5. Resistance Management Strategies Overseas

In most areas where Bt crops are being grown overseas, resistance management is acknowledged as a priority for ensuring the long term viability of the crops. Case-studies in resistance management strategies from Australia and the USA are outlined below.

Australia – Ingard® cotton

The Australian cotton industry has developed a resistance management strategy in conjunction with the licensee of the technology (Monsanto), the Australian Cotton Cooperative Research Centre, and other bodies involved in the Australian cotton industry. The aim of the strategy is to help prolong the life and effectiveness of current and future technology to ensure the sustainability of the Australian cotton industry. The main pest at which Bt is targeted in Australia is Helicoverpa armigera, a cotton bollworm.

Key elements of the resistance management plan for Ingard cotton are:

restricted planting times
hectare restrictions
refuge crops
spray limitations
pupae busting/trap crops
control of volunteer plants

Adherence to the resistance management plan is required under the terms of the Ingard Grower Agreement and under the conditions of registration (Agricultural and Veterinary Chemicals Act 1994). Audits are carried out by Technology Service Providers (the resellers or distributors of the technology – i.e. the people who actually sell seed to growers and to whom Monsanto license or sell the technology) to ensure that growers have a known area of Ingard cotton and that the correct area of refuge is planted. Audits are also carried out by growers. Growers receive a rebate if a spraying record is supplied, and in New South Wales and South Queensland, growers receive a rebate if they fully comply with pupae-busting requirements. Random auditing is also undertaken by Monsanto to ensure good compliance. This includes screening cotton plants for the specific genes to see if the mapping of Ingard areas and refuges carried out by growers has been correct. Wilful non-compliance results in no rebate being paid and the denial of access to the technology for a period.

Restricted planting times

Ingard cotton must not be planted later than November 15. This avoids late maturing and therefore minimises the crop’s exposure to high densities of H. armigera during late summer.

Hectare restrictions

Growers are restricted to a maximum of 400 ha or 30 percent of the total cotton grown per farm unit and in situations where the Ingard area is less than 400 ha but exceeds 30 percent of the total cotton grown there are additional refuge requirements.

Refuge crops

The aim of a refuge is to generate significant Helicoverpa spp. populations which have not been exposed to the Bt protein. Moths produced in the refuge crop disperse to form part of the local mating population where they may mate with moths emerging from the Ingard crops, reducing the chances that two resistant moths will meet and mate. Provided that susceptible moths from the refuges always predominate in the local area they can effectively dilute resistance and slow down the rate at which resistance increases.

For this to be successful, refuge areas must be close to the Ingard cotton crop and must provide sufficient moths. The relative number of adults that emerge per hectare from Ingard cotton and the associated refuge crops determines the required size of the refuge areas. There are different percentage requirements when using different refuge options. There are also different refuge requirements for irrigated and dryland Ingard cotton crops and for the differing growing conditions for cotton in different areas.

The different percentage requirements for the different refuge options are based on data collected from field experiments conducted by the Cotton Cooperative Research Centre over several seasons. The experiments measured the average productivity of several potential refuges. Only refuges assessed in this way are currently approved by the National Registration Authority for use with Ingard crops. Unsprayed conventional cotton and pigeon pea generally produce more moths and over a longer period than sorghum and corn which, while effective, are most attractive at flowering and produce moths over a shorter period.

The movement of Helicoverpa spp. moths during the summer cropping season depends on the mix of crops and their attractiveness at the time of moth emergence. The best location for a refuge crop is close to the Ingard crop and at least within 2km.

For irrigated cotton, the refuge crops must also be irrigated to ensure they maintain attractiveness relative to the cotton. For each 100 hectares of cotton in New South Wales and Queensland, a grower is required to plant a minimum of:

an area of irrigated conventional cotton of 100 hectares which can be conventionally managed for Helicoverpa spp. and other pests (no Bt products may be used); or
an area of irrigated conventional cotton of 10 hectares which will not be treated for any reason with products that control Helicoverpa spp.; or
an area of:

unsprayed irrigated maize of 20 hectares which must be planted in three plantings to ensure attractiveness throughout the season; or
unsprayed irrigated sorghum of 15 hectares which must be planted in three plantings to ensure attractiveness throughout the season; or
unsprayed irrigated pigeon pea of 5 hectares, planted within two weeks of the Ingard cotton and managed to ensure several cycles of flowering throughout the cotton season.

