Accepted for publication 5 September 2000. Published 12 September 2000.
The evolution of genetically modified crops took a major step in the mid-1990s with the approval and commercial release of insect-resistant maize hybrids with transgenes derived from the bacterium Bacillus thuringiensis (Bt maize). The release of Bt maize was met with great enthusiasm by many researchers and crop managers because of its ability to very effectively control European corn borers and other lepidopteran insects without the use of foliar insecticides. Crop producers and the agricultural industry rapidly accepted the technology and began to incorporate it into their crop production practices. But recently, controversy over production and use of genetically modified crop cultivars has focused a great deal of public attention on Bt maize. A number of organizations and individuals have raised questions about the safety and ethics of Bt maize production, despite EPA and FDA approvals that consider environmental impact, food safety, nontarget effects, and pest resistance. The controversy has been fueled largely by the reluctance of European consumers to accept genetically modified crops. Although Bt technology has fairly obvious benefits for maize producers and biotechnology companies, some consumers have found it difficult to perceive the consumer benefits of Bt technology.
In the current atmosphere surrounding Bt maize production, the need for investigation into all potential risks and benefits of Bt technology is more critical than ever. Approval by EPA and FDA carries with it assurance that these products are safe, but additional data may be needed so consumers can make informed choices and convey their preferences to policymakers. One aspect of risk/benefit analysis is the influence that Bt technology may have on maize diseases and mycotoxin-producing fungi in maize.
Because the fungi that produce mycotoxins in maize are frequently associated with insect damage to the plants, insect control has the potential to reduce mycotoxin concentrations in grain. Here we summarize six years of research that indicate that Bt transformation of maize hybrids enhances the safety of grain for livestock and human food products by reducing the plants' vulnerability to mycotoxin-producing Fusarium fungi. Lower mycotoxin concentrations represent a clear benefit to consumers of Bt grain, whether the intended use is for livestock or human foods.
Insects affected by currently available Bt maize hybrids are in the order Lepidoptera, which includes moths and butterflies. The primary target species for Bt maize is a pest imported from Europe, Ostrinia nubilalis, or the European corn borer (Fig. 1). Other Lepidopteran pests of maize that can be controlled or partially controlled by current Bt technology are corn earworm (Fig. 2) (Helicoverpa zeae), common stalk borer (Papiapema nebris), armyworm (Pseudaletia unipunctata), and Southwestern corn borer (Fig. 3) (Diatraea grandiosella). Currently available Bt hybrids are very effective against European corn borer (18), stalk borer, and southwestern corn borer, and they can reduce damage by armyworm and corn earworm, but they have not shown much benefit for controlling damage by black cutworm (Agrotis ipsilon) or fall armyworm (Spodoptera frugiperda) (10,16).
Lepidopterans can influence the development of ear rot and stalk rot diseases in maize. In particular, Fusarium ear rot, caused by F. verticillioides (syn. F. moniliforme), F. proliferatum, or F. subglutinans, and Aspergillus kernel rot, caused by A. flavus, are often associated with insect damage to ears or kernels (6, 19, 20). The major stalk rots are often associated with stalk tunneling (Fig. 4) (1, 2, 3), although the overall importance of insect tunneling in stalk rot development is a matter of some disagreement among plant pathologists (5).
The associations between these insects and maize diseases result from several types of host-insect-pathogen interactions. One type of interaction is a vector relationship. European corn borer larvae carry spores of Fusarium species from the plant surface to the surfaces of damaged kernels (21) or to the interior of stalks, where infection occurs. Viable spores can be found externally, internally, and in the frass of European corn borer larvae. Similar relationships exist between corn earworm or southwestern corn borer and Fusarium or Aspergillus spp. (6). A second type of interaction is the formation of entry wounds for the fungi when larvae feed on stalks or kernels. Even when the larvae do not directly carry the fungi into the stalks, spores subsequently deposited on the wounded tissue are very likely to germinate and infect the plant. Additionally, root and stalk damage by insects causes stress that predisposes the plants to stalk rot development. For these reasons, management of these insects can play a major role in maize root and stalk rot management.
