Corn Rootworms

Western Corn Rootworm, Diabrotica virgifera virgifera LeConte

Northern Corn Rootworm, Diabrotica barberi Smith & Lawrence

Southern Corn Rootworm, Diabrotica undecimpunctata howardi Barber

Appearance and Life History

Corn rootworms are important insect pest of corn in the Midwest. Two species of rootworms that may cause severe damage to corn as both larvae and adults, the western and northern corn rootworms. Southern corn rootworm adults may damage corn leaves, however, because they cannot overwinter in most areas of the Midwest, southern corn rootworm larvae do not present a major threat to corn in this region.

Both sexes of western corn rootworm adults are yellow to green in color with a black stripe along the sides of their wing covers and are about 5/16 inch (7.5 mm) long. However, male wing covers are often nearly entirely black or at least darker in pigmentation than that of the female, which usually appears as more regular stripes. Slight variations in these color patterns may occur. Female western corn rootworm adults have larger abdomens than the males.

The adult of the northern corn rootworm is a tan to pale green beetle about 1/4 inch (6 mm) long. Newly emerged beetles are usually cream or light brown in color, but gradually turn green with age. No marked differences in coloration exist between sexes, but female beetles with their longer and larger abdomens are typically larger than the males.

The adult of the southern corn rootworm, also known as the spotted cucumber beetle, is about 3/8 inch (9 mm) long, yellow to green in color, with 11 conspicuous black spots on its back. Because adults and larvae of this species are considered of minor economic importance in the Midwest, the following discussion on damage, sampling, and management guidelines will not include the southern corn rootworm.

It is extremely difficult to distinguish between larvae of the western and northern species. Rootworm larvae are white and slender, about 1/2 inch (13 mm) long when fully grown, have brown heads, and a dark plate on the top side of their “tails.”

Western and northern corn rootworm have only one generation per year. Eggs of both species are deposited in the soil by female beetles from mid-summer until autumn. The eggs overwinter and begin hatching from late May to early June in most areas of the Midwest. The newly hatched larvae seek out and feed on corn roots. The larvae pass through three stages, or instars, before pupating in the soil. Primary larval damage is caused by the later instars, typically from mid June through mid July.

By late June to early July, adult beetles begin to emerge from corn fields. The males emerge first, the females following in about 5 to 7 days. Mating takes place a short time thereafter. Within 2 weeks of emergence, females begin to lay eggs, usually toward the end of July, with peak egg laying occurring in early to mid August. The beetles in corn fields feed primarily on pollen, green silks, or leaves (see information below concerning western corn rootworm in soybean).

Several factors influence the development of a rootworm population in a particular field, and these should be considered when scouting. Rootworm beetles can readily move between fields and may cause damage in locations other than where they emerged. A late maturing corn field can attract large numbers of beetles since neighboring corn may have stopped producing pollen, the beetles’ preferred food. Beetles may also move into corn and/or soybean fields which have an abundance of pollen-producing weeds, such as volunteer corn, ragweed, or foxtails. Some areas of the eastern Midwest have observed a steady and substantial increase of economic larval damage in first-year corn (corn following another crop). A behavioral variant of the western corn rootworm, that appears to have lost its fidelity for corn, can migrate to neighboring crops such as soybean to feed on pollen, flowers, and foliage, then lay eggs.

Where northern corn rootworm is the dominant species, a trait referred to as “extended diapause” can affect the survivability of the insect. Extended diapause describes a condition whereby a portion of northern corn rootworm eggs are capable of remaining dormant in the soil through two winters and one growing season before hatching in the second season. First-year corn in areas of Minnesota, Iowa, South Dakota, and Wisconsin have been damaged by rootworms exhibiting this extended diapause trait. The western corn rootworm does not show this trait.

Soil texture is an important factor in regard to larval survival. Newly hatched rootworm larvae must move through the soil in search of corn roots and, as they move, they come in contact with soil particles. These particles, if coarse and abrasive, may scratch the cuticle of the larvae, resulting in death by desiccation. As a result, rootworms may be less of a problem in sandy soils. However, the effect of sandy soils can be modified by soil moisture. An irrigated or otherwise moist sandy soil will not affect rootworm populations as will the same soil under dry conditions. In addition, muck soils have shown a lower incidence of rootworm larval feeding damage and may provide some protection against this insect.

Damage

Both corn rootworm larvae and adults may damage corn plants. Newly hatched larvae feed primarily on root hairs and outer root tissue. As larvae grow and their food requirements increase, they burrow into the roots to feed. Larval damage is usually most severe after the secondary root system is well established and brace roots are developing. Root tips will appear brown and are often tunneled into and chewed back to the base of the plant. Larvae may be found tunneling into larger roots and occasionally in the plant crown.

