Corn Rootworm Management

The potential for rootworm damage to corn following corn is greatest in the northern two-thirds of the state. However, moderate to severe damage to corn roots may occur in any field where corn is planted after corn or where corn is planted after soybeans in east-central Illinois.

The abundance of rootworm beetles in a cornfield during the summer of 1997 will largely determine the potential for rootworm larval injury if corn is planted in the same field in 1998. Generally, if beetle numbers reached or exceeded 0.75 per plant at any time during late July, August, or September 1997, rootworm larvae may cause economic damage in that field in 1998.

However, if the field scouted in 1997 was corn after any crop other than corn, the threshold (beetles per plant) is lower. The ratio of female to male beetles in first-year corn is usually higher than in continuous corn. Females migrate more than males, so most of the beetles found in first-year corn are females. As a consequence, the threshold for determining whether to rotate away from corn or to use a soil insecticide in 1998 may be as low as 0.5 beetle per plant. (See "Scouting to Determine Rootworm Potential in 1999," for a discussion about adjusting thresh-olds for different plant populations.)

Fields of corn planted in June 1997 may have exten-sive rootworm damage if they are planted to corn again in 1998. Throughout August and September, rootworm beetles are attracted to late-planted or late-maturing fields. Seeking fresh pollen and silks to feed on, the beetles lay millions of eggs in these fields. Planting the fields to a crop other than corn in 1998 is suggested.

Suggestions for Rootworm Management, 1998

Producers should seriously consider crop rotation as the first choice for rootworm management, particularly in fields with a high probability of rootworm damage. Other management tactics are application of a soil insecticide at planting or during cultivation. A soil insecticide applied at planting is the predominant method of rootworm control in corn after corn.

Crop Rotation
Crop rotation usually is effective (except in east-central Illinois) in preventing damage caused by northern and western corn rootworm larvae. If feasible, do not grow corn 2 years in succession in the same field. First-year corn after soybeans in most areas of Illinois does not require a soil insecticide for rootworm control. In 1998, producers in east-central Illinois are encouraged to consider using a soil insecticide for rootworm control in corn planted after soybeans. Western corn rootworm beetles were com-mon in many east-central Illinois soybean fields in 1997 and are presumed to have laid eggs. In addition, western corn rootworm beetles were found in soybean fields as far west as Logan County and south-ward to Coles County. Although densities of beetles in counties outside of east-central Illinois were not as great, growers should presume that some egg laying within soybean fields may have occurred.

In some instances, rootworm larvae have injured corn planted after soybeans when the bean field was heavily infested with volunteer corn or weeds during August of the preceding year. Adult northern and western corn rootworms were attracted to these fields to deposit eggs, resulting in root damage by larvae the following season. Fields that were infested with 5,000 or more volunteer corn plants per acre in 1997 may warrant treatment for rootworm control in 1998 if they are planted to corn. Good weed control in soy-beans usually prevents rootworm damage in corn af-ter soybeans. The widespread infestations of western corn rootworm larvae in first-year corn in east-central Illinois cannot be attributed to volunteer corn in these fields.

Some east-central Illinois producers continue to experience severe western corn rootworm larval injury in their first-year cornfields (corn rotated with soybean). Evidence suggests that western corn root-worm adults are laying eggs in soybean fields and causing economic injury in some first-year corn-fields. No definitive explanation for why some western corn rootworm beetles find certain soybean fields suitable for egg laying has been determined. Thus far, larval injury to first-year corn has been confined to east-central Illinois counties. However, the geographical range of western corn rootworm beetles found in soybeans expanded during 1997. Producers in east-central Illinois who experienced first-year corn larval injury and found western corn rootworm adults in adjacent soybean fields in 1997 should consider using a soil insecticide in corn after soybeans in 1998. This recommendation will remain in effect until more complete explanations and economic thresholds can be determined.

Corn rootworm beetles generally deposit the vast majority of their eggs in cornfields. The larvae cannot survive on the roots of broadleaf crops (soybeans or alfalfa) or broadleaf weeds. Consequently, when a crop other than corn—soybeans, for example—is planted in a field with soil containing millions of rootworm eggs, the rootworm larvae die from starvation.

