By Ken Hough, OCPA Director of Research and Market Development
"Corn borer resistance to Bt will occur. The real question is ‘How do we manage resistance’ – to delay its occurrence, and to mitigate its effects when it does occur". This statement at the recent Bt Corn symposium at Ridgetown College captures the essence and urgency of the Bt resistance issue.
As with any other pest control agent (i.e. product, or gene), extensive use can increase the rate of development of resistant pest populations. In fact, with Bt resistance management, many of the same principles are involved as with management of weed resistance to a herbicide product or family of products. Some of these principles are:
- increased use (area and/or frequency) of one or more control agents, with a specific mode of action, increases the selection of resistant biotypes within a pest population.
- these resistant biotypes interbreed with any non-resistant biotypes in the local population. This dilutes the frequency of the resistant genes, and thus the frequency of resistant pests.
- when resistant individuals become the predominant biotype within the population, there are few non-resistance individuals in the population to exert the dilution effect, and reversion to the non-resistant population will be slowed.
- the rate of increase, or dilution, of the resistant biotypes within a population is influenced by other factors, including their biological "fitness" relative to the non-resistant types; the number of pest generations per year (more generations = greater potential to develop resistance, assuming no change in the control mode of action), and any alternate control measures exercised within an integrated pest management strategy (i.e. use of controls with different modes of action, cultural management practices, etc.).
Understanding how resistance develops in a population will help growers understand the strategies being proposed to manage corn borer resistance to Bt.
Bt gene mode of action
To date, several different Bt ‘events’ are approved for use in Canadian corn hybrids – ‘Event 176’ in the Novartis (Ciba- ‘Maximizer’ Ô) and Mycogen hybrids, ‘Event Bt11’ in Novarits (NK - ‘Yieldgard’ Ô) hybrids, and ‘Event Mon809’, the Monsanto gene in Pioneer (‘Yieldgard’ Ô) hybrids. However, each of these has the same mode of action – producing the CryIA(b) crystal protein which is activated to its toxic form in the gut of the corn borer larvae. Although not proven, it is expected that corn borers resistant to one of these Bt ‘events’ will likely exhibit ‘cross-resistance’ to the other events, because of the common mode of action. Other Bt events are under development, which produce different crystal proteins. These may provide some promise as ‘alternate’ modes of action. Assuming growers rotate their use of ‘Bt modes of action’ (i.e. the same concept as rotating their use of different herbicide families), this could potentially slow the development of Bt resistance in corn borer populations. Stacking traits with different modes of action within the same hybrid is another strategy being explored.
Corn Borer Populations, High Bt Dosage and Refugia
In any population of corn borers, a few individuals will (probably) carry Bt resistance genes, although these individuals are anticipated to be relatively rare. The relatively high dosage of Bt in tissues of Bt hybrids will kill all but these rare, resistant individuals. If the use of Bt hybrids was widespread enough within a given region that these resistant insects could only find other resistant individuals to mate with (producing resistant offspring), the entire population of corn borers in that region would very quickly become composed of resistant individuals. The further use of Bt hybrids for corn borer control would be ineffective. However, if resistant individuals mate with susceptible individuals, the progeny will be susceptible. Thus, if adult moths (which can travel significant distances) of the resistant corn borers mate with other susceptible individuals (that have matured in nearby ‘refugia’ areas), the overall frequency of resistant individuals will remain low, and ongoing use of Bt hybrids will continue to be effective.
Therefore the concept of refugia, (i.e. areas where Bt susceptible corn borers can survive) for inter-mating with any resistant individuals is a key component in corn borer/Bt resistance management (Figure 1). Refugia can be comprised of non-Bt corn (field or sweet), the primary host for corn borers, and perhaps any other significant host of the corn borer. Current recommendations are for a minimum of 20-30% of the planted acreage should be refugia, and blocks of refugia should be located within 100 rows of the Bt hybrid plantings, to maintain effective inter-mating among susceptible and resistant individuals. Many questions remain, however, regarding these recommendations.
Some Practical Considerations of Refugia
The best scenario for planting refugia is in an adjacent field, or in a distinct block within the same field, at a similar time of planting of the nearby Bt hybrid. Close inter-planting of Bt and non-Bt plants (resulting from a mixture of the two types of seed in the same bag/planter box, or filling some rows of the planter with Bt, and other rows with non-Bt seed) is not recommended. Although corn borer larvae do not travel far, they could move from a non-Bt plant to a Bt hybrid part way through their growth period, and still survive, since they might consume only a sub-lethal amount of Bt toxin. This could elevate the potential for corn borer resistance to develop. Furthermore, in scouting fields, growers would not be able to tell whether any borers seen were indeed resistant biotypes, (whereas in larger block plantings, this should be very clear). Also, the larger, distinct plantings of non-Bt refugia could be managed separately, if necessary, to minimize yield losses or harvest difficulties resulting from the heavier borer infestation. At present, much research and industry-wide planning must be completed before a sound Bt resistance management strategy for corn borer can be implemented. However, growers should start learning about the corn borer/Bt resistance management now, to be better prepared when more definitive recommendations become available. OCPA will participate in, and wishes to encourage, the cooperative efforts among all seed companies, entomology researchers, and others to develop corn borer/Bt resistance strategies that will be effective, practical and cost-efficient.