Fusarium ear rot (Fig. 5) is the most common ear rot disease in the Corn
Belt; it can be found in nearly every maize field at harvest (20). The severity
of this disease is usually low in the north central U.S., but it can reduce
yield and quality. Symptoms of Fusarium ear rot are often highly correlated with
ear damage by European corn borer and corn earworm larvae (Table 1) (3, 14, 19).
Several Fusarium species can infect kernels without causing visible
symptoms, but still affect grain quality and produce mycotoxins. The primary
importance of Fusarium ear rot is its association with mycotoxins, particularly
the fumonisins. Fumonisins are a group of mycotoxins that can be fatal to horses
and pigs, and are probable human carcinogens (13). The importance of fumonisins
in human health is still a subject of debate, but there is evidence that they
have some impact on cancer incidence in some parts of the world (11). Fumonisin
concentrations in maize are or will be under regulatory scrutiny in many parts
of the world (12). In June of 2000, the U.S. Food and Drug Administration
released their proposed recommended maximum levels of fumonisins in human foods
and animal feeds. Although this action does not constitute regulatory
intervention, it does indicate that the agency responsible for food safety in
the United States "believes that human health risks associated with
fumonisins are possible" (22).
Kernel rot caused by Aspergillus (Fig. 6) also is associated with insect damage to ears (6). Aspergillus flavus and A. parasiticus produce the most notorious mycotoxins in maize, the aflatoxins. The economic impact of aflatoxins has been greater than that of other mycotoxins in maize because of low "action levels" set by the Food and Drug Administration for aflatoxin concentrations. The action level (20 parts per billion [ppb]) often is exceeded in maize grown in the southern United States, and occasionally is exceeded in maize grown in the north central United States.
The stalk rot complex comprises the most serious, widespread disease problem in maize. Yield losses occur as a result of premature plant death and lodging. Stalk rot-affected fields usually are damaged by more than one fungal species, but Gibberella stalk rot, caused by Gibberella zeae (Fig. 7), Fusarium stalk rot, caused by Fusarium verticillioides (F. moniliforme), F. proliferatum, or F. subglutinans, and anthracnose stalk rot, caused by Colletotrichum graminicola (Fig. 8) are the most frequently reported (20). The development of stalk rot is greatly affected by plant stress (4) and may or may not be associated with insect damage.
Diseases And Mycotoxins In Bt Maize
During the mid-1990s, as Bt maize approached EPA approval, researchers in several states began to investigate how insect management with Bt maize influenced maize diseases that are associated with insect activity. In most cases, experiments have been conducted comparing disease and mycotoxin levels between Bt hybrids and "near-isogenic" conventional hybrids.
In these studies, differences among types of Bt genes (or Bt events) have become evident. All Bt events are not alike. Currently available Bt hybrids express either CryIA(b), CryIA(c), or Cry9C, which are members of a group of δ-endotoxins originally produced by some strains of the bacterium Bacillus thuringiensis. Table 2 shows some characteristics of currently available Bt events. The expression of Cry proteins in specific maize plant tissues is dependent on the gene promoter used in each transgenic genotype. Proprietary cryIA(b) transformations BT11 and MON810 (YieldGard®) use a CaMV (cauliflower mosaic virus) 35S gene promoter that results in season-long expression of CryIA(b) in all plant tissues, whereas cryIA(b) transformation 176 (marketed as Knockout® and NatureGard®) uses a combination of two maize-derived, tissue-specific promoters: a phosphoenolpyruvate carboxylase promoter that results in gene expression only in green plant tissues, and a pollen-specific promoter. Kernel expression of CryIA(b) appears to be an important factor determining the amount of kernel feeding by European corn borer larvae and subsequently the intensity of Fusarium infection.
