Development of Strategies to Prevent Colorado Potato Beetle Resistance to NewLeaf Potatoes: An Industry First

Jennifer Feldman Riebe
NatureMark Potatoes
Boise, ID 83706

This document is reproduced with the permission of author C. D. Difonzo and Michigan State University. Presented here is Dr. Difonzo's summary statement on the 1996 Food Quality Protection Act from a Michigan State WWW publication ENT 812: Pesticide Policy and Michigan Specialty Crops Food Quality Protection Act

Abstract

This chapter will provide an overview of industry's perspective on insect resistance management (IRM), and steps currently being taken to prevent resistance in the Colorado potato beetle to NewLeaf, Btt-expressing potatoes. Insect resistance management has become an essential part of the development and regulatory review process for Bt-expressing plants, as distinct from other conventional chemistries. This represents a "first" for industry, the regulatory agencies, and the entire agricultural sector. Despite this unprecedented effort, the long-term success of insect-protected crops or other pest control tools ultimately depends on the transfer of information to, and adoption by, individual growers. The effective implementation of preemptive IRM plans will require integration and interaction between regulators, manufacturers of the technology, seed companies, agri-chemical distributors and dealers, agricultural educators, and end-users of the technology.

Introduction

Until recently, the problems associated with resistance and the entire issue of IRM was solely in the domain of growers and Extension educators. In the last few years, however, it has become widely recognized that the impacts of pesticide resistance extend far beyond the limits of the farm. Resistance has become an issue of importance to environmentalists, policy makers, agricultural producers, and consumers. As a result, the responsibility of preventing and managing the development of pest resistance also extends beyond the end-user of pest control technology.

Resistance is simply the process of evolution in response to intense selection pressure. The Colorado potato beetle in particular has a dubious history of developing resistance to virtually every insecticide used against it (Gauthier et al., 1981). The potential for resistance to Btt has also been demonstrated through the repeated application of microbial formulations in the laboratory (Whalon et al. 1993). Although resistance to any pest control technology is possible, the development and release of engineered plants has substantially increased awareness of resistance issues.

Insect-protected crops differ from conventional crop protection chemistry in several ways that may influence the consideration of IRM by industry and others. Unlike broad spectrum insecticides, engineered plants are crop-specific, cultivar-specific, and target pest-specific. This means that the entire value of an insect-protected crop may depend on the susceptibility of a single insect pest. In addition, seed crops may have shorter shelf-lives and longer "manufacturing" times than chemical pesticides. For example, seed cannot be held for long periods or sold for alternative uses in the event that resistance developed to target pests. In the case of potatoes, tubers are generally reproduced for four years before being sold to commercial growers as seed potatoes, so a long-term view of product durability and efficacy is necessary. For these and other reasons, industry has adopted a leadership role in coordinating and commissioning IRM research, and influencing the management and use of their proprietary seed products.

In parallel to the efforts by industry, regulatory agencies are now considering resistance as one element of review by evaluating IRM plans and making registrations of some plant products contingent on preemptive management plans. For example, EPA's regulatory oversight of Bt-expressing potato, cotton, and corn products included a thorough review of resistance management plans by both internal and external independent scientists. Potatoes were approved with a voluntary IRM plan, but subsequent cotton and corn registrations included specific conditions of registration pertaining to IRM implementation.

Practical Considerations and Current IRM Practices for Newleaf Potatoes

There are a number of concepts to consider when designing a practical IRM program. First, the procedures must be simple so that growers can understand and apply them easily. Often times, it's not seed company representatives who are explaining these ideas, but others within the industry such as independent seed growers, Agri-Chemical sales representatives, or consultants. Strategies must be simple enough that these intermediary people can be trained easily, and in-turn provide training to grower customers. Second, the strategies must be practical and easily incorporated into existing management systems. If the recommendations or requirements become complicated or require a lot of extra work, growers will not participate. Third, they must be universal enough to apply to very diverse production environments, while being specific enough to be meaningful on every farm. General IRM plans must have the flexibility to adapt requirements or recommendations to individual grower circumstances. Finally, management programs must allow for competition, without becoming burdensome to individual products or technologies. For example, IRM requirements are currently unique to insect-protected crops, and are evaluated within the regulatory review process on a product-by-product basis. To avoid inequitable marketing advantage, IRM strategies should apply equally to different companies, products, and pest control technologies.

