Insect Resistance Management for Corn RootwormAlan Reynolds, Sharlene Matten, and Tessa MilofskyU.S. EPA/Office of Pesticide Programs, Biopesticides and Pollution Prevention Division (7511C)1200 Pennsylvania Avenue NW, Washington DC 20460reynolds.alan@epa.gov; matten.sharlene@epa.gov; milofsky.tessa@epa.gov

AbstractAs with other Bt crops, EPA has required Insect Resistance Management (IRM) strategies for Bt corn crops targeting corn rootworm (CRW, Diabrotica sp.). IRM is needed due to the risk of resistance from sustained, season-long expression of high levels of Bt toxins. The preferred strategy involves the use of refuges to produce susceptible beetles to mate with any resistant beetles from the Bt crop. For additional information, see EPA’s biopesticides web site: www.epa.gov/pesticides/biopesticides.

CRW IRM Challenges

CRW biology: Behavior (movement, mating, oviposition) Life cycle – both adults and larvae may be exposed to Bt toxins Corn/soybean rotation-resistant western CRW Extended diapause northern CRW

Resistance monitoring – adapting techniques used for lepidopteran pests

Stacked products – harmonizing IRM plans for both CRW and lepidopteran pests

Studying CRW – larvae feed underground and are difficult to observe Mode of action for Bt corn: toxic and/or behavioral? Treatment of refuges for non-CRW pests (i.e. Lepidoptera)

Bt Crop

Resistant Beetles

Non-Bt Crop

Susceptible Beetles

Refuge StrategyA structured refuge is strategic planting of non-transgenic (non-Bt) crops in close proximity to the Bt crop to promote interbreeding of resistant and non-resistant insects. The goal is to reduce chances that resistant beetles mate with each other by providing large numbers of susceptible beetles from the refuge (non-Bt crop).

TARGET = 500 susceptible to 1 resistant

Information Used to Formulate IRM Strategies for CRW

Currently Registered Rootworm Products

Product Toxin Target Pest

Yieldgard Rootworm Cry3Bb CRW

Yieldgard Plus Cry3Bb + Cry1Ab CRW, Lepidoptera

Herculex Rootworm Cry34Ab1/Cry35Ab1 CRW

Herculex Xtra Cry34Ab1/Cry35Ab1 + Cry1F

CRW, Lepidoptera

CRW Refuge Requirements for Bt Corn

Single gene CRW products

- Yieldgard Rootworm

- Herculex Rootworm

Stacked CRW and lepidopteran products

- Yieldgard Plus

- Herculex Xtra

% Refuge 20% (corn regions)

50% (cotton regions)

20% (corn regions)

50% (cotton regions)

Refuge Deployment

Single CRW refuge 2 options:

1) Shared CRW/Lep refuge

2) Discrete CRW and Lep refuges

Refuge Design Adjacent field, Block, Perimeter, In-field Strip

For shared refuge and discrete CRW refuge: Adjacent field, Block, Perimeter, In-field Strip

For discrete Lep refuge: Separate field can be used in addition to other options

Refuge Distance (proximity to Bt crop)

Refuges adjacent to or within field

For shared refuge and discrete CRW refuge: Refuges adjacent to or within field

For discrete Lep refuge: separate refuges must be within ½ mile (¼ mile preferred)

Strip Width (in-field refuges)

4 rows 4 rows (for discrete Lep refuges, 6 rows are recommended)

Dose

Dose is defined as the amount of toxin expressed by the transgenic crop relative to the susceptibility of the target pest.

"High dose" is defined as a level of toxin at least 25 times greater than the amount needed to kill all susceptible insects.

Data submitted for Yieldgard Rootworm and Herculex Rootworm show that both express less than a high dose for CRW, though they are still efficacious against CRW.

Models

Models are predictive tools to estimate the likelihood of resistance given certain assumptions (i.e. they can help evaluate the durability of an IRM strategy).

They can be used to evaluate and compare IRM/refuge options. In the absence of field resistance, models are often the only tool to

evaluate IRM strategies. Models used to evaluate CRW IRM options:

Onstad et al. 2001 (deterministic, multiple patch model)Storer 2003 (stochastic, spatial model)

Refuge Designs

In-field Strips Adjacent Block Perimeter (Border)