Growers choosing sorghum or maize are audited during the growing season to ensure compliance and that sufficient, timely plantings have been made to ensure that the crop is attractive at the stipulated times.

For dryland cotton, the refuge options are restricted to sprayed or unsprayed cotton planted at the time of the Ingard crop. Other crops are not suitable due to the uncertainty of the water supply for dryland crops. Future research may identify other options for dryland refuges.

Where Ingard is more than 30 percent of total cotton, farmers are required to plant an additional amount of refuge.

Spray Limitations

To ensure that adults emerging from refuge crops have not already been selected for resistance to Bt protein, no Bt sprays are allowed on any refuge crops for the entire season. Any management action that negatively affects the population of Helicoverpa spp. is regarded as an ‘insecticidal’ action. Inter-row cultivation destroys pupae and is not allowed in refuge fields unless the same action is carried out in the associated Ingard fields. Food sprays are not permitted in refuge crops. Food sprays will increase the levels of beneficial insects which will decrease the Helicoverpa spp. population, acting as a form of ‘biological’ insecticide.

Pupae-Busting

All Ingard cotton crops must be cultivated after harvest at a level adequate to stop regrowth and to destroy pupae in the soil. In some states, pupae remain dormant in the soil over the winter months. In Ingard cotton, the pupae may comprise a higher proportion of resistant individuals as they will have survived on Ingard and then pupated. By destroying these pupae it is possible to selectively remove the more resistant individuals and reduce the overall frequency of resistant Helicoverpa spp. in the area.

Trap Crops

In Central Queensland, Helicoverpa spp. pupae produced late in the season do not remain in the soil but emerge within 15 days of pupating. Trap crops can be employed to attract any adult Helicoverpa emerging after the cotton has been cut out. After the cotton is harvested the trap crops should be destroyed, removing the food for the larvae which will then die.

Control of Volunteers

The presence of Ingard cotton volunteers within a conventional cotton crop imposes further selection pressure for Bt resistance. Conventional cotton volunteers within an Ingard cotton crop are also of concern. Growers are required to remove volunteers as soon as possible from all fields planted with Ingard cotton following conventional cotton and from all fallowed and conventional fields following Ingard cotton.

USA – insect resistance management (IRM) for Bt corn & cotton

In the USA, the Environmental Protection Agency (EPA) has imposed insect resistance management (IRM) requirements on registered Bt plant-pesticides (USEPA, 2001). It considers that sound IRM will prolong the life of Bt pesticides and that this is in the interest of growers, producers, researchers, and the American public. The EPA’s strategy is two-fold: mitigation of any significant potential for pest resistance development in the field by use of IRM plans; and developing better understanding of the mechanisms behind pest resistance.

The EPA identified seven elements that should be included in a resistance management strategy for genetically modified Bt plants:

knowledge of pest biology and ecology;
appropriate dose expression strategy;
appropriate refuge;
resistance monitoring and a remedial action plan should resistance occur;
employment of integrated pest management;
communication and education strategies on use of the product; and
development of pesticides/constructs that have alternative modes of action.

A dose rate of 25 times the protein concentration necessary to kill susceptible larvae has been defined as the minimum. It is considered that this toxin level should kill the majority of insects that are heterozygous for resistance to Bt. Refuges have been designed to produce a ratio of 500:1 Bt-susceptible insects to Bt-resistant insects. The placement and size of refuges is based on the current understanding of pest biology and Bt technology.

The IRM programmes for Bt corn and Bt cotton include:

requirements for a non-Bt refuge in conjunction with any acreage of Bt crop;
anyone purchasing Bt seeds must sign a grower agreement which contractually binds the grower to comply with the IRM programme and that there will be a mechanism by the year 2003 by which every grower annually affirms their contractual obligations to comply with the IRM programme;
an IRM education programme;
an IRM compliance monitoring system including a third party compliance survey and mechanisms to address non-compliance;
an insect resistance monitoring programme for each target pest;
remedial action plans to be implemented if resistance does develop; and
annual reporting of the IRM and other activities.

Compliance

Grower compliance with refuge and IRM requirements is a critical element for resistance management and significant non-compliance may increase the risk of resistance. However, it is not known what level of non-compliance would compromise the risk protection of current refuge requirements.