As the first Bt-expressing crop to be brought to market, NewLeaf potato provides a good model system for developing and implementing basic IRM strategies. This system is fairly simple in comparison to cotton, corn, or other Bt-expressing crops. NewLeaf potatoes are active against only one target pest, the Colorado potato beetle, which feeds on potato foliage throughout its entire feeding cycle. The Btt (cryIIIa) protein is expressed consistently throughout the foliage at a very high level relative to insect susceptibility. In contrast, Bt-expressing cotton is attacked by multiple lepidopterous target pests that have differential susceptibility to Bt, and very different life histories. Similarly, currently available corn products express the Bt protein at different levels in various plant parts, altering the selection pressure that pest insects may be subjected to during discreet times or feeding stages. Despite the relative simplicity of the potato system and the large volume of existing knowledge on Colorado potato beetle, however, the recent focus on IRM research has increased the understanding of insect biology and behavior tremendously. This is likely to continue as the technology of insect-protected crops is adopted, and we gain experience with implementing IRM plans on a large scale.

The IRM program for NewLeaf potatoes is based on six general components:

1. Baseline estimate of Bt susceptibility in the Colorado potato beetle.

From 1992-1995, cooperating entomologists collected Colorado potato beetle adults from commercial fields in 15 U.S. states and 2 Canadian provinces. Beetles were shipped to the University of MD, where full-series diet incorporation assays were conducted on F-1 neonate larvae, in collaboration with Galen Dively. Results indicated that Colorado potato beetles with no prior exposure to Btt exhibit up to a 7-fold difference in susceptibility, with LC50's from most populations ranging from 1 to 5 ugBtt protein/ml of diet (Galen Dively, personal communication). Significant differences in susceptibility existed among populations with no history of prior Btt exposure, and was assumed to be due to natural variations in sensitivity. No relationship was found between LC50 and previous selection with foliar Btt, suggesting that resistance does not currently exist in any of the field populations tested to date.

Baseline information can be used to develop a discriminating dose assay, and as a benchmark for comparison with suspected resistant populations. Now that the baseline work is completed, individual populations are no longer being tested. Instead, upon the advise of cooperating entomologists, the focus of NatureMark's monitoring efforts has shifted to surveillance and detection.

2. The use of NewLeaf in an overall Integrated Pest Management program.

Integrated Pest Management (IPM) advocates the use of multiple control tactics to reduce the risk of resistance development. Similarly, NewLeaf potatoes should be incorporated into IPM programs as a primary, but not a stand-alone, tool for management of Colorado potato beetle. Growers should continue to employ good pest management practices such as crop rotation to reduce overwintering insect populations. It is recommended that potatoes not be grown on the same ground every year, and that NewLeaf potatoes be planted as far as possible from previous NewLeaf fields (from NatureMark's "Resistance Management Guide"). The use of NewLeaf in a long-term management program, including alternation with other effective chemicals or with no chemicals, is also encouraged. Since NewLeaf will likely reduce potato beetle populations by preventing survival of the overwintering generation, it is expected that growers will use NewLeaf for Colorado potato beetle control only when needed.

Because NewLeaf only controls one insect pest, growers still must scout their fields and apply insecticides when necessary. When possible, selective insecticides should be used to preserve beneficial insects. Many natural enemies, as well as some insecticides targeted at other insects, will provide an additional mechanism of control for Colorado potato beetle if they are surviving in NewLeaf potatoes.

3. High expression

The Btt protein is expressed in NewLeaf potatoes at a consistently high level in the plant, providing the foundation for the "high dose" strategy of resistance management. Assuming that resistance is caused by a single recessive gene, the high-dose strategy predicts that NewLeaf plants will control all homozygous susceptible (SS) and heterozygous resistant (SR) individuals. Only rare homozygous resistant insects are expected to survive. To ensure that resistant insects find susceptible mates, the high dose strategy is always combined with a refuge (see below).

Repeated studies have confirmed that Btt protein expression in NewLeaf plants is approximately 10-50 times higher than the LC99 for Colorado poato beetle larvae. The Btt content does not decline as the foliage matures late in the season, and no Colorado potato beetle larvae have ever been found to survive on NewLeaf plants, either in a research setting or in commercial fields. This strategy is one that can be used successfully with plants, since the Bt protein is not harmful to humans or the environment, whereas it is impractical for use with highly toxic pesticides.

4. Refuge

The high-dose strategy must be combined with a refuge for susceptible insects, to prevent rare resistance genes from accumulating in the population if they arise. By increasing the likelihood of resistant insects mating with susceptible ones, resistance genes can be diluted. There are various methods of incorporating a refuge into the potato production system including seed mixes, field borders, strips, blocks, and whole fields. Currently, entomologists feel that the best strategy is to maintain non-Bt-expressing potatoes as uniform plantings, rather than as mixed seed. This minimizes movement between NewLeaf and unimproved potatoes that might occur as the potato canopy matures and intertwines. If individual insects have the opportunity to feed on both NewLeaf and unimproved potatoes in seed mix, they may obtain a moderate dose of Btt, compromising the success of the high-dose strategy. In order for unimproved blocks to serve as refuges of susceptible insects, they should be grown in close proximity to NewLeaf fields. It is recommended that refuge plantings be placed immediately adjacent to NewLeaf, in split or neighboring fields if possible.

