General Description and Biology of the Corn Rootworm Complex

Recent reports of Bt resistance in the corn rootworm (CRW) complex in Ontario reminds us that integrated pest management is essential to effectively manage this pest. CRW is also present in Quebec though Bt resistance has not been confirmed here. The complex in Ontario and Quebec consists of two species, the western corn rootworm (WCR) and the northern corn rootworm (NCR). The WCR is yellowish with black striped wing covers (Figure 1). NCR is yellowish green to green.

Corn rootworms
Figure 1. Western corn rootworm adult (left), northern corn rootworm adult (right).

The biology of the western and northern corn rootworm is very similar. They produce a single generation per year and overwinter in the egg stage. Egg hatch begins in spring when soil temperature reaches 10°C This usually occurs in late May to mid-June.1 The larvae (Figure 2) go through three growth stages, with the entire larval stage lasting about three weeks. This is where critical plant damage occurs. In eastern Canada, adults emerge late July to early August.1 After mating, females deposit eggs in the soil, with the majority of eggs deposited in the upper 6-inches of the soil profile.

Corn rootworm larvae
Figure 2. Corn rootworm larvae. Note the brown head capsule and brown anal plate. Photo courtesy of UNL Entomology.

There is a variant of the WCR in the United States that feeds on and deposits eggs in soybean crops. Since the lifecycle continues, this variant can cause damage to corn following soybean. This variant may also be present eastern Canada but has not been confirmed. The same technique used to sample WCR in corn can be used to sample soybean fields in areas known to have the WCR variant. Place sticky traps at least 18-inches above the canopy. If a daily average per trap equals or exceeds 1.5 WCR, a control tactic is recommended for the next cropping season. Management tactics are the same as managing the non-variant type. Since this variant can survive in soybeans, planting soybeans following soybeans is not recommended; however, planting winter wheat would be an excellent choice to extend the rotation away from corn as it is not a larval host for corn rootworm.


Management of Larval Injury by Sampling Adults

Managing potential injury caused by the larval stage begins with monitoring the adult activity during late July through August of the previous year. Counting adults in corn fields, either on plants or captured on sticky traps, can provide an indication of the risk of larval damage the following year. The use of sticky traps is becoming the standard method to assess the risk of injury. Follow-up with root digs the following year and assess the actual level of root feeding using the node injury scoring system.

Counting Adults Using Sticky Traps in Corn2

For every 4 to 20 hectares (10 to 50 acres) of corn, select two corn rows, separated by at least 100 metres (330 feet) and at least 50 metres (165 feet) from the field margin. Place six sticky traps (e.g. Pherocon® AM unbaited), in each row separated by 50 metres (165 feet). Attach the trap directly above the ear, removing leaves that may get tangled in the trap. Mark the row, so it can be easily found. Place the traps at silking and count the total number of WCR and NCR every week and replace the trap. Determine the average number of corn rootworm adults per trap per day. For example, if a total of 100 beetles captured on 12 traps over a week would be an average of 1.2 beetles per trap per day (100/12 = 8.3; 8.3/7 = 1.2 beetles/trap/day). If at any time during the sampling period the number of beetles exceeds an average of 2 beetles per trap per day, a control tactic is recommended for the next growing season. The trapping period should last at least four weeks.

Sticky trap used to sample corn rootworm adults

Figure 3. Sticky trap used to sample corn rootworm adults. Photo courtesy of Cory Tilstra.
Sticky trap placement in the field
Figure 4. Sticky trap placement in the field. Figure courtesy of Erin Hodgson, Iowa State University.

Counting Adults on Sticky Traps in Soybeans

In areas where the WCR variant is present, soybeans can be sampled using the same technique that is used in corn. Place sticky traps at least 45 centimetres (18 inches) above the canopy. If a daily average per trap equals or exceeds 1.5 WCR, a control tactic is recommended for next cropping season.

Management Tactics if Adult Counts Exceed the Threshold3

Follow the management recommendations from the Ontario Ministry of Agriculture, Food and Rural Affairs and the Canadian Corn Pest Coalition.

