Northern and western corn rootworms are native to North America. Populations of the northern corn rootworm, first recorded in 1824, were confined to the north central United States until the mid-1950's. During the 1960's, the range of this species expanded into Pennsylvania. Western corn rootworm was first recorded in 1868 in Kansas and, by the mid-1950's, had spread only to Nebraska and portions of Colorado, South Dakota, and Iowa. For reasons unknown, population distribution greatly expanded between 1955 and 1970. Not until 1980, however, was western corn rootworm first observed in Pennsylvania. Since then, it has spread throughout the state and is now the predominant rootworm species in western Pennsylvania.
Corn rootworms have long been the major economic pests of corn in the United States. In Pennsylvania, corn rootworm damage has been sporadic. Over the past several years, however, the invasion of western corn rootworms and the expansion of northern corn rootworm populations in the state have greatly increased the potential for economic losses.
The influx of western corn rootworm into Pennsylvania has necessitated development of a management guide that encompasses both rootworm species. The objective of this publication is to inform corn producers, field scouts, and county extension agents of the biology of both rootworm species, ways to identify the species and the damage they inflict, as well as scouting methods, economic injury levels, and control strategies.
Life Cycle of the Corn Rootworm
Figure 1 depicts the life cycle of the corn rootworm. Both species of corn rootworm complete one generation per growing season. Adult corn rootworms begin emerging between mid-July and mid-August, depending on weather conditions during their summer developmental period. Adult emergence continues for four to six weeks. Adults of western corn rootworm usually begin to emerge five to seven days before those of northern corn rootworm. After emergence, the eggs of adult females mature within about two weeks. Once a female begins laying, she deposits 75 percent of her eggs over a period of 30 to 35 days. A female rootworm may live as long as 80 days and deposit more than 1,000 eggs during her life. She deposits her eggs mainly during August and September.
Both species of rootworm prefer to deposit their eggs in cornfields. Females deposit most eggs near the base of corn plants, within the top six inches of the soil surface. (Western corn rootworm females may deposit their eggs throughout the field.) Small holes and cracks in the soil give the females access to areas below the soil surface.
Once corn rootworm eggs are deposited, they remain unhatched until spring. The eggs of both species must be exposed to a period of cold before larvae can hatch. This physiological requirement is known as diapause. Any larvae that hatch in the fall die along with adults from the cold winter temperatures. Larvae begin to hatch from overwintering eggs during June and feed on corn roots. Depending on spring temperatures, eggs may begin to hatch anytime from early to late June. Western corn rootworm larvae tend to hatch five to seven days before northern corn rootworm larvae. The larval stage lasts from four to six weeks, after which pupation occurs. Adults begin to emerge five to ten days after pupation and feed on corn silks. Northern corn rootworm adults can be found feeding on the flowers of a number of plants, but they deposit their eggs primarily in cornfields.
The adult is the only developmental form of northern and western corn rootworm that can be distinguished visually without the aid of a microscope (Figure 2). Adult northern corn rootworm beetles are only about ¼ inch long and are solid yellowish green (tan when newly emerged). Western corn rootworm beetles are about the same size and color, but they have three dark stripes on their wing covers. On some beetles the stripes overlap, making the wings appear dark brown or black. Adults are very active and fly rapidly if disturbed.
The spherical, whitish or yellowish eggs are deposited in the soil and are almost impossible to see. Larvae of both species are white, measure about 1/8 to 1/2 inch long, and have brown heads, six small forelegs, and slightly wrinkled skin (Figure 3). Larvae also have a dark plate on the top side of the last abdominal or tail segment. The pupae are white, but otherwise similar in appearance to the adult.
Figure 2. Adult Western (left) and Northern (right) Corn Rootworm
Figure 3. Corn Rootworm Larva.
In Pennsylvania, western corn rootworm is now predominates across the majority of the state. The northern corn rootworm out numbers the western in some of the cooler and more northern regions of the state. Relative proportions of each species vary from site to site and between years at a site.
Both rootworm species can cause damage in their larval and adult stages. Newly hatched larvae feed on root hairs and outer root tissues, but also tunnel into the soft root tissue. Larger larvae tunnel into roots and occasionally into the plant crown. Tunnel openings are visible on the roots, and root tips appear brown and chewed back.