By reducing water and nutrient uptake of plants, larval root pruning places severe physiological stress on corn. Yield reductions may result, especially in corn also suffering from moisture, compaction, or fertility stress. Roots damaged by rootworms may be weakened to the extent that the plants lodge or grow in a curved “sled-runner” or “gooseneck” shape. These lodged and misshapen plants often pollinate poorly and are difficult to harvest, contributing to yield losses. Also, rootworm damage sites are often pathways for infection by pathogens. Thus, rootworm damage may be compounded by root and/or stalk rots.

High adult rootworm populations may interfere with corn plant pollination by severely clipping silks during pollen shed. This can result in poorly filled ears.

Western corn rootworm beetles can also cause damage by feeding on the green tissue of the leaves. Leaves may be partially or totally stripped by adults, resulting in a parchment-like appearance to the damaged area. Unless these beetles are unusually abundant, only scattered plants are severely damaged. However, such foliar damage can be indicative of a large adult population which may be a serious threat to pollination and next season’s corn roots.

Though considered of minor importance in other crops such as soybean and alfalfa, the variant of the western corn rootworm can feed on pollen, flowers, and foliage in these other crops. The presence of beetles and the extent of damage to crops such as soybean may indicate the potential of economic damage to next year’s corn roots, if rotating to corn.

Sampling Method

Larvae in Corn – It will not be necessary to survey all corn fields for corn rootworm larvae. Sample those corn fields with suspected insecticide failures or fields in which a high adult rootworm population was observed, the previous July-August. Also sample corn fields that were in other crops like soybean the previous season, if beetles were observed or suspected as being present in those crops the previous year.

Randomly select 1 plant in each of at least 10 areas of a field. Using a spade or shovel, cut a 7 inch (18 cm) cube of soil around the base of each plant, making certain that the blade of the tool enters the ground vertically to avoid cutting roots. Lift the plant and soil out of the ground and place them on a small piece of dark canvas or plastic. Slowly break the soil away from the roots and carefully examine the soil and roots for larvae. The dark background will make it easier to find any of the small white rootworms. The soil and root sample can also be washed in a pail of water to extract the rootworms. The rootworms will float to the top and can be counted. The addition of salt to the water will float the worms to the top more easily.

Record the number of larvae found on each plant examined, repeating the procedure for each of the plants sampled. Determine the average number of rootworm larvae per plant by dividing the total number of larvae found in the field by the number of plants examined. Also note whether the numbers were the result of hand sorting or washing the soil and roots.

Adults in Corn – Surveys for rootworm beetles should begin shortly before corn silking commences. Several factors must be considered when planning adult rootworm scouting activities. Rootworm beetles are extremely active and can be readily observed feeding, mating, and flying about an infested field. However, such activity varies considerably with the time of day, and such behavior patterns can greatly influence field counts. Beetles are most active in the morning and late afternoon, limiting their activity during the normally higher temperatures of mid-day. Thus, to get the best indication of true population levels, sample in the morning or late afternoon.

Also, corn hybrids, even of the same maturity, vary to some degree in their time of pollination. Therefore, if a particular field has been planted to more than one hybrid (not blended together), each hybrid must be scouted separately.

To determine if rootworm adult sampling is necessary in a particular field once silking has or is about to begin, quickly walk across the field. If no beetles are observed in the ear zone area or on the ears, no further scouting will be necessary at this time. However, return to the field in 2 to 3 days to again check for beetles. Even if no beetles are found, you should check a field twice weekly until the silks have been pollinated and then once weekly until early September for egg laying activity. If you do see beetles in the field, definite counts, using one of the two following scouting methods, should be made immediately.

1. Determining Potential for Silk Damage and Interference With Pollination
   Randomly select and check 5 plants in at least 5 areas (25 total plants) of a field for silk clipping. If silk clipping is noted, determine (in inches) what length of the silk remains on each plant sampled. Determine the average silk length per plant by totaling the length for all plants sampled and dividing by 25.

   Record the number of beetles noted for each plant and determine the field average. Also, estimate and record the percent of pollination that has taken place. This can be done by carefully unwrapping the husk leaves from an ear and then gently shaking the ear. The silks from fertilized ovules will readily drop off the ear. As well, note if pollen is being shed by inspecting the tassels of each plant sampled. Remember that weather can greatly affect whether pollen in being shed. Record the results.

2. Determining Potential for Larval Problem in Next Year’s Corn
   Two randomly selected plants in each of at least 20 areas of the field should be checked. As you carefully approach each plant, watch for and record any rootworm beetles that drop from the plant or fly away. Grasp the silks in one hand to keep any beetles from leaving them while you inspect the remainder of the plant. After sampling the whole plant, slowly open your hand and count the number of beetles on or within the silks. Examine the silks thoroughly since beetles may be deep in the ear tip area. If there is a second ear on the plant, look through its silk. Repeat the procedure for the second plant that is within 10 feet of the first plant. After sampling all plants, determine the average number of beetles per plant. A face shield, or goggles, will protect your eyes from leaves and pollen while scouting corn fields for beetles.