Refuge blocks should be treated with conventional foliar insecticides for control of Colorado potato beetles. Foliar insecticides can provide adequate economic control while allowing some insects to survive, and will increase the time during which potato beetles are active in refuge fields. In addition, growers should vine-kill their NewLeaf potato plants before killing unimproved potatoes. This will force any surviving adults out, and allow them to mate with susceptible adults on other unimproved potatoes.

Since engineered insect control traits are variety-specific, the incorporation of refuges often occurs naturally through standard potato production practices, making this a very practical approach. Most growers already produce several varieties to accommodate different markets. NewLeaf is not available in all varieties, so growers are necessarily gaining experience with NewLeaf as only a portion of their acres. This is quite different from chemical pesticides that are typically applied broadly to all farm acreage, leaving no distinct refuge.

Prior to 1997, the refuge concept was included in NatureMark's IRM plan as a recommendation. Growers were instructed to maintain at least 20% of farm potato acres as unimproved, non Btt-expressing potatoes that could be treated with conventional insecticides for potato beetles. Data from 1996 suggests that compliance with this recommendation was very high. Acreage information was gathered from 94 of 112 total customers in 1996, or 84% of NewLeaf commercial growers. The proportion of NewLeaf on these farms ranged from 0.1% to 69% of total potato acreage (Figures 1-3). Beginning in 1997, the refuge requirement is included as an element of NatureMark's Technology Agreement, so commercial growers are legally bound to maintain at least 20% of their potato acreage as unimproved varieties. This type of mandatory compliance with IRM is unique to proprietary seed technology, and offers new opportunities for instituting IRM on a large scale.

5. Communication and Training

The introduction of Bt-expressing plants has raised the awareness of IRM throughout the agricultural community. Resistance management is a discussion topic in virtually all potato industry trade articles, and other communications regarding biotechnology and insect-protected crops. As a result, introduction of the concept of preemptive IRM strategies has been well accepted.

Education and training is a primary component of NatureMark's IRM plan. This is accomplished through individual contacts with current and potential customers, through the distribution of written materials, and through third party educators. Each of these methods are discussed below.

  1. Personal contacts. One-one-one interaction with customers is a high priority, as this is the most effective way to provide the information necessary to ensure a successful experience with NewLeaf potatoes. In 1996, each NewLeaf customer was contacted before planting to review recommended pest management practices, and was visited at least once during the growing season. In addition to providing information on recommended Best Management Practices, including IRM, personal contact with NewLeaf growers facilitates monitoring of product efficacy and compliance with IRM practices. NatureMark also maintains a direct phone line (1-800-3-TATERS) for customer inquiry or response. This number is printed on all technical materials, and growers are encouraged to call at any time with questions or concerns.
  2. Written materials. In consultation with cooperating entomologists, NatureMark has summarized the current IRM recommendations in a one-page document that describes the grower's role in Colorado potato beetle resistance management. Additional documents describe regional pest management recommendations, including non-target pest scouting and choices for economical and selective pesticides to use in combination with NewLeaf potatoes. Since seed purchase decisions are made well in advance of the growing season, there are many opportunities to disseminate such written technical information. Pest and resistance management recommendations are distributed widely throughout the winter at trade shows, grower meetings, and through direct mailings. Seed growers who sell NewLeaf are given a packet of technical information, including IRM recommendations, that they can provide to their potential customers in advance of seed sales. Since the 20% on-farm refuge is now a legal requirement for use of NewLeaf seed, the IRM requirements are considered by commercial growers before signing NatureMark's Technology Agreement. Finally, the pest and resistance management recommendations are mailed to every NewLeaf grower again when the seed sale is complete so that they can use this information as they plan their production and rotation system.
  3. Third party educators. Extension and research entomologists have been key contributors to the development of NatureMark's IRM program, and continue to play an important role in grower education of IRM strategies. In addition to Extension educators, however, chemical sales representatives and dealers are often a primary avenue for the transfer of pest management information to growers. Through a unique partnership with the Agri-Chemical industry, NatureMark works closely with field representatives from three major distributors. Trained field representatives are not only able to supply growers with technical information, but also provide an additional mechanism for NatureMark to monitor IRM compliance, since they are in close contact with growers concerning overall pest management inputs on NewLeaf and unimproved potato varieties.