Soil Insecticide Application at Planting
Certain granular soil insecticides may be applied at planting time to prevent damage by corn rootworm larvae. The granules should be applied directly over the row in a 7-inch band ahead of the planter press wheel or firm-ing wheels and incorporated with spring tines or drag chains mounted behind the planter units. Some insecticides also may be applied in the seed furrow, but others are not labeled for in-furrow application because they either won’t provide adequate root protection or will cause seedling injury. Consult Tables 2 and 3 for recommended application rates and proper placement.

Some planting-time treatments applied in early April may provide only marginal root protection. Consider an application during cultivation in late May or early June in such fields, rather than a treatment at planting time. The rates suggested in Table 2 should not be exceeded for rootworm control. Research has shown that increasing the rates of soil insecticide applied does not improve the level of root protection. In-creasing the rate of application does not solve root-worm control problems and may accelerate the onset of resistance in the rootworm population. Proper calibration, placement, and incorporation of rootworm soil insecticides improves the likelihood of effective root protection. See "Calibration for Granular Soil Insecticides,".

Suggestions for Alternating Rootworm Soil Insecticides
Avoid using the same soil insecticide for several consecutive years. Continuous use of one insecticide may allow soil microorganisms to break it down more rapidly or may hasten the onset of insecticide resistance. Illinois entomologists encourage growers to alternate rootworm soil insecticides following these suggestions:

• If performance of a soil insecticide has been poor in a particular field in recent years, do not use the same insecticide in that field in 1998.
• Avoid using carbamates in consecutive years.
• Avoid using the same organophosphate or pyrethroid for several consecutive years.

Soil Insecticide Application at Cultivation
A soil insecticide may be applied during cultivation either as an alternative to a planting-time application or as a "rescue" treatment if the planting-time insecticide fails to protect the roots from rootworm larvae. In either case, you should dig up several plants in June to examine the roots and surrounding soil for rootworm larvae and injury. If you find three or more larvae per plant and the field was not treated at planting, a cultivator application may be warranted. Some soil insecticides may be applied only once per season (consult the proper label). If the field was treated at planting and rootworm larvae and damage are obvious in June, you might want to apply a cultivator treatment. "Obvious" rootworm injury is characterized by brown root tips and roots that have been tunneled in or chewed back toward the base of the plant.

The insecticide should be applied on both sides of the row at the base of the plants just ahead of the cul-tivator shovels. Cover the insecticides with soil. The best time to apply a basal treatment of a soil insecticide at cultivation is usually in late May or early June if evidence of rootworm feeding damage is noted.

Soil moisture may affect both application and effectiveness of cultivation-time treatments. Fields that are too wet may never be cultivated. On the other hand, the insecticide may not perform satisfactorily if the soil is too dry.

Control of Rootworm Beetles to Prevent Egg Laying
Properly timed sprays that prevent rootworm beetles from laying eggs may eliminate the need for a soil insecticide the following year. However, the procedure requires supervision by properly trained pest management personnel. Beetle migration and certain weather conditions may minimize the treatment’s effectiveness. Atochem has been very successful in encouraging growers to treat their fields with Penncap-M, not only to achieve a suppression of egg laying by corn rootworm adults but also to "pick up" control of other insect pests, like corn leaf aphids and European corn borers. Many consultants in northern and north-central Illinois have embraced Atochem’s marketing scheme and are urging clients to accept this approach for rootworm control. Our chief concern is that economic thresholds of corn leaf aphids, corn borers, and corn rootworm adults may never be reached in many fields. This approach to pest management will likely increase the frequency of unnecessary treatments and perhaps hasten the development of insecticide resistance. Recent research conducted in Nebraska suggests that some localized populations of rootworms may be developing resistance to methyl parathion, the active ingredient in Penncap-M.

Another product, called SLAM, may be applied for control of rootworm beetles in field corn, sweet corn, seed corn, and popcorn. SLAM contains the active ingredient carbaryl (13 percent), root powder of Cucurbita foetidissima (65 percent), and carriers (22 per-cent). SLAM does not contain an attractant for corn rootworm beetles. This product reduces beetle densities significantly and rapidly in treated fields; however, we have not seen any convincing evidence that SLAM protects the roots of next year’s corn crop. SLAM was used as the main insecticide during a 1997 areawide suppression program against corn rootworms. Preliminary reports suggest that although initial "knock-down" activity resulted from the use of this product, residual effectiveness was not evident. Further research will be conducted during 1998 to evaluate the potential of an areawide program for managing corn rootworm populations.