To minimise the effects of non-compliance, the EPA considers that it may be necessary to develop a broad compliance strategy as part of the IRM strategy. Ideally this programme would include four major objectives:

an understanding of the effect of non-compliance on IRM;
identification of compliance mechanisms to maximise adoption of IRM requirements;
measurement of the level of compliance; and
establishment of an enforcement structure to ensure compliance and penalise non-compliance.

Various compliance mechanisms have been proposed, including techniques such as grower contracts, grower certification tests, fines and other penalties, community refuges, sales incentives, crop insurance of the refuge, deposit/refund for planting refuges, databases of non-compliant growers, county/area-wide compliance goals and sale restrictions, intensified grower education, and grower audits.

At present, the EPA’s authority is over the registrants and not over individual growers. Registrants have been responsible for compliance through the use of grower contracts. However, it has been noted that the EPA’s reliance on industry to monitor and enforce compliance is "seen as a major problem". It has been recommended that a third party compliance monitoring programme should be developed. This would be accompanied by an appropriate enforcement programme. Penalties for non-compliance might include: sales restrictions; sales prohibitions to specific growers; registrant fines and warnings; and increased refuges for specific non-compliant growers (through grower contracts).

Refuges for Bt Corn

The EPA put in place a consistent set of required refuge strategies for all Bt field corn products beginning with the 2000 growing season. Registrants are required to provide the EPA with data on how much Bt corn is sold, county-by-county, and have to provide educational material to growers in order to promote responsible product use.

The strategies require a 20 percent non-Bt refuge to be planted within ½ mile of the Bt crop. In certain southern counties and states where most Bt cotton is grown, the requirement is 50 percent non-Bt field corn refuge for Bt Cry1Ab field corn products to mitigate the development of resistance by corn earworm populations that feed on both corn and cotton.

Outside cotton growing areas, agreements specify that growers must adhere to refuge requirements as described in the grower guide/product use guide. The specific refuge requirements for Bt corn are:

Growers must plant a refuge of at least 20 percent non-Bt corn that may be treated with insecticides as needed to control pests.
Refuge planting options include separate fields, blocks within fields, or strips within fields.
External refuges must be planted within ½ mile of the Bt crop (¼ mile or closer is preferred).
When planting a refuge in strips across a field, strips must be 4 rows wide and preferably 6 rows.
Insecticide treatments for control of pests may only be applied if economic thresholds are reached for one/more of the target pests. Instructions to growers specify that microbial Bt insecticides must not be applied to non-Bt corn refuges.

For Bt sweet corn, no specific refuge requirements are necessary because of harvesting times. However, the EPA has determined that crop residue should be destroyed within 30 days, which should kill any live larvae remaining in the corn stubble.

Refuges for Bt Cotton

The refuge options for Bt cotton crops are:

A 5 percent external untreated non-Bt cotton refuge that must be 150 feet wide and preferably 300 feet wide. The refuge must not be treated with sterile insects, pheromones, or insecticides and must be within ½ mile of the Bt crop.
A 20 percent external refuge within 1 mile (preferably ½ mile or closer) from Bt cotton fields that may be sprayed or treated with sterile insects, insecticides (excluding foliar Bt products), or pheromones. There are no guidelines concerning the insecticides to be used or a policy of rotation.
A 5 percent embedded refuge that must be 150 feet wide and preferably 300 feet wide embedded as a contiguous block within the Bt cotton field (i.e. the block must be surrounded by Bt cotton). The refuge may be treated with sterile insects, insecticide or pheromones whenever the entire field is treated and must not be treated independently of the entire field at any other time.

The 5 percent external, untreated refuge option is due to expire in 2004 unless new information becomes available. During the next two years, the registrant of Bt cotton will be required to develop considerable new data on alternative host plants as possible effective refuges. The registrant will also be required to submit protocols to begin field tests on alternative hosts and chemical insecticide sprays on Bt cotton.

Contact for Enquiries

Dr Sharon Adamson
Manager, Innovation Policy
Ministry of Agriculture and Forestry
PO Box 2526
Wellington
NEW ZEALAND

Phone: +64 4 894 0618
Fax: +64 4 4 894 0741
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