Adults in Soybean and Some Other Crops – Two sampling methods, sweeping and sticky traps, can be used in soybean and other non-corn crops to determine the potential for western corn rootworm larval damage to next year’s corn roots. Both methods need to be implemented and carried out from the end of July through the first week in September.

1. Sweeping
   Sweeping soybean (refer to page C-78), and other crops like alfalfa, requires the use of a 15 inch (38.1 cm) diameter sweep net. While walking the field in a “M” pattern, take 20 continuous sweeps in 5 different areas of a field (keeping the net closed between the 5 areas). It is important to take sweeps from areas that represent differences in topography, i.e., high ground, low ground, near waterways, wetter areas, dryer areas, dark soil, light soil, etc. Areas of a field within approximately 50 feet (15.2 m) of a corn field(s) should be sampled separately since a natural “spillover” of beetles can often be observed in these areas. A sweep consists of one quick movement of a sweep net through the foliage from one side of the body to the other with the top of the net slightly above the top of the foliage. In soybean, the length of the sweep covers an area equivalent to about two 30-inch (76.2 cm) rows. As each sweep is completed, the net is reversed and brought down through the foliage from the opposite direction as one walks at a normal pace. Once all 100 sweeps have been taken, either immediately count and record the number of western corn rootworm beetles in the net or place the contents of the net in a plastic bag and freeze until beetle counts can be made. Sweep samples should be taken on a weekly basis for 6 weeks.
2. Sticky Traps
   Pherocon AM® yellow sticky traps placed on stakes throughout soybean and/or other fields like alfalfa is a passive sampling method for determining western corn rootworm beetle numbers. There are no volatile attractants on these cards. The beetles are attracted to the bright yellow color and then become entangled in the sticky substance on the surface of the cards. A minimum of six traps are placed in different locations of a field and allowed to remain for a week before removal and counting; for ease of collecting traps in drilled soybeans, consider placing them along wheel tracks, skipped rows, etc. The traps are positioned on stakes or posts so that the traps show just above the crop canopy. Each week, for 6 weeks, the old traps are removed, new traps installed, and the stakes or posts are adjusted so that the traps are just above the growing crop. The captured beetles are counted and the numbers recorded weekly.

Management Guidelines

Corn Insect Control Recommendations: 

For years, crop rotation has been a major management strategy for corn rootworm. As a general rule, rootworms cannot successfully complete their larval development on crops other than corn. Thus, if rootworm eggs are laid in a corn field one season and a soybean field the next season, the young larvae find themselves without a suitable host plant and soon starve to death in the soybean field. However, where the western corn rootworm variant or the northern corn rootworm extended diapause trait exists in the Midwest, crop rotation by itself is no longer an effective management strategy in all regions. Sampling for larvae and adults, as described previously, is imperative to apply the following management guidelines.

Larvae in Corn

1. Hand-Sorting of Soil
   Two or more rootworm larvae/plant prior to lay-by may signal the need for rootworm larval control.
2. Washing the Roots and Flotation
   Eight or more rootworm larvae/plant prior to lay-by may signal the need for rootworm larval control.

Adults in Corn

1. For Prevention of Silk Clipping and Interference with Pollination
2. Rootworm Larval Prediction for Next Year’s Corn
   Prediction of next year’s larval problem, for those fields returning to corn, is based on the current year’s average number of beetles per plant during late July through August.

• a) Continuous Corn – Beetle counts in fields planted to corn for two (2) or more years. Next season rotate to another crop or treat for rootworm larvae at cultivation or planting, if the average beetle count per plant equals or exceeds the number listed below for your average number of plants per acre.

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• b) First-Year Corn – Beetle counts in first-year corn fields. Next season rotate to another crop or treat for rootworm larvae at cultivation or planting if the average beetle count per plant equals or exceeds the number listed below for your average number of plants per acre.

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• c) Sequential Sampling – This sampling method, modified from the above, is based on sampling only enough beetles to determine the likelihood of an economic larval population developing in next year’s corn, if going back to corn. Pairs of plants should be sampled as one walks a field in a U- shaped pattern. If fewer than 3 beetles are found on the first 10 plants, sampling may be terminated until the next week. If 3 to 17 beetles can be found on the first 10 plants, one should continue sampling. If more than 17 beetles, one may cease sampling since an economic population is present. If 54 plants have been sampled and counts are still in the intermediate range, stop sampling and return to the field and repeat the sampling procedure in one week. Weekly scouting should continue until an economic population is present or until the numbers of beetles decline in late August or early September.

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Adults in Soybean and Other Crops – Data is not available for the exact number of western corn rootworm beetles needed in soybean or other crops to cause economic larval damage in corn the following year. Consult with university researchers in the Midwest for new developments on the establishment of an economic threshold(s). Finding “numerous” western rootworm beetles versus a “few” in these crops during the sampling period may help one determine whether an economic problem in the following year’s corn crop may occur.