Removal of root tissues restricts the water and nutrient uptake of the corn plant. Soil moisture largely determines the extent of rootworm damage to corn plants. Under moist soil conditions, most corn plants can compensate for rootworm damage by rapidly growing new root tissue. As long as the plants do not blow over during immediately after the roots are pruned, they typically will not experience significant yield reductions. Dry conditions restrict the plant's ability to compensate for root pruning. No-till fields with soil compaction problems may be stressed even more severely by rootworm damage, because the root system is restricted to the upper six inches of soil where rootworm numbers are highest and the soil in more likely to dry out.
Severe root pruning of small plants (four to six leaves) can kill the plant. Larger, more vigorously growing plants typically produce new root growth faster than larvae can prune the roots. At high rootworm densities, root pruning may be severe enough to cause plant lodging and reduce yields. Lodging occurs during the vegetative stages of corn development, so lodged plants continue to grow, orienting themselves toward the sun. As a result of this growth habit, the plant eventually looks "goosenecked." By the time "goosenecking" is evident, root pruning is nearly complete. Because western corn rootworm larvae tend to hatch earlier in the season and feed more vigorously than northern corn rootworm larvae, they are more likely to cause severe damage to corn roots.
Adult rootworms feed on corn silks by clipping them off, sometimes before pollination. If the corn silks are not clipped back inside the husk and at least 1 inch of silk is extended beyond the tip of the husk, the kernels can still be fertilized. When clipped, the silks continue to grow. Therefore, as long as the rate of silk growth exceeds that of silk clipping, adult feeding does not interfere with pollination. Severe silk clipping during pollen shed, however, results in barren or poorly filled ears. Poor pollination from silk clipping is more likely to occur when plants are under drought stress. Once the silks turn brown, severe clipping does not affect yield, since browning indicates that pollination is complete.
Scouting is the procedure recommended for monitoring pest population densities. An inadequate scouting procedure can lead to improper management decisions. For this reason, it is important that corn producers develop a good scouting program that is as accurate as possible.
Counts of corn rootworm beetles in cornfields during August help corn producers decide which management alternative will be appropriate the following spring. Sampling the egg and larval stages has proven too time-consuming and costly to be economically feasible. On the other hand, August beetle counts do allow a longer period of time for errors to build up as a result of differential mortality. Therefore, a tradeoff between sampling cost and accuracy is necessary.
The scouting procedure described in this section minimizes the chances of inaccurately estimating corn rootworm beetle populations in field corn. Corn producers must bear in mind, though, that no population estimate is 100 percent accurate. Walking into a corn field and counting the number of rootworm beetles on the first five plants you see is not an accurate way to estimate rootworm numbers. Like cattle grazing in a pasture, rootworm beetles are not evenly distributed throughout the field. Individual adults have a tendency to group together, so some areas in a field may have higher numbers than others. To compensate for the chance of sampling in a "hot spot" or in an area with very few beetles, sampling should be done throughout the field. Research has shown, however, that a completely random sample is unnecessary.
Sampling 40 sets of two plants using an "M" or a "W" sampling pattern is currently recommended for up to 40 acres of corn. In fields where this pattern is not feasible, a representative sample from all areas of the field is recommended. In fields with more than one corn variety or planting date, each area should be treated as a separate field.
The previously recommended sampling method was to observe rootworm beetles on 100 plants, either as ten 10-plant samples or twenty 5-plant samples selected at random throughout the field. While this method is adequate and can still be used, research indicates that sampling 2 plants at forty locations within a field provides a more accurate estimation of adult rootworm numbers. The advantage of the 100-plant sampling technique over the forty 2-plant sampling system is that it takes less time.
The estimated cost in human hours per acre using a forty 2-plant sampling is shown in Figure 5. Although the overall labor cost does not change with field acreage, an economy of size is realized when costs are calculated on a per-acre basis. The scouting cost for a forty-acre field is approximately 1.5 labor hours, or 0.038 labor hours per acre. A scout hired at $5.00 an hour to sample a forty-acre field would cost approximately $0.19 per acre, or $7.60 per field for one visit. Each field would need to be sampled for beetles only one to three times between August 10 and August 20. Therefore, the overall sampling cost would be $0.19 to $0.57 per acre. The cost to sample a ten-acre field would be 0.15 labor hours per acre, but the cost to sample a field would be the same as the forty-acre field. These costs do not include overhead costs associated with running a consulting business or the costs associated with sampling for other corn pests.
Figure 5. Estimated cost per acre (human-hr/acre), by field size, to scout a corn field for corn rootworm beetles.
