Canadian Corn Pest Coalition (CCPC)

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What is Insect Resistance Management (IRM)?

IRM Topics

To understand Insect Resistance Management and why it is necessary, one must first understand the European corn borer (ECB).

The ECB, in its larval or juvenile stage, is a caterpillar and a member of the order Lepidoptera. In Canada, this insect is widespread, ranging from the southern Prairies in the west to the Maritimes in the east and is most predominantly but not exclusively present in corn-growing regions.

In most parts of Canada, the ECB population produces a single generation each growing season. In southwestern Ontario and southern Quebec, however, there can be two generations of ECB each year.

Schematic of resistance development potential in corn borer populations as hybrids with corn borer resistance are introduced (from Ostlie, et al., 1997).
Prior to the introduction and widespread use of Bt hybrids, individual corn borers with resistance to the Bt protein are rare in insect populations.
With the introduction of hybrids with the Bt trait, selection against susceptible types increases the proportion of resistant moths in the population.
If resistant moths find each other and mate, their offspring are all resistant. The proportion of resistant moths in the population will increase rapidly.
If resistant moths mate with susceptible moths, their offspring are susceptible and die from Bt feeding. An effective refuge will supply an overwhelming number of susceptible moths to mate with (rare) resistant moths.

ECB has four stages of development: egg, larva, pupa and adult. Only the larval stage causes damage in corn. ECB overwinters as full-grown larvae in corn stalk stubble, cob residues, weeds and other host plants.

In areas that experience a single generation each growing season, adult moths emerge from corn stalks from mid June to August. In two-generation areas, first generation moths emerge from late May to July and second generation moths emerge from late July to early September. A third generation, known as a "suicide" generation, may begin development in mid-September in hot years but cannot complete its development before the crop is harvested.

Bt corn and how it controls ECB

Bacillus thuringiensis or Bt is a naturally occurring soil-borne bacterium that produces crystal-like proteins that selectively kill a few specific insect species, including ECB. More than 100 different proteins have been identified and commercialized for controlling specific pests in a variety of crops.

The Bt protein currently in Bt corn hybrids only affects a specific group of insects whose juvenile stage are caterpillars (Lepidoptera) that have an alkaline gut. The Bt in corn hybrids has a direct effect on corn borers when ingested by binding with specific sites on the gut wall. This process ruptures cell walls, allowing gut contents to leak into the body and cause death.

Resistance: how it could develop

An insect is resistant to the Bt used in Bt corn hybrids if the insect does not respond by converting the Bt protein to the toxic form and allowing it to bind to sites on the gut wall to form crystals and rupture cell walls.

Resistance could occur because:
- the protein cannot bind to the specific site of the gut wall
- some factor in the gut breaks the protein down
- some other factor changes how the insect might respond to the protein

Resistance may arise within populations through natural genetic variability and increased selection pressure by continual exposure to Bt protein. ECB resistance to the Bt protein is carried on a recessive gene. That means both copies of the gene must be present in an individual to confer resistance. On the other hand, the gene for susceptibility is considered dominant, which means only one copy of the gene is required to confer susceptibility.

In any population of ECB, resistance genes are rare. That means a few borers will have two copies of the resistance gene, some will have one copy and most will have none. In Bt corn, ECB with one or more copies of resistance genes could survive and produce more offspring. Improved survival results in a selective advantage. As Bt corn acreage increases, so will the populations of ECB exposed to Bt corn. Over time, more larvae carrying resistance genes could survive to adulthood and produce offspring with resistance genes. Keeping good records and implementing a comprehensive insect resistance management strategy is critical to delaying resistance development.