Results of field studies have consistently demonstrated that hybrids
containing the MON810 and BT11 Bt events have significantly lower incidence and
severity of Fusarium ear rot and produce grain with lower fumonisin
concentrations than their non-Bt counterparts (Figs. 9 - 11.). Similar
results have been obtained in studies conducted in Iowa, Illinois, and North
Carolina (7, 9, 14, 15). When conventional hybrids were subjected to high
populations of European corn borers, Fusarium ear rot severity and fumonisin
concentrations were elevated, often to levels considered unsafe for swine and
horses (10 ppm and 5 ppm, respectively). Safe fumonisin levels for humans are
unknown (13). Fusarium ear rot and fumonisin levels in MON810 and BT11 hybrids
were uniformly low (usually less than 10% of the concentrations in the non-Bt
hybrids) and they were unaffected by European corn borer populations. In hybrids
with Bt events DBT418 and 176, ear rot severity and fumonisin concentrations
were similar to the conventional counterparts. This is probably a result of the
lack of kernel expression in event 176 hybrids and the generally poorer
late-season corn borer control demonstrated by event DBT418 hybrids. Hybrids
with Bt event CBH351 displayed ear rot and fumonisin levels similar to MON810
and BT11 hybrids. Over all hybrids, there are highly significant correlations
among insect damage, Fusarium ear rot severity, and fumonisin concentrations
Some field studies also have shown reduced kernel infection by A. flavus and lower aflatoxin concentrations in BT11 and MON810 hybrids compared with their non-Bt counterparts. However, these reductions have been less dramatic and less consistent than those seen for fumonisins. Studies have been conducted in Iowa, Illinois, Mississippi, Texas, Georgia, and other locations. In Iowa and Illinois, A. flavus infection and aflatoxin concentrations have typically been too low to discern any differences among hybrids. In contrast, aflatoxin concentrations in the Mississippi and Texas studies have been very high. Windham et al. (23) reported that when plants were infested with southwestern corn borers, a BT11 hybrid had more than 75% reduction in aflatoxin compared with its non-Bt counterpart (5 ppb vs. 41 ppb). This level of control is significant because the FDA action level for aflatoxin is 20 ppb. When plants were infested with southwestern corn borer and inoculated with A. flavus, aflatoxin concentrations in the BT11 hybrid were about 50% lower, but the concentrations were well above the FDA action level in both hybrids (290 and 650 ppb, respectively, for the Bt and non-Bt hybrids). In plants that were not manually infested with insects, there were no differences in aflatoxins concentrations. In Texas in 1998, a significant reduction in aflatoxin concentration was reported for BT11 and MON810 hybrids compared to the non-Bt hybrids, but aflatoxin concentrations were well above the action level in all hybrids (J. Benedict, Texas A&M University, personal communication). Studies in Georgia have not demonstrated significant differences in aflatoxin concentrations between Bt and conventional hybrids (D. Wilson, pers. comm.).
The relationship of insect damage to maize stalk rot is less clear-cut than the relationship to Fusarium and Aspergillus ear rots. The fungi causing stalk rots often enter plants through the roots (4); under these conditions, resistance to lepidopteran pests is unlikely to be of much benefit. However, some proportion of stalk rot incidence is related to stalk-boring insects and there is evidence for reduced stalk rot in Bt hybrids. In New York, Bergstrom et al. (1) reported significant reductions in anthracnose stalk rot for hybrids with MON810, BT11, and 176 Bt events. In Iowa and Nebraska, results have varied among experiments. In Iowa fields that experienced considerable predisposing stresses and had little insect damage, stalk rot developed equally in Bt and non-Bt hybrids. Where European corn borer populations were moderate to high, significantly less stalk rot (primarily Gibberella) occurred in the Bt hybrids, and the effect differed among Bt events (8). Reimers et al. (17) reported that there were no differences in stalk rot between Bt and non-Bt hybrids in a 1997 experiment. We are continuing to investigate the relationships among European corn borers, stalk rot, and stalk strength in Bt hybrids.
