibrating applicators to deliver the suggested amount of granules per 1,000 feet of row, the amount will be correct regardless of row spacing.
Soil conditions and type of planting equipment can greatly affect the placement of soil insecticides, which is an important factor in rootworm management. Soil insecticides are more effective if lightly covered with soil during application. Granules or liquids remaining on the surface may rapidly volatilize or break down, which can result in poor performance. Also, leaving insecticide on the soil surface may increase the chances of environmental contamination and the poisoning of nontarget organisms.
Fertilizer combined with a soil insecticide should be applied in bands on both sides of the seed furrow at seed level. Certain insecticides (e.g. Thimet 20G, Dyfonate 20G, Mocap 10G, and all liquid formulations) should not be allowed to enter the seed furrow, or stand reduction may occur. If soil is moist, rough, or cloddy and the furrow fails to close properly, the possibility of phytotoxicity is increased.
Regardless of material or placement, some feeding on roots will occur, and some larvae will survive to adulthood. Extensive adult emergence does not necessarily indicate that the insecticide did not perform adequately.
Soil insecticide performance should be evaluated annually by comparing corn root ratings in treated and untreated areas. Use of the same corn rootworm soil insecticide in continuous corn over several consecutive years in the same field has generally been successful in Nebraska. In a few fields, however, poor and/or erratic performance has occurred. While many of these failures can be attributed to application problems, planting dates, calibration errors, or environmental factors, some may have resulted from continuous use of the same soil insecticide.
If problems have occurred with planting time applications, consider using a cultivation application or rotating to another crop. When there is no alternative to planting time application in continuous corn, consider the following suggestions.
After about June 10 (or earlier depending on timing of egg hatch), rescue treatment at lay-by time can be made by applying any of the suggested cultivation-time materials to the soil at the base of plants. Cover the insecticide with 1-2 inches of soil. This treatment will not guarantee root protection because the insecticide will not thoroughly penetrate the soil. It may help reduce further root damage by establishing a barrier between the rootworms and developing roots. If applications are made by aircraft, use granules, cultivate into rows immediately, and irrigate if possible. Considerable variation in insecticide performance has occurred where broadcast applications have not been incorporated into the soil.
Research at several midwestern universities, including UNL, has shown that use of reduced rates of corn rootworm soil insecticides usually provides acceptable levels of protection from corn rootworm feeding injury. If you are planning to use this approach, consider the following points:
