By: Kristi Waterworth
There’s a belief among gardeners that the best corn you’ll ever have is plucked from the garden and immediately taken to the grill — kids on farms sometimes have races to see who can get the maple-honey sweet ears from the field to the cook first. Of course, being kids, they may not know to watch for corn rootworm injury, a potentially serious problem of corn stands large and small.
If you’re looking for corn rootworm information, you’ve come to the right place. Read on to learn more about the corn rootworm beetle and how to control it on your home-grown corn.
Corn rootworms are the larval stage of the corn rootworm beetle, a pollen-feeder that can cause considerable damage to corn and soybeans. These yellow-green beetles are elongated, measure up to about 5/16 inches in length and bear black stripes of various widths or spots on their wing covers.
Larval rootworms remain in the soil, feeding on the roots of maturing corn and soybeans. Sometimes, these pests tunnel into the root itself, causing them to turn brown, or chew them back to the crown of the plant. Occasionally, rootworms burrow into the crown of the plant as well. All this damage reduces available water and nutrients, causing the plant considerable stress as it’s trying to develop corn or soybeans.
Adults feed on corn silks, attracted by pollen shed. They often clip the silks, causing poor development of corn ears. Adult corn rootworm beetles also feed on foliage, stripping a single layer of tissue from affected leaves, and causing white, parchment-like areas of dead tissue to occur.
Control of corn rootworm beetle is difficult in the home garden, since many control methods are limited to commercial producers. But, if your corn stand is small, you can always handpick adults as soon as they appear on your silks and drop them into a bucket of soapy water. Check every day, carefully looking under each leaf as well as in silks. Hand-picking requires some determination, but if you can break the life cycle of the corn rootworms, you’ll have a better corn crop.
Crop rotation is very effective prevention, provided you don’t rotate with soy or other legumes. Corn rootworms in some areas have developed a taste for these wholesome beans and their cousins, so choose something significantly different to rotate with your corn. Tomatoes, cucumbers or onions may be better choices, depending on your garden configuration.
Planting early corn is another way many home gardeners avoid these pesky insects. Corn that pollinates from late April to mid May avoids trouble from the adult beetles, which emerge in late May or June.
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Western corn rootworm (CRW), Diabrotica virgifera virgifera LeConte, is one of the most damaging insect pests to U.S. agriculture with damage and control costs exceeding $1 billion annually. Historically, this insect species has been managed using soil insecticides, crop rotation and most recently, corn varieties that are genetically modified to contain a toxin to kill feeding corn rootworm larvae (CRW-GM). Currently, all of these technologies are failing in areas across the US corn production. While CRW populations are not economically damaging every year in the Northeast, the damage is economically significant in the years of moderate to high populations. As a result, farmers growing corn for multiple years in a field, use CRW management technology each year at planting to prevent potential losses.
With rapidly increasing CRW-GM seed costs, reduction in efficacy, continued eastward movement of the rotation resistant variant, low milk prices and increasing pressure on the milk producers by milk processers to produce GM free milk for public consumption, producers are looking for alternatives for rootworm control. On organic farms, biological control of soil insects (corn rootworm. wireworm) will decrease damage and increase yield.
In a 2014 research project, native NY EPNs were inoculated in a first-year corn field and continuously planted to corn through 2018. In 2016, two years after inoculation, an economic population of CRW larvae were present in the field plots with the untreated check plots suffering almost two nodes of roots destroyed, an economically damaging level. In the EPN treated plots (inoculated just once in 2014), EPNs protected corn roots from CRW larval feeding and plants only suffered 0.25 of a root node damage. This level of protection rivaled that provided by CRW-GM events planted in the same trial. Within this research project, economically damaging CRW populations did not materialize in 2017 or 2018 seasons. However, the EPN populations in the soil have remained high enough to respond to the next economically damaging outbreak of CRW larvae. An ongoing set of experiments in the Hudson Valley, NY under organic production indicate that persistent biocontrol nematodes also reduce root feeding damage from wireworms.
We anticipate the cost to inoculate corn fields in the Northeast will range between $50-100 per acre. Typical differential cost between GM and non-GM corn varieties is $50 per acre. Under organic production, the cost of biocontrol nematodes would be returned to the farmer through reduced damage from both wireworms and corn rootworm and increased yield.
Fifty farms in NY, VT, and PA will implement biological control on 500 acres of corn to manage corn rootworm and as a result will reduce production costs by $50 per acre by reducing corn rootworm management costs (using non-Bt-CRW corn seed or eliminate soil insecticide).
By Dr. Brent Tharp, Technical Product Manager
Selecting and placing hybrids is one of the most important decisions corn growers make each year. Successful hybrid selection starts with knowledge of the past pest and disease pressure in your fields. Specifically, corn rootworms (CRW) pose a significant threat to yield and profit, making it a pest that cannot be ignored. Understanding the level of risk from corn rootworm in your area and specific fields will help you choose the best hybrids to lower risk and maximize profit.
Corn rootworm pressure, and the risk of rootworm feeding, is very hard to predict from year to year. The risk of significant rootworm feeding depends on many factors, some of which are hard to measure but there are actions a corn grower can and should take to better understand the risk in their specific fields. Past field history is one predictor of future risk, and continuous corn fields are typically at a higher risk than first year corn fields.
The best way to predict rootworm risk the following year is to monitor corn rootworm beetle activity during the growing season. In order to better understand the rootworm pressure in our marketing area, and to help our customers make better decisions, Wyffels has conducted a rootworm beetle monitoring program the last seven years using yellow sticky traps.
Traps were placed in over 875 fields across our marketing geography, replaced each week, and the number of beetles on each trap was recorded. The number of beetles in each trap was averaged across the field and then divided by the number of days the trap was deployed to result in the number of beetles per trap per day (beetles/trap/day) in each field.
University and extension research has established thresholds for the number of beetles/trap/day that would lead to significant damage the following year. According to Iowa State University, fields that recorded peak beetle activity at or above 2 beetles/trap/day are at a high risk of damage the following year. While it’s hard to know exactly what level of beetle activity will lead to damage the following year, this monitoring provides a guide of beetle activity to better assign risk of rootworm damage the following year.
In 2020, Wyffels monitored 877 fields for adult rootworm activity. Traps were placed in corn fields to monitor northern and western rootworm beetle levels in continuous corn production and in soybean fields to measure the population of the western corn rootworm variant that can affect corn planted after soybeans.
The bar chart below shows peak beetle activity across these crops. In soybeans, 91% of the fields had very little beetle activity with 33% of fields never capturing a beetle. In first-year corn fields, 91% had very little activity and 35% had zero beetles. Beetle activity in continuous corn fields was much more robust, with 35% of fields over threshold and another 11% near threshold.
The following maps illustrate where beetle activity occurred and can serve as a guide to better understand risk in your area. Black symbols are fields with no beetle activity, green symbols represent minimal activity, orange symbols are fields where peak activity was below but near threshold, while red symbols are fields with above threshold activity. Areas with orange and red symbols indicate increased beetle activity and carry more risk of rootworm damage next year.
Beetle activity in soybean fields was low in many areas this year. In these areas, non-GMO or VT Double Pro® hybrids with or without a soil insecticide could be viable options for first year corn fields in 2021. However, beetle activity was higher in parts of NW Illinois, SW Wisconsin, and NE Iowa signifying more risk of rootworm damage next year in first year corn. If using a non-rootworm traited hybrid in these areas, be sure to use a soil insecticide or plant a SmartStax® hybrid.
In corn fields, beetle activity varied and was randomly distributed across our marketing area. We do not recommend planting an unprotected hybrid in continuous corn, especially long-term continuous corn. Planting a SmartStax hybrid or a VT Double Pro hybrid with insecticide is recommended on second-year corn acres in 2021. Fields with multiple years of continuous corn naturally pose a higher risk than a second-year corn field, so planting a SmartStax hybrid is recommended in fields with 3 or more years of continuous corn.
If you’re in an area that has shown high CRW pressure in continuous corn situations, or you’ve had CRW damage in your fields, the best option is to plant a non-host crop, like soybeans, to break the life cycle of the CRW beetles and reduce pressure. It’s not uncommon to have above average soybean production in fields after long-term continuous corn, so you’ll benefit from that, and break the cycle to reduce CRW populations. If you must plant corn, you should consider other management practices like adult beetle control during the growing season. In very high corn rootworm pressure, traits alone will not be able to provide adequate protection.
With lower commodity prices, some corn growers may consider selecting hybrids without rootworm protection to lower input costs. However, selecting non-RW traited hybrids without monitoring pest pressure to understand risk of CRW can expose growers to risk of yield loss and standability issues, costing significantly more than is saved on seed costs. Understanding the risk of corn rootworm damage in your specific fields will help you make the best management decisions to manage risk and maximize profit in 2021.
Stunted, yellow cornstalks that are prone to toppling over are the most serious damage caused by corn rootworms.
These beetle larvae are 1/2?3/4″ white grubs that tunnel into the roots of corn plants to feed.
Adult beetles also feed on corn silks, pollen, tassels, and occasionally, leaves. One type of rootworm beetle may target the flowers and leaves of squash-family plants and beans as well, especially in late summer. Corn rootworms are most likely to be a problem in gardens where corn has grown for two seasons or more.
There are three different species of this pest:
We have three species of rootworms on New Hampshire corn. Their effects can sometimes be serious enough to require action. In states farther west or south, that might routinely include an insecticide treatment. In New Hampshire, the need for insecticide treatment is rare. Field corn is more commonly affected than sweet corn, since field corn is not often rotated, nor treated with insecticide, as opposed to sweet corn plantings that are more commonly rotated and treated with insecticides for pests such as earworms and European corn borers. Field corn that is chopped for silage is more seriously affected by rootworms than field corn grown for grain, which is common further south.
All three species have similar life histories with a single generation per year. Females lay eggs in the soil in late summer, near the bases of corn plants. They prefer to lay their eggs in cracks and crevices in the soil. The eggs overwinter in the soil and hatch in early summer. Most just-hatched larvae die if no corn roots are near them. However, if a field is again planted to corn, then many of the larvae survive, chew on fine roots, and bore inside larger roots. By late July or August, the larvae have reached full size and pupate in the soil. The adult beetles emerge in August, feed on the silks and leaves, and mate. Soon after mating, the females lay eggs in the soil.
Northern Corn Rootworm (Diabrotica barberi) is the most common of our three species. It was first collected in New Hampshire in 1968, and now lives throughout the state. It is usually tan or green in color, lighter when first emerged. Like the other species, adults are about 1/4" long. In late summer it is frequently found on the silks or in the ear zone.
Southern Corn Rootworm (Diabrotica undecimpunctata) is the least common rootworm in New Hampshire. Also called the spotted cucumber beetle, it is yellow-green with 11 or 12 large black spots. It is so uncommon that it is not of much concern. Out of the 260 rootworm sticky traps we set across the state in summer of 2000, we caught only one southern corn rootworm!
Western Corn Rootworm (Diabrotica virgifera) is the most destructive of the three species, by far. It is also the most recent one to invade New Hampshire. We discovered our first specimens in 1991. Males of this species are yellow with three lengthwise black stripes. Females show more black than the males, and the stripes are often merged together. In addition to the larval root chewing, adults of this species feed extensively on silks and foliage.
Plants that have severe root injury will often partially fall over (lodge), creating a goose necked form. Attacked plants can be stunted and have much lower yield. (Also, at harvest time the cutter misses that portion of the lodged stalk that is laying on the ground.) Sometimes the fresh silks can be severely chewed, and this can affect pollination. In the corn belt, western corn rootworms are sometimes in high enough numbers that the leaves can appear ragged from chewing.
Rootworms are least abundant in the far north and the mountains, where northern corn rootworm is usually the only species found. The highest rootworm populations seem to be where lots of corn is grown, and not rotated. In our 2000 survey, we found high populations in particular fields in Hollis and Haverhill. The previous year, hot spots were in Hollis, Antrim, Plainfield, Lebanon, and Hudson, and so, local conditions affect buildup. Research in New York showed that soil type plays a role in rootworm numbers. Soils that are poorly drained tend to have lower rootworm numbers, compared to well-drained sites. This may be due to poorer survival of the larvae or pupae under wet conditions. The effect disappears in dry years.
Western corn rootworm is most likely to be encountered in Grafton, Hillsborough, Cheshire, or Sullivan counties. It has not yet been confirmed in Carroll County, but this is probably because we have not searched intensively there.
Southern corn rootworm has so far been reported (in very low numbers) in Strafford, Rockingham, Grafton, and Hillsborough counties. It probably occurs across New Hampshire, south of the White Mountains.
The standard method for monitoring rootworms involve counting the beetles in a field in late summer to predict potential problems next year, in the same field. The following method was devised by Integrated Pest Management (IPM) workers in New York. This technique (called a sequential sampling method) minimizes the amount of effort you need, while maximizing reliability of the information. Table 1 is set for an action threshold of 2.5 rootworms per plant. Fields with sandy soils may be able to tolerate higher numbers than this. More detail is in the fact sheet by Shields et al., listed in the acknowledgments. It is unlikely that monitoring will be needed if you grow corn that is genetically modified to resist rootworm attack.
Two weeks after pollination, cross the field, stopping at several places to count the number of rootworm adults that you find per plant. Pay particular attention to the silks and ear zone. The beetles often are partly hidden in the silk, or hide in cracks between the ear and stalk, or leaf and ear. In hot weather, some beetles may fly away as you approach each plant try to include those in your count. You can keep track of the plants you count and the score (“running total”) on a small pad of paper.
The first time or two that you use the method, you may prefer to write down 4 columns, as in Table 2: Plant Number, Number of Northern Rootworms, Number of Western Rootworms, and Running Total. Running total is not a simple sum of the number of rootworms you find. You count each northern corn rootworm as one half, and each western corn rootworm as one. For each new stalk, you add the score for that plant to the running total for the previous plant, so “Running Total” is a cumulative value. You keep moving to new stalks and count, until the running total either 1) falls below the “too low” value or 2) exceeds the “threshold reached” figure. That is when you stop. If you get all the way through the table (to plant #55) and the running total still has not reached either the “too low” or “threshold reached” level, then this is a borderline case, where it might be worthwhile to treat, or it might not. In such a situation, sampling again a week later may help. This sampling method is designed to require very few samples in fields with very high or very low rootworm populations, so the actual number of stalks you count depends on the number of rootworms you find.
For the last several years, we have been monitoring rootworm numbers with the use of Trece brand unbaited yellow sticky traps. This program is designed to give us information on rootworm levels over a wide area, because the method described above is impractical to simultaneously check rootworm numbers in dozens of fields in 6 counties! The sticky traps are placed around corn stalks in the ear zone, when silks have appeared. Ten to 14 days later the traps are examined, and the beetles are counted. This method can give us a very rough idea of the rootworm population, and we are tentatively using 100 beetles per trap as the threshold to consider some form of treatment for the next year.
In August 2000, we placed traps (most fields had two) in 129 fields in Coos, Grafton, Sullivan, Cheshire, Merrimack, and Hillsborough counties. The overall average catch was 10.5 rootworms per trap (8.6 Northern, 1.9 Westerns). Only 3 fields reached our rough threshold of 100 rootworms per trap. Compare these results with those from four years of trapping in Ohio, a state with significant rootworm populations: using the same yellow sticky traps, hung in the same way, an average of 114 rootworms were captured per trap in first year corn, and 289 per trap in non-rotated corn. This underlines the inappropriateness of automatically treating our fields with rootworm insecticides, or relying on pesticide advertising that is targeted for the corn belt.
• Cultural Practices -The simplest and least expensive control method is to rotate fields. Corn that is grown in soil that grew some other crop last year is unlikely to suffer any root injury from rootworms. The major disadvantage of rotation is the lack of rotational crops that can produce a similar yield of energy for livestock. You may wish to delay planting fields that you plan to rotate next year. Later planted fields often attract rootworm adults from adjacent plantings (they like the younger plants and fresh silk). They will lay plenty of eggs in this later planted field, but since it is going to be rotated to another crop next year, the eggs will die.
• Biological Control - You can purchase insect-attacking nematodes to control rootworm larvae, but this approach is currently far too expensive.
• Genetic Control - Many acres of field corn grown in New Hampshire in 2016 have been genetically modified to resist injury from rootworms. If you are growing these varieties, there is no need to apply a rootworm insecticide, and probably limited need to monitor for rootworms.
• Chemical Control - For situations where rotation is impractical, the corn variety is not genetically modified to resist rootworm attack, and rootworm populations are high enough, insecticide treatment may be useful. In the corn belt, rootworm damage is a much greater risk than here in New Hampshire, and insecticides are more often used. Generally, insecticides are applied to the soil (usually in granular form) to kill the young larvae. Some labels instruct to do this at planting time others say after planting.
Insecticides should only be applied if you are certain that populations are high enough to pose a risk. (See comments on thresholds above.) Soil insecticide use carries a risk of acute toxicity to the applicator and possibly other farm workers and livestock. Application to soil may also carry a significant risk of contaminating groundwater, and a risk to surface waters if heavy rainfall comes right after treatment.
Soil insecticides are also highly destructive to insect predators that live on or in the soil, such as ground beetles and predator mites. Granular forms of several insecticides are so dangerous to small birds, that their registrations have been canceled or severely limited in the U.S. (The birds apparently mistake granules for seeds, and die after eating only a few.) Repeated use of the same pesticide may cause rootworms to become resistant. Lastly, consider the cost, which could be $30 or more per acre. Adding up the various risks and costs, applying insecticide to a field where it is not needed can be a serious mistake.
For the 2001 version of this fact sheet, Keith Waldron provided suggestions and assistance. The sequential sampling chart is from Cornell Cooperative Extension Field Crops Insect fact sheet 501 by Elson Shields, Roger Sher, Paul Taylor and Phillip Sutton. George Hamilton reviewed the manuscript and provided considerable assistance with rootworm surveys. Tom Buob also did much work on the surveys, and was joined by Steve Turaj, David Seavey, and Bruce Clement. Part of the funding for the rootworm survey came from an IPM grant from the New Hampshire Department of Agriculture, Markets & Food. Carl Majewski made valuable suggestions for the 2016 re-formatted version.
Stop! Read the label on every pesticide container each time before using the material. Pesticides must be applied only as directed on the label to be in compliance with the law. All pesticides listed in this publication are contingent upon continued registration. Contact the Division of Pesticide Control at (603) 271-3550 to check registration status. Dispose of empty containers safely, according to New Hampshire regulations.
Download the Resource for the complete fact sheet and a printable version.
The best way to deal with corn rootworms is to prevent it. One of the best things that can be done is crop rotation. The right planting time is also a preventive measure that will work. As much as possible, plant the crop early. It will be more attractive to the pest if you plant late. The application of post-emergent insecticide will also be helpful. Furadan 4F is one of the best brands to consider. However, it is critical to choose the right time for its application. Spraying it too early will cause leaching and when too late, on the other hand, it will lack effectiveness.
Both corn rootworm larvae and adults are capable of damaging corn plants. Larvae begin by feeding on root hairs and outer tissue before burrowing into the roots, causing the roots to appear brown. This larval root pruning reduces water and nutrient uptake of the plants, and significant root lodging and yield reductions may occur as a result.
Corn rootworm beetle populations primarily feed on pollen, clipping silks during pollination and causing poorly filled ears. The western corn rootworm beetle is also known to feed on the leaves of the corn plant, but this isn’t particularly damaging unless there are unusually high numbers of the pest.
While considered only a minor threat to crops such as soybean and alfalfa, the western corn rootworm variant can lay eggs in these crops which could result in potential damage to next year’s corn roots if rotating to corn.