Beetle sampling serves two purposes: to determine if beetle populations are high enough to interfere with pollination and to determine if a soil insecticide, commercial seed treatment or corn rootworm transgenic corn hybrid should be used the following planting year. The following scouting procedure is recommended for sampling adult rootworm populations:
- Once the first beetles are observed in an area, begin looking for gravid females in the field. Gravid females have a swollen abdomen full of eggs. A gravid female can be identified by squeezing her abdomen and observing the discharge of eggs. Females become gravid approximately two weeks after emergence.
- When 10 percent of the females observe are gravid (usually about August 10 in Central Pennsylvania), begin intensive counts of beetles.
- Examine two plants in each of 40 areas within a field. If populations appear to be high, 20 areas may be sufficient.
- Sample the plants at random or use the "M" or "W" pattern. To use the "M" or "W" pattern, move across the field in an M or W and stop periodically to sample two plants until you have 40 samples. Move slowly and quietly as you approach the plant to avoid disturbing the beetles. If the corn silks are green, grasp the silks at the tip of the ear in one hand and, without jostling the corn plant, cut off the ear tip. Hold the silks tightly in your clasped had. Then count the beetles on the remainder of the plant. Start at the bottom of the plant and work up. Count the beetles resting on the upper and lower leaf surfaces all the way up to the tassel. When the weather is hot, pull the leaves away from the stalk and count the number of beetles hidden in the sheath collar region of the leaf. Sampling behind the leaf axis is unnecessary if sampling is done early in the day. Finally, open your hand slowly and count the number of beetles in the silks. Record the number of beetles found on the plant.
- Resample the field seven to ten days later if the economic threshold was not reached during the first sampling. Once the average number of beetles per plant begins to decline, further sampling is not necessary (Figure 6).
Figure 6. Economic and noneconomic buildup of corn rootworm populations through the growing season.
Timing Scouting Activities
Knowing when to scout for corn rootworm adults is critical to keeping the cost of scouting down. Regional maps that provide prediction of the beginning of adult emergence for Pennsylvania and surrounding states are available on the Penn State Department of Entomology Website. These maps are generated using a high-resolution temperature prediction model that interpolates temperature to a 10 square KM grid (6.2 square miles) and a temperature driven mathematical model of corn rootworm development. Because insects proceed through their life cycle at different rates depending on whether it is cooler or warmer, the timing of major events in their life cycle can vary by up to four weeks at one location and up to five weeks across the state in the same year. For instance, peak adult emergence of corn rootworm has occurred anywhere from mid July to late August in Central Pennsylvania. By knowing when 5% adult emergence is predicted, a field scout can save unnecessary trips to the field. Adult scouting should start about 10 days after 5% adult emergence is predicted for a given geographic location. Figure 7 is shows an example of the maps for corn rootworm development.
Knowing when adults are laying eggs is only part of the information needed to determine when to scout a field. A second important piece of information is the stage of corn development. Adult corn rootworm females are particularly attracted to silking corn fields. Fields in the vegetative stage of corn development and brown silk and beyond are significantly less attractive to adult females. The reason for this is females require the amino acids found in corn pollen for proper egg development. These amino acids are present in the leaves, but at significantly low concentration. Thus, a female feeding on leave tissue must consume considerable amount of tissue to get the necessary amount of critical amino acids. Therefore, it is much more efficient for a female to move to pollinating field or areas of fields.
This important behavior of corn rootworm fields can be used to advantage to determining when and which field to scout. Scouts should concentrate of fields that are silking in the area and avoid vegetative stage fields until they reach pollination. Note : In extremely dry years, high number of adults can be found feeding on whorl stage corn and causing significant defoliation and an insecticide application may be needed. However, spraying at this time can lead to the need for a second application once the field begins to pollinate as new beetles are attracted into the field. For this reason, it is usually a good idea to wait until pollination to apply a treatment. Scouting can end once the field has reached brown silk. Late planted fields can attract very high number of adults because few silking fields remain in the area.
A decision concerning the control of rootworm populations can have a major impact on the profitability of corn production. As discussed earlier, scouting provides a measure of the likelihood that the field will experience a yield reduction. Whether this reduction is great enough to warrant an insecticide application depends on a number of biological and economic factors. Because research has not been able to develop methods accurately projecting what biological and economic conditions will influence the control decision, conservative economic thresholds have been established. As listed in Table 1, these thresholds are static and do not vary with changed economic and biological conditions. In reality, though, the corn rootworm thresholds would change whenever a biological or economic condition changed.
|Species||Beetles per plant|
First-year corn fields
|Beetles per plant|
Continuous corn fields*
*Fields in which corn is planted for two or more consecutive years.
