Bt maize: less pest damage, fewer mycotoxins

Heavy pest infestation makes maize plants more susceptible to fungal infections. As a result, food and animal feed is more severely contaminated with fungal toxins. Effective measures to combat pests therefore often have a positive side-effect in that they also reduce mycotoxin levels. This applies to Bt maize as well. The finding from a trial series in Germany is confirmed by an international metastudy.

Like many types of grass and cereal, maize is often attacked by mould fungi of the genus Fusarium. Some of these fungi produce mycotoxins . If maize contaminated with mycotoxins is fed to animals, these highly poisonous substances can lead to severe health problems in the animals, e.g. to reduced fertility or digestive problems. Mycotoxins can also be harmful to human health. EU-wide maximum limits have therefore been set for three different mycotoxins (see table).

Fusarium fungi, which produce these toxins, can enter a maize plant through the stigma of the female flower, but also through sites where the plant has been attacked by chewing pests. The fungi then spread inside the maize plant.

How severely a maize plant is infected with fungi and whether this leads to mycotoxins being produced depends on a large number of factors. Humidity and temperature during the growing season, soil cultivation and the susceptibility of the variety in question all play a role, as does the time chosen for harvesting. The complex process, which is not fully understood, makes it difficult to control mycotoxin formation in individual cases and to reduce contamination reliably. Studies in Germany, for instance, have shown that mycotoxin levels vary widely between individual maize plants, even on the same site. There are many indications that stress – both for the maize and for the fungus – results in higher mycotoxin production.

But it is also clear that a heavy infestation of chewing pests leads to higher mycotoxin contamination. In many maize-growing regions of Europe the European corn borer is the main maize pest: the larvae bore their way into the maize plants, leaving holes through which Fusarium and other fungi can enter. The more effectively the corn borer is controlled, the fewer chewing sites there are that can be used by the fungal pathogens to colonize the maize plant, in addition to the stigma route.

Cultivation trials: Bt maize has lowest mycotoxin levels

Prof. Andreas Schier of Nürtingen-Geislingen University has been conducting systematic cultivation trials with genetically modified maize for years. Among other things, these involved measuring mycotoxin levels.

Although the results vary depending on the site, variety, trial design and type of mycotoxin, the general trend is clear (see diagrams)

     * There is a clear correlation between the severity of the corn borer infestation and the mycotoxin level.

     * Cultivation of the conventional maize variety without any measures to control the corn borer produced
        both the highest pest infestation and the highest mycotoxin levels. This effect is more pronounced on
        sites with high corn borer infestation.

     * Chemical and biological methods of controlling the corn borer, e.g. the use of insecticides or parasitic
        wasps (Trichogramma), can reduce both the number of corn borer larvae and the level of mycotoxin
        contamination.

     * On all sites, the Bt maize varieties used showed the best results: only isolated corn borers were found
        in the crops. On almost all the trial fields the mycotoxin values measured were lower in the Bt maize
        plants than in the conventionally grown control plants.

Summary evaluation of international studies

The results published by Andreas Schier in the latest issue of MAIS are also confirmed by international studies. Felicia Wu of Pittsburgh University (USA) evaluated a large number of studies from various countries that had investigated the relationship between Bt plants and mycotoxin levels in harvest produce.

     * Ten out of thirteen studies on Bt maize came to the conclusion that Bt maize is less contaminated with
        mycotoxins than the conventional control variety in each case. This applies to the mycotoxins DON,
        ZEA and FUM.

     * In the case of aflatoxins, a particularly toxic group of mycotoxins, the lessening effect of Bt maize is
        not as pronounced, and was found in only three out of seven studies. Aflatoxins are produced by
        various types of Aspergillus, which enter the plants primarily through the stigma. In North America,
        Aspergillus fungi are mainly transferred from pests that are not controlled by Bt maize.

Maize cobs with mould

Mycotoxins: maximum levels for unprocessed maize

Fusarium toxin	µ/kg
Deoxynivalenol (DON)	1750
Zearalenone (ZEA)	200
Fumonisin B1 and B2 (FUM)	2000

Corn borer infestation (number of larvae /100 plants; left axis) and DON mycotoxin levels in harvested grain maize (µg/kg; right axis) at various sites. All data: 2007 growing season.

Site: Altreetz (Brandenburg), practical evaluation. The study looked at MON810 Bt maize and a conventional check variety. No further measures for corn borer control were taken.

Site: Kitzingen (Bavaria), practical evaluation. Also included a conventional variety on which an insecticide (Steward 125 g/ha) had been applied to combat the corn borer.

Site: Nürtingen (Baden-Württemberg), large-plot trial. Comparison: MON810 Bt maize, conventional check variety, insecticide treatment (Steward 125 g/ha), biological treatment (two applications of the parasitic wasp Trichogramma)

All diagrams are simplified. All show the level of DON mycotoxins (Deoxynivalenol). ZEA (Zearalenone) and FUM (Fumonisin B1 and B2) levels were also investigated.