Field Layouts Options

	Large Block(s) Refuge Advantages: - Good mixing of insect populations - Can harvest Bt and refuge acres separately Limitations: This layout requires more time to change seed and clean seed boxes
	Split-Planter Refuge Advantages: - Good blending of insects - Less cleaning of seed boxes Requirements: - Individual planter boxes - Plant at least four rows of non-Bt hybrid per strip - Match hybrid maturities Limitations: -Can't harvest the refuge separately.
	Perimeter Refuge Advantages: - Good mixing of insects - Refuge accessible for early harvest if pest pressure is high - Can harvest Bt and refuge acres separately Limitations: - Changing seed within field is required
	Pivot Corners Refuge Advantages: - Maximizes production on irrigated acres. Limitations: - Suitable only in regions where a dry-land corn crop can be consistently produced - Difficult to plant corners or spray pests in refuge
	Separate Field Refuge Advantages: - Greater flexibility with hybrids, insect and weed management, planting and harvesting Limitations: - Only fields within 400 metres (¼ mile) of Bt field qualify as a refuge. Recommendations: - To achieve optimum mixing of insect populations, plant refuge as close as possible to the Bt field (at least within 400 metres or ¼ mile.)

Insect Resistance Management strategy explained

The Canadian Corn Pest Coalition developed an Insect Resistance Management strategy approved by the Canadian Food Inspection Agency in 1997. The objective of the strategy is to minimize the potential for resistance development by implementing a high Bt dosage within Bt corn hybrids while providing refuge non-Bt corn hybrids that contain no Bt proteins within close proximity of the Bt crops.

Bt corn hybrids are designed to produce very high levels of Bt proteins, much higher than levels found on corn treated with Bt insecticide sprays. The intent is to control all ECB larvae with no genes for resistance and those with one resistance gene.

In close proximity to Bt corn plants, growers are required to grow non-Bt corn hybrids. Movement of corn borer adults between Bt and non-Bt fields is important for genetic mixing of resistant and susceptible types. The concern is that corn borer moths do not move very far, generally staying within 400 metres (1,500 feet) from where they emerge. That means refuge non-Bt corn must be grown within 400 metres (1/4 mile) of a Bt corn crop to be effective. In addition to the 400-metre requirement, the industry also agreed that a minimum of 20 per cent of corn planted in any one field must be non-Bt corn.

For a complete description of the industry-approved Insect Resistance Management Strategy, please refer to A Grower's Handbook, Controlling European Corn Borer with Bt Corn Technology published by the Canadian Corn Pest Coalition.

Key steps for European corn borer IRM plan

As a requirement of regulatory approval for Bt events in corn in both Canada and the USA, companies are required to educate their customers on the importance of an IRM plan to maintain the long-term effectiveness of Bt technology. Customers are required to sign an agreement stating they understand the importance of planting a refuge. Companies must also demonstrate that its customers are complying with the IRM requirements.

The approved IRM strategy contains the following five requirements:

Grow Bt corn hybrids only in fields that are at risk from European corn borer
Plant a non-Bt corn refuge within 400 metres (1/4 mile) of your Bt crop. This will ensure that some European corn borer moths that have NOT fed on Bt corn are within close proximity of potentially resistant moths from the Bt field for mating. The genetic mixing of susceptible and resistant borers decreases the potential for fully resistant populations to develop. The general requirements for "refuge" are as follows:
- Have 20 per cent of total corn acres planted to non-Bt hybrids, implemented as complete fields, blocks, headland or alternate strips that are at least six rows wide.
- Bt corn fields must be planted within 400 metres (1/4 mile) of the refuge. Do not mix Bt and non-Bt seed as this practice actually increases the chance of resistance development. The use of ECB insecticides is NOT permitted in the refuge planting. Bt and non-Bt hybrids should be of similar maturity.
Keep careful and accurate records as to where Bt and non-Bt corn has been planted for effective insect monitoring and hybrid performance evaluation purposes.
Scout for insects and diseases in both your Bt and non-Bt fields.
Monitor for resistance of European corn borer to Bt corn. If feeding damage is detected, investigate the cause. Verify from field records that a Bt hybrid was planted where the surviving larva and damage was found. Get help in accurately identifying the feeding insect. Notify seed company representatives immediately if you suspect a problem. Appropriates diagnostic protocols and tools are in place to determine if resistant ECB have developed.

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Updated 01/06/06 :: CCPC 2006