Although the results described here support the utility of Bt hybrids for management of Fusarium ear rot and possibly Aspergillus ear rot and stalk rots of maize, it should be emphasized that these diseases all require an integrated management approach involving other tactics. Although Bt hybrids appear to be an effective tool for reducing fumonisins, this control tactic might not be enough when conditions are very favorable for disease development. In the southeastern United States in years favorable for severe ear rot, Bt hybrids can have levels of ear rot and mycotoxins similar to those in non-Bt hybrids. Ear rot diseases and their associated mycotoxins can occur in kernels in the absence of insect damage because they have other pathways for infection. Alternative infection pathways are an even greater limitation for stalk rot management with Bt hybrids, because the primary pathway for infection (through roots) is independent of lepidopteran feeding damage.
Another limitation of Bt maize hybrids is their spectrum of activity. Currently available events are very effective against European corn borer, but not as effective against corn earworm and fall armyworm. In the southern United States, where aflatoxin problems are chronic, corn earworm, fall armyworm, and southwestern corn borers are the primary lepidopteran pests feeding on maize ears. Damage to ears of Bt hybrids by these insects probably leads to A. flavus and F. verticillioides (F. moniliforme) infection and mycotoxin contamination.
Bt hybrids can be an important tool in the integrated management of Fusarium ear rot and possibly Aspergillus ear rots and maize stalk rots. New Bt hybrids now under development promise to provide more complete control of corn earworm and fall armyworm, which should enhance their effects on insect-associated fungi. New events also are being developed for control of coleopteran pests such as corn rootworms (Diabrotica spp.). Control of corn rootworms has the potential to reduce stalk rot by maintaining better root health and reducing physical damage to the roots where the stalk rot fungi can enter the plant. Coleopterans that feed on maize ears and silks, such as adult corn rootworms and sap beetles (family Nitidulidae) can contribute to ear rot (6). If new transgenic hybrids are resistant to these insects, there could be further contributions toward mycotoxin management. Transgenic control of insects and diseases offers an alternative that is much more effective, consistent, economical, and environmentally sound than foliar insecticides. For example, in sweet corn for fresh market sales, 12 or more insecticide applications may be made in a single season to control kernel-feeding insects and subsequent mold development. Even with currently available partial resistance to corn earworms in Bt hybrids, insecticide use can be drastically reduced (10).
Debate surrounding the use of genetically modified crops should be based on an assessment of all risks and benefits that can be measured, in comparison to the risks and benefits of other approaches to crop production. These risks and benefits include environmental impacts, livestock impacts, and potential human health threats. Available data show that Bt transformation of maize hybrids enhances the safety of grain for livestock feed and human food products by reducing the concentrations of toxic, carcinogenic fumonisins in the grain. Lower mycotoxin concentrations represent a tangible benefit to grain consumers, whether the intended use is for livestock or human foods. Consumers and regulatory agencies should consider these factors in decisions regarding Bt maize use.
1. Bergstrom, G. C., Davis, P. M., and Waldron, J. K. 1997. Management of anthracnose stalk rot/European corn borer pest complex with transgenic Bt corn hybrids for silage production. Biol. Cultural Tests 12:13.
2. Chiang, H. C., and Wilcoxson, R. D. 1961. Interactions of the European corn borer and stalk rot in corn. J. Econ. Entomol. 54:850-852.
3. Christensen, J. J., and Schneider, C. L. 1950. European corn borer (Pyrausta nubilalis Hbn.) in relation to shank, stalk, and ear rots of corn. Phytopathology 40:284-291.
4. Dodd, J. L. 1980. The role of plant stresses in development of corn stalk rots. Plant Dis. 64:533-537.
5. Dodd, J. L. 1997. Gray leaf spot tolerance, Bt resistance, stalk rot and yield of corn. Professional Seed Research, Inc. February 4, 1997.
6. Dowd, P. F. 1998. Involvement of arthropods in the establishment of mycotoxigenic fungi under field conditions, pp. 307-350 in Mycotoxins in Agriculture and Food Safety (Sinha, K. K., and Bhatagnar, D., eds.) Marcel Dekker, NY.