Economic thresholds depend on which rootworm species is present and on whether the field has been in corn for one or more years. If both species are present, use the economic threshold for the dominant species. If western and northern numbers are similar, use the western corn rootworm thresholds since they are the most damaging species. Separate economic thresholds have been established for the two species because western corn rootworm is known to feed more vigorously and emerge earlier in the season. Since this species emerges earlier in the season, it begins feeding on the roots while the corn plants are still small. Because the root system at this stage is usually quite small and not extensive, root feeding significantly slows the plant's growth rate. By contrast, the northern corn rootworm begins feeding when corn plants are larger and are producing root tissue more rapidly. Therefore, more feeding is necessary to affect the root system significantly.
Thresholds are further delineated according to how many years a field has been in corn. Thresholds are lower for first-year corn because the ratio of female beetles to male beetles is 70:30, as compared with 50:50 in those fields that have been in corn for greater than one year. Since females produce and deposit the eggs in a field, more eggs will be deposited in first-year corn. Typically, two to three years of continuous corn are necessary before economically harmful numbers of rootworm build up in a field. Economic rootworm populations seldom occur after a field has been in continuous corn for seven or more years.
Under some conditions, the adult stage can be a significant problem. If five or more beetles per plant are present when 50 percent or fewer of the plants are pollinated, an insecticide application may be worthwhile. When economic populations of adult rootworms are present, they clip the green silks and interfere with pollination. By reducing the number of pollinated kernels, adults reduce the plant's yield potential. Once the silks have turned brown, clipping by adults does not reduce yield. Late-planted fields are most likely to have problems with silk clipping. But since western corn rootworm is becoming more prevalent in Pennsylvania, reduced yields due to silk clipping may become more common.
Corn rootworm damage is seldom a problem in field corn when the corn is rotated to a different crop within three years after the field is first planted to corn. In the larval stage, neither rootworm species is known to survive on broadleaf plants. When corn is rotated to alfalfa, red clover, soybeans, or any other legume, the majority of rootworm larvae die from starvation within one year. Although a small number of eggs may hatch two years after deposition in a field, the resulting rootworm population is insignificant. Note : In areas of the Midwest extended diapause of northern corn rootworm has lead to problems in first year corn. This has not been observed in Pennsylvania. Rotation to wheat, rye, barley, oats, or sorghum is safe, because rootworm larvae do not survive well on these grass species. Some survival on a few common grass weed species occurs, but individuals that survive tend to be smaller and produce fewer eggs.
Where a crop rotation system is used, rootworm populations are extremely low or nonexistent in first-year corn. Therefore, application of a rootworm insecticide is unnecessary. Even so, adult rootworm populations should still be sampled in late summer to estimate their numbers and determine if an insecticide application is necessary the following year. (See the above sampling section and Table 1.) Unless rootworm populations are extremely high in the area and large numbers of females have migrated into the field, rootworm numbers in second-year corn should be below the economic threshold. In a three-year rotation, the third year of corn is the most likely to have economic levels of rootworm. Again, sampling of second-year corn can determine the need for a rootworm insecticide. If the field will be rotated out of corn the following year, scouting the field is unnecessary.
In very rare cases, damage by economic levels of rootworm has been reported in first-year corn. This damage may be due to poor grass control in the previous year's crop, particularly if a field has had a volunteer corn problem. In years when adult populations are extremely high, enough females may deposit their eggs in fields having a volunteer corn problem to cause economically harmful levels of rootworm larvae in first-year corn.
Another possible explanation for economic rootworm damage in first-year corn is that extreme competition for egg-laying sites forces females to deposit their eggs in less favorable habitats, such as soybean or alfalfa fields. At any rate, the probability of having an economic problem in first-year corn is so low that the best strategy is to accept any losses that occur and prepare for the following year.
In areas of western Indiana and eastern Illinois, a variant of the western corn rootworm has developed a behavioral resistance to crop rotation. In these areas a corn and soybean rotation has been practiced over large areas for up to 35 years. In recent years, females have adapted to laying their eggs in soybean fields so their offspring hatch in cornfields the following year. Through natural selection, those females with this behavioral trait have produced more surviving offspring that also have the trait, while female that continue to lay their eggs in cornfields have experienced significant mortality in their offspring. Thus, individual corn rootworms with the behavioral trait of laying eggs in soybean fields have become dominate over those laying eggs in corn fields. It is unlikely that this western corn rootworm variant will become a problem in Pennsylvania because of our predominate dairy systems with long rotations.
