Integrated Pest Management of Corn Rootworms in North Dakota
(E1852, Revised March 2022)This publication summarizes Integrated Pest Management of corn rootworms in field corn including identification, life cycle, crop damage, field scouting, thresholds, root injury ratings, cultural control, Bt corn hybrids, biological and chemical control.
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Identification
Northern and western corn rootworms belong to the order Coleoptera (beetles) and the family Chrysomelidae. Like all Coleopterans, rootworm species go through four life stages: egg, larva, pupa and adult.
Eggs
Eggs (Figure 1) are oval, white to cream and approximately 1/50 inch (0.5 millimeter [mm]) long. They are extremely difficult to find without magnification.
Larvae
Larvae (Figure 2) of both species are slender and white to cream with a brown head capsule and brown anal plate at the posterior end. They pass through three instars of development. Newly hatched (neonate) larvae are slightly more translucent white than later, more solid cream-colored second and third instars. The full-grown third-instar larva is about 1/2 inch (13 mm) long.
Pupae
The pupa (Figure 3) is translucent white to cream, and is similar in size and shape to the adult; however, it lacks fully developed legs and wings. The pupa is a resting and nonfeeding stage.
Adults
Adult corn rootworm beetles can easily be identified in the field. Northern corn rootworm adults (Figure 4) are tan to pale green, 1/4 inch (5.6 mm) long beetles. Western corn rootworm adults (Figure 5AB) are yellow to yellowish-green and black beetles. Most female western corn rootworm beetles (Figure 5A) have three longitudinal stripes on their forewings. In contrast, the forewings of most male western corn rootworm beetles (Figure 5B) have a nearly solid black marking. Western corn rootworm adults vary from 3/16 inch (4.2 mm) to 5/16 inch (6.8 mm) long. Female beetles of both species are usually slightly larger than males, and female abdomens become enlarged when carrying eggs inside.
Life Cycle
Northern and western corn rootworms typically produce one generation per year. Although corn is the preferred host of northern and western corn rootworms, weeds such as volunteer corn in other crops and foxtail grasses can also be attractive to egg-laying females and can serve as alternate hosts for larvae.
Corn rootworm adults usually begin emerging in mid-July in North Dakota. Northern corn rootworm beetles usually emerge first; western corn rootworm beetles emerge one to two weeks later. In North Dakota, 50 percent of adult emergence occurs by late August to early September, and it typically is completed by mid-October.
Male beetles of both species emerge about five to seven days earlier than females. Female northern corn rootworm beetles mate within one to two days after emerging, whereas most western corn rootworm females mate within 24 hours after emergence.
Females feed for about two weeks after mating, after which they are ready to lay eggs. Most egg laying occurs from late July through September. A single northern corn rootworm female can lay more than 1,000 eggs during the growing season, whereas western corn rootworm females are capable of producing more than 1,800 eggs.
Females lay eggs within the upper 8 inches (20 centimeters [cm]) of the soil surface. Some eggs will fail to survive due to several mortality factors, including predation, soil factors, extreme cold temperatures and excessive rainfall. Before eggs will hatch, they must undergo a long period in diapause, a slowed physiological state that allows them to overwinter. Eggs overwinter in the soil from September to June of the following year.
After overwintering, eggs hatch into larvae from late May through June in North Dakota. Soon after hatching, the tiny first-instar larvae orient to volatiles emitted from growing corn roots to find their host plants and begin feeding.
Larvae continue to feed as they develop, and reach maturity in the third instar. Mature third instars subsequently enter the pupal stage. Full-grown (third-instar) larvae pupate in earthen cells in the soil during early July. The transformation from pupa to adult takes about one week, with beetle emergence from soil beginning in mid-July.
Variation in the timing of egg hatch and developmental rates can result in overlaps of different corn rootworm life stages within a field.
Crop Injury
The most significant corn rootworm injury to corn occurs when larvae feed on the root system (Figure 6). First-instar larvae usually feed on root hairs and outer root tissue. Second instars feed on lateral roots and sometimes burrow into the roots. Mature third instars cause the most damage by feeding on lateral roots and often burrowing into the roots, often making longitudinal tunnels through the central core of the brace roots.
Tunneling injury can result in completely severed individual roots. Feeding scars and the remaining tissues from severed roots (Figure 7) turn brown shortly after the injury. Injured root systems, especially those in which brace roots are heavily injured and/or severed, have reduced capacity to take up water and nutrients from soil.
Severe larval root feeding injury can cause up to 50 percent yield losses. Severe corn rootworm feeding injury to roots also can cause plants to become weak and exhibit symptoms of water stress.