6. Surveillance and tracking.

Some form of monitoring is necessary to be able to detect resistance. Following completion and analysis of the baseline susceptibility work, participating entomologists determined that the best approach to long-term monitoring is a program of coordinated surveillance, combined with a plan of action if surviving insects are discovered. This is expected to be much more effective than sampling individual insects from random populations. Because NewLeaf controls all stages of Colorado potato beetles all season, it is clearly evident if any surviving insects are present in the field. As a result, monitoring is straight-forward, simple, and easy to respond to quickly. This method is more feasible with high-expressing plants than with topically-applied chemicals, and may offer new opportunities for early detection of resistance. Because chemical pesticides decline after application, surviving insects are commonly found following treatment. Therefore, it is often difficult to detect the early stages of resistance. Consequently, resistance to chemical pesticides often builds gradually in the population, becoming widespread before growers or pest managers can adjust their practices.

NatureMark has developed a coordinated surveillance program through regional NatureMark representatives, Agri-Chemical partners, Extension representatives, and growers whereby full-season efficacy of NewLeaf potatoes is confirmed in all commercial fields. Growers, scouts, and other field representatives are instructed to notify NatureMark immediately if any larvae are seen on NewLeaf plants. If larvae are found, NatureMark will take action to investigate the situation, and minimize the potential damage. First, it must be confirmed that host plants are indeed NewLeaf, as seed mix-ups occasionally happen. NatureMark has developed a rapid serological test that can be used in the field to identify plants containing the Btt protein. If plants are confirmed to be NewLeaf, then surviving larvae can be collected and shipped to the University of Maryland for bioassay to determine actual susceptibility to Btt. If Colorado potato beetles are found to be resistant to Btt, they can be treated immediately with a conventional insecticide to prevent further reproduction and movement.

In two years of commercial production, current surveillance techniques have alerted company representatives to only two situations where CPB larvae appeared to be surviving on NewLeaf plants. Both turned out to be plantings of non Btt-expressing plants that were mistakenly thought to be NewLeaf, due to planting errors. In both cases, NatureMark was able to respond immediately and confirmed that the plants did not contain the Btt gene.

The preceding points are all steps that NatureMark has taken to incorporate IRM strategies as essential, and accepted, components of NewLeaf potato production. However, even the best IRM program will fail if growers do not participate. Since they actually handle the product, they are ultimately responsible for carrying out the recommended practices. They must comply with refuge requirements, use the product within an integrated management program, and follow through with scouting for Colorado potato beetle survivors.

While instituting the first of such IRM programs is a challenge, the potato industry provides an excellent test system. Potato growers have first-hand experience with insecticide resistance, so they already have heightened awareness of the importance of IRM. NatureMark's recommended practices also fit well with current practices, and will require only minor modifications to incorporate. Finally, the seed increase process is very gradual in potatoes, and most commercial growers have begun growing NewLeaf on an experimental basis as only a small portion of their acres. This has given both growers and NatureMark the opportunity to experiment with new IRM practices, including maintaining diverse insect management programs on each farm that may require different scouting and treatment regimes. It is anticipated that such IRM practices will be considered commonplace and accepted by the time NewLeaf potatoes become widely adopted.

Conclusions

In summary, resistance management has been brought to awareness with the introduction of Bt-expressing plants. Industry has taken unprecedented steps prior to commercialization of these products to determine optimal IRM strategies, and is now influencing how the products are used by growers. Regulatory agencies have also begun considering IRM plans as one component of the registration process.

Strategies of "high expression" of the Btt protein, combined with a nearby refuge for susceptible insects, are the foundation of NatureMark's resistance prevention program for NewLeaf potatoes. Large refuges already exist in most areas through the planting of multiple potato varieties, most of which do not contain the Btt gene. As NewLeaf acreage increases, compliance with refuge requirements will be secured through NatureMark's grower license, or Technology Agreement. In collaboration with the Agri-Chemical distribution industry, a coordinated surveillance program has been initiated to ensure rapid detection of surviving insects that may be resistant to NewLeaf. A broad-based communication and training program has also been developed to support implementation.

Citations

  • Gauthier, N.L., R.N. Hofmaster, and M. Semel, 1981. History of Colorado potato beetle control. In: J.H. Lashomb and Casagrande, R.A. (eds.); Advances in potato pest management. Hutchinson Ross, Stroudsburg, Pennsylvania. Pp 13-33.
  • Whalon, M.E., D.L Miller, R.M. Hollingworth, E.J. Grafius, and J.R. Miller. 1993. Selection of a Colorado potato beetle (Coleoptera: Chrysomelidae) strain resistant to Bacillus thuringiensis. J. Econ. Entomol. 86: 226-233.