At-Planting Insecticide Application
For some farm operations, growing continuous corn is more profitable than rotating corn to other crops. Where continuous corn is grown, two at-planting management strategies are practiced by farmers for rootworm control. One strategy is to apply a soil insecticide on a prophylactic basis. This strategy reduces the risk of losing corn yields to corn rootworms, but it may not be the optimal strategy. Many years of rootworm insecticide trials at The Pennsylvania State University indicate that while prophylactic insecticide applications are sometimes necessary to control rootworm, they usually are cost-effective in about 15% of fields. However, economic thresholds suggest that about 35-40% of fields could justify management. Determining which fields are most likely to develop economic numbers of rootworm is necessary in order to minimize costs and maximize profits.
A second management strategy for at-planting insecticide applications is to base the need for rootworm control on counts of adult beetles from the previous summer. (See the section on scouting procedures above.) Thresholds established for making a decision on rootworm insecticide treatment are conservative, favoring an insecticide application (Table 1). Because it is not possible to be 100 percent accurate when making rootworm insecticide recommendations, conservative economic thresholds are established to minimize the chances for large yield reductions. Conservative estimates decrease the chance of not applying an insecticide when one is needed but also increase the chance of applying an insecticide when one is not needed.
Commercial Seed Treatments
Several insecticide products are now available that come applied to the corn seed. A decision is to use these products must be made at the point of seed purchase. The advantage of this approach is that the grower does not need insecticide applicator and does not need to store and handle bags of insecticide. Although these products will provide adequate control under low to moderate pressure, they tend to be more variable in their protection of the crop then the traditional soil insecticides when corn rootworm pressure is high. All products provide protection against a spectrum of soil insects.
Post-emergence Insecticide Application
Furadan 4F is labelled for use as a post-emergence application. When timed properly, this method provides excellent control. However, the timing is critical. Timed to early the product will leach and degrade before the corn rootworms are present to control. Timed too late and the product may not have time to reach the root zone to control the pest before it damages the crop. This material should be timed to 1 week before to 1 week after 5% egg hatch (entrance into the 1st instar). As mentioned in the timing of scouting section, 5% hatch can vary by up to four weeks between years and geographic locations. Therefore, for a grower to effectively use this program they need to understand and have access to tools for timing corn rootworm development. As a rule-of-thumb, some individuals are using the observation of fireflies to indicate that corn rootworm have begun to hatch. This rule, however, may result in different outcomes depending on the firefly species in your area.
The advantage of this strategy is that the insecticide is applied nearer to the period of larval hatch. Insecticides applied at planting must stay in the soil for four to ten weeks before rootworm larvae begin to emerge. For registered insecticides, six weeks is near the limit of effectiveness. The objective of at-planting applications is to protect the roots sufficiently for the plants to produce an acceptable yield, not to eliminate rootworms. If the period from planting to rootworm larval emergence is longer than six weeks, the insecticide may fail. Moving the application date closer to larval emergence reduces the chance of insecticide failure and allows for more effective control. In addition, the application rate can be reduced, resulting in a lower control cost.
In 2003, the first genetically modified corn hybrids were commercially released by Monsanto. These hybrids contain a gene that codes for an insecticidal protein that is toxic to corn rootworm larvae that feed on their roots. Field trials with experimental materials, in Pennsylvania, have shown the gene to provide outstanding protection of the root systems. Although not statistically different, root rating on Bt-corn hybrids were always numerically lower than those protected with an insecticide application. However, because the hybrids have just entered the market place, it is not known how they will yield in comparison to non-Bt hybrids protected with a commercial insecticide. A decision to use this technology will come down to cost, root protection and convenience. As with commercial seed treatments, growers choosing this technology will eliminate the need to insecticide applicators and storage and handling of insecticides.
Control of adult rootworms is usually not necessary. When corn is planted during late April and May, pollination is typically complete by the time adult beetle populations are large enough to cause problems. But in corn planted after June 1, beetle feeding can cause pollination problems. Control of corn rootworm adults in corn that has completed its vegetative growth phase requires aerial application (airplane or helicopter) or application by a high clearance sprayer. In many areas, the ability to control populations or rootworm adults is limited because either the equipment is not available or the topography is too hilly. Other management tactics, such as early planting, help to control problems with adult corn rootworms. Corn producers who want to plant corn following a small grain should be aware of the possibility of reduced pollination due to adult rootworm feeding.
Check the current Penn State Agronomy Guide for insecticides registered for control of larval and adult corn rootworm.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Dennis Calvin Professor of Entomology