The prerequisites for a successful beetle suppression program are very complex. One must be able to identify both species (western and northern), distinguish between the sexes, and determine whether or not the females are gravid (have eggs present). Frequent scouting trips and precise scouting techniques also are required. For more specific information about this rootworm management alternative, contact the Extension entomologists at the University of Illinois (Information Resources). An Extension publication (MP63-C) titled Adult Corn Rootworm Management, from the University of Nebraska, should be a valuable resource for people interested in learning more about managing corn rootworms.

Control of rootworm beetles to prevent egg laying is an alternative to applying a soil insecticide at planting. Ideally, one properly timed spray should replace a soil insecticide. Unfortunately, some fields require two sprays to combat extended beetle emergence and egg laying. Two sprays or a spray plus a soil insecticide the following season may hasten the onset of rootworm resistance to insecticides.

Controlling rootworm beetles to prevent egg laying and controlling beetles to prevent silk clipping do not overlap in time. Most eggs are laid from mid-August through early September, well after the time when treatments to prevent silk clipping would be necessary.

An adult management approach to prevent egg laying by western corn rootworms in soybeans currently is not recommended. Until sampling strategies and economic thresholds can be developed, growers are encouraged not to attempt this strategy to prevent corn rootworm larval injury in corn planted after soybeans.

Summary: Planning Your Rootworm Management Program
A management plan for rootworms should be long-range (not a year at a time) and include crop rotation, possible use of insecticides, and scouting to determine the need for rootworm control. Soil insecticides applied at planting or cultivation time usually protect roots from severe rootworm larval damage. Alternating soil insecticides is encouraged. Aerially applied insecticides to control rootworm adults can be targeted to prevent significant egg laying, eliminating the need for a soil insecticide the following year. Consider the following for a long-range rootworm management program.

• Alternate corn with another crop when possible, particularly in fields where rootworm beetles averaged 0.75 or more per plant last summer, or if the soil insecticide did not protect the roots adequately in 1997.
• If you intend to grow corn after corn and if rootworm beetles averaged 0.75 or more per plant in corn after corn or 0.5 per plant in first-year corn last summer, apply a rootworm soil insecticide at planting time. Apply the rate suggested in Table 2, and consider our suggestions or alternating rootworm soil insecticides.
• Another alternative is controlling rootworm adults to prevent them from laying eggs. If the number of beetles reaches or exceeds 0.75 per plant, apply an insecticide when 10 percent of the females are gravid (with eggs). Continue to monitor fields weekly after treatment for rootworm beetles. A second application of an insecticide may be necessary if the number of beetles reaches or exceeds 0.5 per plant.
• Consider a cultivation-time application of a rootworm soil insecticide if you intend to plant in early April or if your planting-time insecticide does not provide adequate root protection.
• Scout for rootworm beetles from mid-July through early September 1998 to determine the potential for rootworm larval damage in 1999.

Scouting to Determine Rootworm Potential in 1999
The abundance of rootworm beetles in a cornfield during the summer is an indicator of potential rootworm problems the following year. You can determine the potential for rootworm damage in 1999 by counting western and northern corn rootworm beetles from mid-July through early September 1998 in this way:

1. Scout weekly between mid-July and early September in fields that might be planted to corn again in 1999.
2. Examine two plants selected at random in each of 25 areas of the field. Count all the western and northern corn rootworm beetles each time. The counts take about 45 minutes in a 40-acre field.
3. As you approach a plant, move quietly to avoid disturbing the beetles. Count the beetles on the entire plant, including the ear tip, tassel, leaf surface, and behind the leaf axils.
4. Record the number of beetles you find per plant. If the average is more than 0.75 beetle per plant in corn after corn or 0.5 beetle per plant in first-year corn for any sampling date, plan to rotate away from corn or apply a rootworm soil insecticide to corn in 1999. If populations do not exceed an average of 0.5 beetle per plant for any sampling date, a soil insecticide will not be needed the following season.

Thresholds for Corn Rootworm Beetles for Different Plant Populations and Cropping Sequences
Table 1 shows the thresholds for different plant populations and cropping sequences. If the beetle count exceeds these thresholds, management of next year’s larval population through crop rotation or insecticide application (corn following corn) is recommended.