Management tactics include:

  • Crop rotation to a non-corn crop
    • Corn is the only agronomic crop in which the corn rootworm complex can complete its life cycle. If adult counts exceed the threshold, rotating to non-corn crop is the most effective known control.
    • Manage corn volunteers in the rotational crop
    • If possible, rotate away from corn for two years
    • If crop rotation is not possible then plant a non-CRW hybrid in second year corn to reduce the selection pressure on the trait and use another management option to control the CRW such as insecticide.
  • Insecticides
    • Insecticides can be applied as seed treatment or soil-applied prior to egg hatch to suppress economic injury by the corn rootworm larvae. The local recommendation is to use insecticides in the second year of a continuous corn system.4 Consult your local provincial extension office for product recommendations and application requirements.
    • Foliar insecticide treatments for adult control are not recommended as there is little economic benefit.
  • Corn product selection that contains a pyramided below-ground trait package
    • A pyramided corn product is one that has been genetically modified to produce at least two proteins as directed towards a single pest. For products containing SmartStax® technology, there are two Bacillus thuringiensis (Bt) proteins that are active against corn rootworm larvae, Cry34/35Ab1 and Cry3Bb1. This trait is not recommended in first or second-year corn in order to reduce selection pressure on the trait. Following other crops, CRW is expected to be low. Scout first year corn for adult CRW beetles. Use seed treatment or soil applied insecticides in year two of a continuous corn system. There is a testing program in Ontario with biocontrol nematodes as a potential solution for controlling CRW. Use a pyramided below-ground trait in year three of a continuous corn system with no insecticide seed treatment or soil applied insecticide. Rotate to a non-corn crop in year four or sooner.4
    • *note: if CRW are resistant to Bt proteins then a pyramided below-ground trait will not be effective to control the population. Consider rotating to a non-corn crop and follow the Mitigation Measures set in place by the Ontario Ministry of Agriculture, Food Rural Affairs and the Canadian Corn Pest Coalition.5

Visit the Canadian Corn Pest Coalition website for the most recent recommendations

Resistance to Bacillus thuringiensis

In 2020 several counties in Ontario experienced unexpectedly high levels of CRW damage in pyramid Bt-RW hybrids. This is similar to what corn growers in the US Corn Belt experienced a decade ago.5 In the early 2010s, it was reported in Iowa that populations of WCR had become resistant to the Cry3Bb1 Bt protein.6 Additional resistant populations have been identified in Nebraska7, Illinois8, and Minnesota.9 Some populations with resistance to the Cry3Bb1 protein are also resistant to the mCry3A and eCry3.1Ab Bt proteins. Recently, there has been confirmed resistance to the Cry34Ab1/Cry35Ab1 Bt protein.10

Common Questions and Answers on Corn Rootworm Management

Should I sample my first-year corn field if I am planning on planting corn there next year?

Yes. Corn rootworm females usually mate in the field where they emerge. Females usually disperse more readily than males, therefore first year corn fields usually contain a higher percentage of female beetles. Fields planted later than surrounding corn fields may be more attractive to dispersing females if the field is beginning to silk and shed pollen compared to earlier-planted fields in which silks have begun to brown.

Can I use larval sampling to determine the need to use a rescue treatment?

Larval sampling can be used to detect the presence of corn rootworm larvae. The general economic action threshold is if 2 or more larvae per plant are found, a treatment may be warranted. Usually by the time larvae are detected, there isn’t enough time to treat to prevent economic loss.

Can I use an insecticide to kill the adults before enough eggs are deposited to result in economic injury?

This tactic has been used in the US with some success in the past though it is currently not recommended as an economically viable option in Canada. Timing is critical with this tactic. Adults are counted on plants and if a threshold is reached, usually 0.7 to 1 beetle per plant and 10% of the females are gravid (ready to deposit eggs) an insecticide is recommended.

Does tillage have an impact on the population?

Fall or spring tillage will not have an impact on egg survival.

Will a cold winter kill the eggs?

Yes, but it depends on how cold, duration of the cold spell, rootworm species, soil texture, and possibly snow cover and tillage system. In general, NCR can withstand colder temperatures for a longer period than WCR. Fields without snow cover or crop residue may experience colder temperatures. Research has found that if soil temperature is maintained at or below -10°C for two to four weeks substantial mortality of WCR eggs can occur.11