Severe injury to brace roots can cause plants to lodge (Figure 8). Lodging is often followed by “goose-necking”, which is a curved or bent-appearing stalk condition that results from plants attempting return to a vertical orientation after lodging (Figure 9). Larval feeding injury can also predispose roots to plant pathogen infections, which can further reduce overall plant health.
Extensive plant lodging within a field can make harvesting difficult, resulting in additional losses. It should be noted that lodging can also be caused by other factors, such as severe winds, especially after a rainfall event, or feeding injury from other root-feeding insect pests. This underscores the importance of scouting for corn rootworm presence in fields exhibiting lodging symptoms.
Larval feeding injury can also predispose roots to plant pathogen infections, which can reduce overall plant health further.
Adults of both species will feed on corn leaves (Figure 10), silks (Figure 11), tassel/pollen (Figure 12), and kernels (Figure 13).
Corn rootworm beetle feeding on leaves results in feeding scars (Figure 14) that can appear as scratch marks on the leaf surface or holes (Figure 10), or leaves can be completely stripped. High corn rootworm beetle infestations can cause additional yield losses by damaging corn silks, which interferes with pollination, and by directly feeding on developing kernels.
Monitoring and Thresholds
Monitoring Adults With Traps
Corn rootworm adults can be monitored using sticky traps. During early August, deploy 12 unbaited Pherocon AM® yellow sticky traps per 10 to 50 acres of corn. Traps should be arranged into two linear transects separated by at least 330 feet (100 m). The transects also should be at least 100 feet (30 m) away from field edges and/or waterways. Each transect should contain six sticky traps that are at least 165 feet (50 m) apart from each other.
Each trap should be attached to a corn plant stalk at ear height (Figure 15). Replace traps once per week for four to six weeks, or until the threshold is reached. Count and record the number of beetles caught on each trap.
Lastly, calculate the average number of beetles per trap per day (add total number of beetles captured from all traps and divide by total number of sticky traps, then divide that result by the total number of days traps were deployed in the field).
Threshold
A capture rate of two or more beetles (either species or any combination of the two species) per trap per day during the week of peak abundance indicates that a high corn rootworm population is expected the following year. As such, a corn rootworm management tool likely will be necessary to protect the following year’s corn crop.
Larval Root Feeding Injury Ratings
Quantifying rootworm larval feeding injury to corn roots can be helpful for verifying the success or failure of a soil insecticide or Bt corn hybrid, and it also can help determine the best pest management strategy for a field in the subsequent year.
To estimate the extent of larval feeding injury in a corn field, randomly select 10 plants throughout the field and excavate their roots in late August or early September. Most corn rootworm adults will have emerged by this time, so root feeding injury should mostly be completed.
Selecting plants that have been treated with the same management tool(s) is important. For example, in a field planted with Bt corn and its required refuge (non-Bt corn) in a block or strip near the field, select
10 plants from the Bt corn area and 10 plants from the non-Bt corn. Be careful not to mix plants from areas that received different management tools.
Dig roots from soil, taking care to avoid damaging them with the shovel. Soak roots in a bucket of water for 30 to 60 minutes, and then wash the soil from them. Make sure to avoid causing mechanical damage to roots during washing.
Once roots are cleaned, quantify larval feeding injury by using the Iowa State 0-3 node-injury scale (Table 1). Calculate the average root injury rating and assess it according to thresholds. One node pruned generally equates to a 15 to 18 percent yield loss.
Threshold
| Average Root Ratings | Integrated Pest Management Decision |
|---|---|
| <0.25 | Bt corn hybrid is effective and protects against economic yield loss |
| 0.25 - 0.75 | Possible economic loss may depend on other plant stresses as well |
| >0.75 | Potential economic yield loss can occur; the current management strategy is not effective and should be improved next year |
| Average Root Ratings | Circumstance |
|---|---|
| >1.00 | In Bt corn hybrids expressing a single toxin protein |
| >0.50 | In pyramided Bt corn hybrids |
Table 1. Node-injury scale to quantify corn rootworm larval feeding injury.
| Injury score | Description of the injury level to corn root system |
|---|---|
| 0.00 | No damage |
| 1.00 | One complete node or equivalent of one node consumed |
| 2.00 | Two complete nodes consumed |
| 3.00 | Three or more nodes consumed |
A single root (for example, brace root) is counted as consumed if it is pruned back to within 1.5 inches (38 mm) of the stalk. Feeding between nodes is scored as the percentage of the complete node consumed; for example, 1.50 is equivalent to 1½ nodes consumed (Oleson et al., 2005). An uninjured corn root is shown in Figure 16A, and it can be contrasted with the root in Figure 16B that sustained severe corn rootworm larval feeding injury (rating of about 1.8). To view an interactive version of the node-injury scoring system, follow the internet link by VanDyk (2005) in the Selected References section.