7. Dowd, P. F., and Munkvold, G. P. 1999. Associations between insect damage and fumonisin derived from field-based insect control strategies. Proc. 40th Annual Corn Dry Milling Conf., June 3-4, 1999. Peoria, IL.
8. Gatch, E. W., and Munkvold, G. P. 1999. The role of transgenic Bt hybrids in the management of the maize stalk rot complex. Proc. 111th Session, Iowa Acad. Sci., April 23-24, 1999, Ames, IA.
9. ILSI Health and Environmental Sciences Institute. 1999. An evaluation of insect resistance management in Bt field corn: a science-based framework for risk assessment and risk management. ILSI Press, Washington, DC.
10. Lynch, R. E., Wiseman, B. R., Plaisted, D., and Warnick, D. 1999. Evaluation of transgenic sweet corn hybrids expressing CryIA(b) toxin for resistance to corn earworm and fall armyworm (Lepidoptera: Noctuidae). J. Econ. Entomol. 92:246-252.
11. Marasas, W. F. O. 1995. Fumonisins: their implications for human and animal health. Natural Toxins 3:193-198.
12. Miller, J. D. 1999. Reducing the impact of mycotoxins on the agricultural economy: A perspective on regulation. http://www.scisoc.org/meetings/abstract/1999/sp99ab19.htm APSNet Publication P-1999-0118-SSA.
13. Munkvold, G. P., and Desjardins, A. E. 1997. Fumonisins in maize: can we reduce their occurrence? Plant Dis. 81:556-565.
14. Munkvold, G. P., Hellmich, R. L., and Rice, L.G. 1999. Comparison of fumonisin concentrations in kernels of transgenic Bt maize hybrids and non-transgenic hybrids. Plant Dis. 83:130-138.
15. Munkvold, G. P., Hellmich, R. L., and Showers, W. B. 1997. Reduced Fusarium ear rot and symptomless infection in kernels of maize genetically engineered for European corn borer resistance. Phytopathology 87:1071-1077.
16. Pilcher, C. D., Rice, M. E., Obrycki, J. J., and Lewis, L. C. 1997. Field and laboratory evaluations of transgenic Bacillus thuringiensis corn on secondary Lepidopteran pests (Lepidoptera: Noctuidae). J. Econ. Entomol. 90:669-678.
17. Reimers, C. I., Clark, T. L., Kamble, S. T., and Foster, J. E. 1998. Relationship of European corn borer and stalk rots in Bt and near isoline non-Bt maize hybrids in southeastern Nebraska. (Abstr.) 1998 Entomol. Sci. Am. North Central Branch Abstract D-7.
18. Rice, M. E., and Pilcher, C. D. 1997. Perceptions and performance of Bt corn. Pp. 144-156 in Proc. 52nd annual Corn & Sorghum Research Conf., Dec 10-11, 1997, Chicago, IL.
19. Smeltzer, D. G. 1958. Relationship between Fusarium ear rot and corn earworm infestation. Agron. J. 50:53-55.
20. Smith, D. R., and White, D. G. 1988. Diseases of corn, pp. 701-766 in Corn and Corn Improvement, Agronomy Series #18 (3rd ed.) (Sprague, C.F., and Dudley, J.W., eds.) ASA-CSSA-SSSA, Madison, WI.
21. Sobek, E. A., and Munkvold, G. P. 1999. European corn borer (Lepidoptera: Pyralidae) larvae as vectors of Fusarium moniliforme, causing kernel rot and symptomless infection of maize kernels. J. Econ. Entomol. 92:503-509.
22. United States Food and Drug Administration, Center for Food Safety and Applied Nutrition. 2000. Background paper in support of fumonisin levels in corn and corn products intended for human consumption. Online. FDA CFSAN home page.
23. Windham, G. L., Williams, W. P., and Davis, F. M. 1999. Effects of the southwestern corn borer on Aspergillus flavus kernel infection and aflatoxin accumulation in maize hybrids. Plant Dis. 83:535-540.