Corn grain can be naturally contaminated by mycotoxins, natural toxins produced by fungi.
Wounding of the kernel (Figure 2) makes it easier for infection by the fungi that produce mycotoxins. European corn borer and other caterpillars commonly produce wounds that favor infection and mycotoxin contamination.
That being the case, does the use of genetically modified Bt corn—genetically engineered to provide control of certain insect pests—result in reduced mycotoxin contamination?
Figure 1. Fusarium ear rot of corn, usually associated with fumonisin contamination.
To address this question, field studies have been conducted comparing Bt-hybrids to non-Bt hybrids that are otherwise genetically very similar. Thus far, the overall results are that meaningful mycotoxin reductions sometimes occur due to the Bt trait. A more detailed discussion follows.
Field studies in the USA and Europe have shown reduced Fusarium ear rot and reduced fumonisin contamination from Bt corn. These reductions have been associated with reduced insect feeding on kernels containing the Bt toxin.
In studies where reductions in fumonisin concentration occurred, these ranged from 20% to 90%, often bringing the grain below concentrations that pose risks to humans and the most sensitive animals.
Figure 2. Aspergillus ear and kernel rot of corn, often associated with aflatoxin contamination. The fuzzy appearance inside the kernel is the sporulation of the invading fungus.
It is interesting to note that the application of synthetic insecticides to control kernel-feeding insects on non-Bt plants also sometimes reduces insect feeding and fumonisin contamination. This raises the question as to which is more sustainable: use of the Bt trait or application of insecticides. Either way, reductions in fumonisin contamination are highly desirable.
As with fumonisins, field studies have often documented reductions in aflatoxin contamination due to the Bt trait. Significant reductions were most common when aflatoxin levels were at moderate to high levels in the non-Bt corn.
In studies where reductions were documented, these commonly ranged from 50-90%. In some cases, aflatoxin reductions from the Bt trait were sufficient to bring the corn below 20 parts per billion, an important regulatory threshold for use of the grain in human foods. As with fumonisins, aflatoxin reductions from the Bt trait have generally been linked to reductions in kernel injury from insect feeding. In some cases, the Bt trait did not result in aflatoxin reductions. This may be due to several reasons:
In those fields, drought stress may have been more important in promoting aflatoxin accumulation than insect damage to kernels;
As noted below, the Bt trait isn’t effective against all kernel-feeding insects.
There are several caveats and complicating factors in this line of research, as follows:
Bt corn isn’t a “magic bullet”, eliminating all mycotoxin contamination. Indeed, sometimes reductions in mycotoxin contamination do not occur. This is true for both fumonisins and for aflatoxins. However, reductions of fumonisins and aflatoxins have been documented commonly in field studies, especially under conditions moderately to highly favorable for ear rot and mycotoxin contamination. Such reductions occur commonly enough—with no known “downsides” relating to consumption of the grain—that the appropriate use of Bt corn is considered to be beneficial both for food safety as well as for animal health.
The Bt toxin must be expressed in the corn kernel in order to reduce insect injury and, ultimately, mycotoxin levels. In some Bt corn hybrids, the Bt toxin is not expressed in the kernel. Such hybrids have no protection against wounds created by insect feeding, and therefore fumonisin contamination is not reduced in these hybrids.
While the Bt trait is important in reducing mycotoxin contamination, the magnitude of mycotoxin reduction can depend on the genetic background of the corn hybrid. In other words, if you compare two hybrids that have the same Bt trait, one may have less fumonisin than the other, simply because of its genetic background.
The effectiveness of the Bt trait in reducing mycotoxin contamination depends on the insect pest present. For example, Bt corn is often effective at reducing feeding damage from the European corn borer, but not the corn earworm. Consequently, reductions in fumonisin contamination may occur if the European corn borer is the principal pest in a field, but not if the corn earworm is predominant.
Corn hybrids that express the Bt trait in kernels sometimes exhibit reduced kernel contamination from fumonisins and aflatoxins. This is thought to be due to reduced insect feeding on the kernels.
The Bt trait is not a “silver bullet”, eliminating all mycotoxin risk. However, reductions occur commonly enough, with no known “downside”, that the Bt trait is thought to contribute to food safety and livestock health in both developed and developing countries. While it is well-documented that the Bt trait can reduce mycotoxin contamination, it is best used wisely, and only in fields with a moderate to high risk of damage from the target insect pests.